KEVIN A. CASSADY AND RICHARD J. WHITLEY
Viral infections of the central nervous system (CNS) occur infrequently and most often result in relatively benign, self-limited disease. Nevertheless, CNS infections have tremendous importance because of the potential for death and neurologic damage. The highly specialized brain tissue is exquisitely sensitive to metabolic derangements. Injured brain tissue recovers slowly and often incompletely. Even in patients who recover fully from viral encephalitis, months may be required for return to normal function (1). The brain and spinal cord provide diagnostic and therapeutic obstacles. On an anatomic level, the brain is housed in a closed skull with the spinal cord suspended within a bony columnar cage. A unique immunologic surveillance system and the blood–brain barrier further distinguish infections of the CNS from those involving other organ systems. Pathologic processes in the CNS have limited clinical expressions and frequently share pathogenic mechanisms. Tumors, infections, and autoimmune processes in the CNS often produce similar signs and symptoms (2). Clinical presentation and patient history, though frequently suggestive of a diagnosis, remain unreliable methods for determining the specific etiology of CNS disease (2,3). Understanding the pathogenic mechanism of a disease provides a rational basis for the development of antiviral medications and strategies for the prevention of viral CNS infections.
The pathogenesis of viral infections is multifactorial: age, immune status, cultural practices, and genetic makeup can influence the clinical manifestations of viral infection as readily as viral load, gene polymorphisms, receptor preference, and cell tropism. Although asymptomatic enteroviral infection predominates, some patients progress to viral meningitis or, rarely, fulminant encephalitis (1,4). A detailed description of the pathogenesis of the individual viral encephalitides is beyond the scope of this chapter. Instead, general concepts of viral infection and the pathogenic mechanisms of viral CNS infection are reviewed and specific examples developed where applicable.
DEFINITIONS
Viruses display tissue tropism and cause illness with a characteristic temporal course. The definition of viral CNS disease is often based on both viral tropism and disease duration. Encephalitis refers to inflammation of parenchymal brain tissue. Acute encephalitis occurs over a relatively short period of time (days), whereas chronic encephalitis presents over weeks to months. The temporal course of slow infections and spongiform encephalopathies of the CNS (kuru, visna, variant Creutzfeldt-Jakob disease) overlaps with that of the chronic encephalitides. These progressive CNS diseases are distinguished by a long incubation period, eventually resulting in death or extreme neurologic disability over months to years (1,5).
Viral disease in the CNS can also be classified by pathogenesis. Neurologic disease is frequently categorized as either primary or postinfectious (1). A primary encephalitis results from direct viral entry into the CNS that produces clinically evident cortical or brainstem dysfunction. Subsequent damage occurs as a consequence of the host immune response, but invasion by the pathogen initiates CNS damage (6). The parenchyma exhibits neuronophagia, and viral antigen or nucleic acids can be detected (6). Postinfectious or parainfectious encephalitis is an acute demyelinating process temporally associated with a systemic viral infection but without evidence of direct viral invasion in the CNS and is included as one of the causes of acute disseminated encephalomyelitis (ADEM) (6,7). Pathologic specimens demonstrate demyelination and perivascular aggregation of immune cells but no evidence of virus or viral antigen, suggesting an immune-mediated etiology (1). The presence of immune cells distinguishes primary and postinfectious encephalitis from an encephalopathy.
Inflammation occurs at multiple sites within the CNS and accounts for the myriad of clinical descriptors of viral neurologic disease. Inflammation of the spinal cord, leptomeninges, dorsal nerve roots, or nerves results in myelitis, meningitis, radiculitis, and neuritis, respectively. Aseptic meningitis is frequently used to refer to a benign, self-limited, viral infection causing inflammation of the leptomeninges (1). The term aseptic meningitis is used instead of viral meningitis because a pathogen fails to grow in conventional culture media and reflects the historic ability to diagnose and treat only bacterial and fungal CNS infections (6,8). This misnomer hinders epidemiologic studies, because the definition fails to differentiate between infectious (fungal, tuberculous, viral, or other infectious etiologies) and noninfectious causes of meningitis. Meningitis and encephalitis can represent separate clinical entities; however, a continuum exists between these distinct forms of CNS disease (1). A change in a patient’s clinical condition can reflect disease progression, with involvement of different regions of the CNS making it difficult to predict the extent of CNS infection early in the clinical course. A patient may present with meningismus and be diagnosed as having viral meningitis and then progress to meningoencephalitis with altered consciousness and focal CNS changes (6). Epidemiologic data in many cases provide clues to the viral etiology.
EPIDEMIOLOGY
Epidemiology studies of meningitis and encephalitis potentially underestimate the true incidence of viral CNS infections. Even when aseptic meningitis was a reportable disease, not all patients having a cerebrospinal fluid (CSF) pleocytosis or symptoms consistent with a viral meningitis had viral cultures or other diagnostic studies performed. An overview is difficult, because each pathogen fills a different ecologic niche with unique seasonal, host, and/or vector properties (1) (Tables 4.1 and 4.2). Instead, it is useful to analyze the individual agents responsible for viral brain infections in an effort to find population patterns and trends.
Historically, laboratory techniques for identifying neurologic infections were insensitive, invasive, and required brain biopsy. Over the last two decades, molecular detection techniques have improved the detection of pathogen’s nucleic acids in the CSF (8,9). Despite the improved sensitivity of these techniques, the pathogen remains unidentified in the majority of cases of encephalitis. Depending on the study and diagnostic methods used, investigators fail to identify an agent in the majority of presumed CNS infections (10,11). CSF viral culture rates differ based on etiology. They can often be diagnosed only presumptively by acute and convalescent serologic testing or isolation of virus from another location in the body (6,12). In a retrospective review of patients who had positive bacterial CSF cultures, 1 of 20 had a concomitant virus isolated from the CSF (13,14). Historically, the definitive method for virus detection in encephalitis was brain biopsy and viral culture (1,2). Polymerase chain reaction (PCR) techniques and other molecular biologic methods from CSF samples have replaced culture and brain biopsy as the standard for diagnosing encephalitis for some viruses (herpes simplex virus [HSV], enterovirus, varicella-zoster virus [VZV], and JC virus) (8,15,16). PCR has exquisite sensitivity; however, the technique’s sensitivity can lead to erroneous diagnosis, because PCR may detect latent or integrated viral DNA potentially unrelated to the pathogenic process (1). The introduction and testing of new antiviral drugs will likely provide an impetus for accurate and timely diagnosis.
Acute viral meningitis and meningoencephalitis represent most viral brain infections and frequently occur in epidemics (1). Enteroviruses cause an estimated 60% to 90% of cases, whereas arboviruses constitute the majority of the remaining reported cases (1,8). The Centers for Disease Control and Prevention (CDC) received notification of approximately 7,200 to 14,500 cases of “aseptic meningitis” annually (1). Most of these cases occurred from the late spring to autumn months, reflecting the increased incidence of enteroviral and arboviral infections during these seasons (17,18). The incidence and etiology of encephalitis varies based on geography, environmental factors, and frequency of exposure to vectors responsible for viral transmission (19,20).
The CDC received 740 to 1,340 annual reports of persons with encephalitis from 1990 to 1993 (1). Herpes simplex virus infection of the brain occurs year round without seasonal variation, affects all ages, and constitutes most fatal cases of endemic encephalitis in the United States (21). Arboviruses, a group of more than 500 arthropod-transmitted RNA viruses, are the leading cause of encephalitis worldwide and in the United States (1). Arboviral infections occur in epidemics and show a seasonal predilection, reflecting the prevalence of the transmitting vector (22). Asymptomatic infections vastly outnumber those that are symptomatic. Patients with symptomatic infections may develop a mild, systemic febrile illness or a viral meningitis. Encephalitis occurs in a minority of persons with arboviral infections, but the case-fatality rate varies extremely from 5% to 70%, depending on viral etiology, age of the patient, and unique host differences (1,23).
Japanese B encephalitis and rabies constitute most cases of encephalitis outside of North America. Japanese B encephalitis virus, a member of the genus Flavivirus, occurs throughout Asia and causes epidemics in China despite routine immunization for the virus (24,25). In warmer locations, the virus occurs endemically (26). The disease typically affects children, although adults with no history of exposure to the virus are also susceptible (27). As with the other arboviral infections, asymptomatic infections occur more frequently than symptomatic infections. However, the disease has a high case-fatality rate and leaves half of the survivors with a high degree of neurologic morbidity (27). Of note, West Nile virus (WNV) encephalitis, a member of the Flavivirus family, has increased in incidence strikingly in the United States (28). In 2002 alone, the CDC reported more than 3,989 cases and nearly 250 deaths. WNV infection declined in the United States such that between 2008 and 2011, only 712 to 1,356 cases were reported. There was an increase in cases (5,387) in the United States in 2012. Many of the cases occurred in the Mississippi Valley and Southern and Central United States, suggesting an evolving epidemiology for this introduced pathogen (Fig. 4.1) (http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount12_detailed.htm). Rabies virus, a bullet-shaped RNA virus of the family Rhabdoviridae, remains endemic around the world (29). Human infections in the United States decreased over the last decades to one to three cases per year because of the immunization of domesticated animals. Bat exposure is increasingly recognized as the source of infection. Fifteen percent (685 of 4,470) of bats tested carried the rabies virus in one study analyzing risk of bat exposure and rabies (30). In most cases, (22 of 24) there was no evidence of bite; however, in half of the cases, direct contact (handling of the bats) was documented (31). There is experimental evidence that bat-associated rabies virus variants transmit across the dermis and potentially through hair follicles (29). Alternatively, bat bites may not have been recognized (1). In areas outside the United States, annual cases of rabies encephalitis number in the thousands.
Postinfectious encephalitis, an acute demyelinating process, accounts for approximately 100 additional cases of encephalitis reported to the CDC annually in historical studies (32,33). The disease historically produced approximately one third of the encephalitis cases in the United States and was associated with measles, mumps, and other exanthematous viral infections (1). Postinfectious encephalitis is now associated with antecedent upper respiratory viral infection (noticeably with influenza virus) and varicella in the United States (32). Measles continues to be a leading cause of postinfectious encephalitis worldwide. In addition to the postinfectious process, patients with paraneoplastic syndrome and autoantibodies to the N-methyl-D-aspartate (NMDA) autoantibodies have also been recently described (34). Recent studies suggest that antigenic variation in the N-terminal domain of the NMDA receptor may predispose these patients to the autoimmune encephalitis (35). The slow infections of the CNS and transmissible spongiform encephalopathies (TSEs) occur sporadically worldwide (5). The prototypical TSE is Creutzfeldt-Jakob disease (CJD); it occurs at high rates within families and has an estimated incidence of 0.5 to 1.5 cases per million populations. In 1986, cases of a TSE in cattle, bovine spongiform encephalopathy (BSE), were reported in the United Kingdom. In addition to affecting other livestock throughout Europe that were fed supplements containing meat and bone meal, cross-species transmission of BSE has been documented, leading to a ban in the use of bovine offal in fertilizers, pet food, or other animal feed (5). Increases in atypical CJD cases coincided with the peak of BSE cases, suggesting animal to human transmission (5). The report of atypical CJD (unique clinical and histopathologic findings) affecting young adults (an age at which CJD rarely has been diagnosed) led to the designation of a new disease, variant Creutzfeldt-Jakob disease (vCJD). From 1996 to 2011, there have been 224 cases of vCJD reported, with 175 of these occurring in Ireland and Great Britain (World Health Organization [WHO] Web site: www.who.int). The numbers have declined since the ban in the use of bovine offal in fertilizers, pet food, or other animal feed. Active monitoring is still important, and detection of BSE continues to be reported in North America (Fig. 4.2) (CDC BSE Web site: http://www.cdc.gov/ncidod/dvrd/bse/).
Environmental factors influence infections of the CNS. Changes in behavior, cultural beliefs, and modification of the environment result in changes in disease patterns and exposure to new infectious agents. Arboviral infections will likely increase as populations encroach on wilderness habitats and flood plains (1). Vaccination has further changed the character of viral CNS disease. In 1952, poliomyelitis affected 57,879 Americans (1). Widespread vaccination has eradicated the disease currently from the Western Hemisphere. As social and environmental changes occur globally, the character and prevalence of CNS viral infections will also change. CNS infections must be examined in a geographic, cultural, and environmental context as well as at the cellular, molecular, and genetic levels.
PATHOGENESIS
Viral Spread
Viruses use two basic pathways with fundamentally different steps to gain access to the CNS: hematogenous and neuronal. Viruses must survive and multiply at the cellular level efficiently and in sufficient quantity to infect the CNS. The mechanism of spread to the CNS is in large part determined by viral factors, site of entry, and successful replication in intermediate cells (1). The local immune response at the site of entry, the systemic immune responses, and the limited vascular access afforded by the blood–brain barrier further reduce the opportunity for viral neurologic infections (1). Differences in host physiology and mechanism of spread to the CNS further influence the clinical manifestations of neurologic disease (1). For example, adults with herpes simplex encephalitis (HSE) have different presenting signs and symptoms than newborn babies with HSV infection of the CNS. The route of viral spread and areas of neurologic involvement differ based on the age of the patient and mechanism of exposure (36). The subsequent neurologic damage and poor outcome, however, are similar (1). Subtle differences at the epidemiologic, host, tissue, cellular, and genetic levels can alter this balance between viral exposure and symptomatic infection.
Hematogenous Spread
Enteroviruses and arboviruses are prototypes for viremic spread to the CNS. Although the location of viral entry differs for each family, both cause primary and secondary viremia prior to infecting the CNS. Reviewing the necessary steps and the numerous barriers to hematogenous neurologic infection explains the low incidence of symptomatic viral infection and the even lower frequency of viral neurologic infections. A virus must first bypass or attach to and enter host epithelial cells to produce infection (37) (Fig. 4.3). In addition, the cell must be permissive, providing an adequate environment for viral replication. The initial steps involved in hematogenous spread of virus to the CNS consist of replication at the local site of entry and primary viremia (1). Infection of a secondary tissue frequently ensues, permitting secondary replication and an extensive viremia that seeds the CNS. Not all viruses follow this sequence, and genetic factors of both virus and host influence the route of viral spread.
The cornified layers of dead skin cells provide a structural defense for the greatest potential infective surface area of the human body. Layers of keratin protect the underlying epithelium from viral contact, thereby decreasing the incidence of viral entry (1). Breaks in this defensive layer can result in higher frequency of infection as well as more severe disease. Some vector-borne viruses bypass the cornified epithelial layer by inoculation into the subepithelial layer or directly into the blood (1). The nonkeratinized epithelial layer that constitutes the conjunctival, respiratory, oral, and nasopharyngeal surfaces provides an ideal entry point for aerosolized viruses or pathogens transmitted by large droplets. Parainfluenza and adenovirus, although uncommon, can cause primary encephalitis (1). More frequently, however, the respiratory viruses are associated with postinfectious encephalitis (32). A mucous layer composed of mucopolysaccharides, secretory immunoglobulins, and inflammatory cells provides a mechanical, chemical, and cellular defense against pathogens (1). In the gastrointestinal and urogenital systems, constant transit protects the mucosa. As in the respiratory mucosa, leukocytes and secretory factors augment this mechanical defense. The enteroviruses tolerate stomach acid, bile salts, proteolytic enzymes, and alkaline infusions to infect the host. Certain viruses (coxsackievirus A9) actually require exposure to proteolytic enzymes in the gut before they can infect select cell types (1).
Once virus breaches the epithelial barrier and finds a permissive cell, primary replication occurs. Virus then can spread and replicate in the lymph node, or it can bypass the node and enter the circulatory system, where it seeds other tissues (arbovirus, enterovirus, measles virus, or varicella virus) (1). Local immune responses are crucial in limiting systemic viral infection. The generation of a swift inflammatory response can limit viremia and symptoms of infection. Some viruses resist phagolysosomal degradation, allowing them to circulate and replicate within the protective sheath of a macrophage (38,39). Antigenic changes and the sequestration of viral receptors provide additional mechanisms that enable viruses to evade lymphocytes. For example, human rhinovirus 14, influenza virus, and poliovirus have receptors embedded in a recess or “canyon” in the viral membrane (1). The virus is able to evade the immune response by altering the molecules on the surface surrounding the highly conserved, immunologically inaccessible receptor molecules lining the canyon. Other viruses have hypervariable sequences surrounding a small, molecularly conserved binding sequence. The viral binding site may be smaller than the antigenic sequence recognized by the immunoglobulin. Changes in the hypervariable molecules surrounding the binding site allow the virus to evade immune responses without disrupting the fidelity of the receptor binding site (40,41).
Primary viremia allows virus to seed distant locations of the body and frequently marks the onset of clinical illness. Virus circulates in the vascular system attached to or within host cells such or as free virus within the plasma (1). Viruses have limited access to the CNS via cerebral vessels and require sufficient numbers of progeny to overcome the improbability of contact and entry into a permissive cell. In rare circumstances, such as disseminated neonatal herpes infection, virus infects the CNS after primary viremia. However, most infect an intermediate tissue prior to reaching the CNS. Viral genes may be as important as host physiology in determining the route and degree of viral dissemination. For example, the reovirus S1 gene determines the mechanism of viral spread in the host. The S1 gene codes for a capsid hemagglutinin, σ1, that binds to neuron receptors. Serotypes with an intact σ1 gene spread to the CNS by neuronal pathways, whereas σ1-deficient mutants gain access to the CNS via the hematogenous route (42).
The liver and spleen provide ideal locations for secondary viral replication because of their highly vascular structure. The high degree of parenchymal contact and large number of fixed mononuclear macrophage cells also provide an excellent opportunity for host eradication of viremia (1). Viruses infect tissues other than the liver and spleen, such as muscle, endothelium, and blood cells. These sites provide an environment for viral replication in highly vascular locations that facilitate extensive viremia. Secondary viremia produces high titers of virus in the bloodstream for prolonged periods of time, facilitating the seeding of target organs. Viral genetics and host physiology determine the location and extent of infection at these secondary sites (1).
Virus must localize in the vessels of the CNS before crossing the blood–brain or blood–CSF barrier, a network of tight endothelial junctions sheathed by glial cells that regulate molecular access to the CNS (43). The pathophysiology of viral transport from blood to brain and of viral endothelial cell tropism is poorly understood. Virus infects endothelial cells, leaks across damaged endothelium, passively channels through endothelium (pinocytosis or colloidal transport), or bridges the endothelium within migrating leukocytes (1). Cell-associated and cell-free viruses can cross the endothelium and enter the parenchyma or CSF. This bridging of the endothelium occurs in choroid plexus vessels, meningeal blood vessels, or cerebral blood vessels (1) (Fig. 4.4). Once in the CSF, virus may remain limited to the meninges or may enter the brain parenchyma across either ependymal cells or the pial linings.
Neuronal Spread
Rabies and HSV infection are prototypes of viral CNS infections that access brain by peripheral neuronal spread. Historically, the peripheral neural pathway was considered the only pathway of viral neurologic infection. Experiments with HSV and rabies virus performed in the nineteenth and early twentieth centuries, combined with the discovery of the blood–brain barrier at the turn of the century, led most investigators to conclude that all viral neurologic infections occurred by neuronal spread (1). Contemporary data, however, show that the bloodstream provides the principal pathway for CNS infections in humans. Some viruses (poliovirus and reovirus), previously thought to infect the CNS by the hematogenous route, have been detected in peripheral neurons in experimental models (44). Viremia and neuronal spread to the CNS can occur concurrently and are not mutually exclusive (1).
Neuronal spread occurs along peripheral or cranial nerves. The nerve shields the virus from immune regulation and allows access to the CNS. Rabies virus classically infects by the myoneural route; however, infection has been documented from corneal transplantations, and aerosolized entry has occurred following spelunking in caves contaminated with infectious bat guano (29,45). These sources of infection are infrequent and employ the same axonal mechanism of spread within the nerve, albeit from a different location than the myoneural route.
Rabies virus replicates locally in the soft tissue following a rabid animal bite, although entry into sensory nerves prior to soft tissue replication has also been documented (1). Protection by antibody-mediated immune mechanisms in the soft tissue provides the only known method of preventing neurologic disease and death (29). After primary replication, the virus enters the peripheral nerve. Experimental evidence demonstrates acetylcholine receptor binding as the mechanism of myocyte entry (1). However, viruses have also been documented in cells lacking these receptors. Once in the muscle, the virus buds from the plasma membrane and may cross myoneural spindles or enter the nerve by the motor endplate. The virus then travels by anterograde and retrograde intraaxonal transport to infect neurons in the brainstem and limbic system. Viruses appear to cross the transsynaptic space between neurons by passive transport rather than receptor-mediated transport. Recent evidence suggests rabies virus enters projections in the postsynaptic neuron that extend into invaginations on the presynaptic side. These projections pinch off and fuse with the presynaptic membrane, allowing the virus to spread along motor or sensory neural pathways (1,45).
Paresthesias near the location of the animal bite and change in behavior follow over the next weeks. These signs and symptoms correlate temporally with the axoplasmic transport of virus and infection of the brainstem and hippocampal region (1). The infection spares cortical regions during this phase, allowing animals to vacillate between periods of calm, normal activity and short episodes of rage and disorientation (45). Eventually, the virus spreads from the diencephalic and hippocampal structure to the remainder of the brain, killing the animal. Experimental rabies infections in animals demonstrate that the mode of acquisition influences the neuroanatomic location of initial infection (1).
Viruses also infect the CNS through cranial nerves. The olfactory system is unique among cranial nerves in that the neurons regenerate and have approximately a 1-month life span. The olfactory neurons are not protected by the blood–brain barrier, theoretically providing direct neuronal access to the brain (1). Animal studies have shown that HSV can infect the brain through the olfactory system as well as the trigeminal nerve. Moreover, the inferomedial temporal lobe, the initial location of early HSV encephalitis, contains direct connections with the olfactory bulb. The association of viral latency in the trigeminal ganglia, the relative infrequency of HSE, and the confusing data regarding encephalitis from HSV reactivation suggest that the pathogenesis is more complex than described earlier (1).
Host and Viral Factors Influencing Neurotropism
As illustrated in Tables 4.3 and 4.4, viruses exhibit differences in neurotropism (1). Strain and serotype differences influence viral neuroinvasion and neurovirulence. For example, reovirus types 1 and 3 produce different CNS diseases in mice based on serotype differences in receptor affinities (1,42). Escape mutant B4 of tick-borne encephalitis (TBE) virus also demonstrates viral differences in mouse neuroinvasiveness. A single amino acid substitution (Tyr to His) in domain 2 of viral surface protein E eliminates viral neuroinvasiveness without affecting neurovirulence (1). Receptor difference is only one determinant of viral neurotropism. Other viral factors may influence neurotropism. For example, enteroviruses in the same receptor family produce very different diseases. Coxsackieviruses B1 through B5 readily produce CNS infections, whereas type B6 rarely produces neurologic infection. Viral genes influence neurovirulence of HSV-1. Mutant HSV-1 viruses with either γ134.5 gene deletions or stop codons inserted into the gene have a decreased ability to cause encephalitis and death following intracerebral inoculation in mice as compared with wild type virus (46,47). Upon entering mouse neuronal cells, these γ134.5 (−) mutants trigger the shutdown of protein synthesis and elicit interferon signaling responses that limit efficient viral replication (48).
Host physiology is also important in determining the extent and location of viral CNS disease. Age, sex, and genetic differences between hosts influence viral infections and clinical course. With respect to HSV infection, host mutations in pattern recognition receptors important for type I interferon production predispose patients to HSE (49). Host age influences the clinical manifestations and sequelae of a viral infection (50). Differences in outcome are twofold: mature neurons resist virally induced apoptosis, and younger patients can have more immature immune response to infection (51,52). Differences in macrophage function can alter infections and disease. Moreover, macrophage processing capacity can change with age in humans (1,38). Enteroviral infections exemplify the difference that host physiology plays in determining the extent of viral disease. Enterovirus infections in children younger than 2 weeks of age can produce a severe systemic infection, including meningitis or meningoencephalitis (53). Ten percent of neonates with systemic enteroviral infections die, and as many as 76% are left with permanent sequelae (1). In older children, however, enteroviral infections produce less severe disease. In addition to age, physical activity may be another important host factor that determines the severity of infection. Exercise and trauma have been associated with increased risk for paralytic poliomyelitis and may result in an increased incidence of enteroviral myocarditis and aseptic meningitis (1,54). The frequency of infections in groups frequently reflects epidemiologic differences in exposure. Increasingly, host differences are recognized as equally important determinants of disease at the cellular and molecular levels.
Central Nervous System Physiology
The blood–brain barrier limits chemical and environmental exposure to the CNS by a series of tight endothelial junctions bound and maintained by glial cell foot processes. This barrier provides a physiologic boundary between the metabolically sensitive neuronal cells and the chemical changes outside the CNS (43). In addition, the endothelial cells and tight junctions provide a physical barrier to most pathogens, limiting viral access to the CNS. As with most biologic systems, the blood–brain barrier is more complex and heterogeneous than previously imagined. The blood–brain barrier was first described during the late nineteenth and early twentieth century, when scientists noted that various dyes administered intravenously failed to penetrate the CNS (1). In the 1960s, experiments certified that the tight junctions between endothelial cells lining the cerebral vessels blocked the passage of small protein molecules.
The tight junctions between endothelial cells provide a relatively impermeable layer to most polar substances. Unique transport systems and enzymes further distinguish the CNS capillaries from blood vessels in other organs. The asymmetric distribution of transport proteins in the endothelial cell membrane creates a highly resistant, polarized cell layer that limits paracellular diffusion (55). Hydrophilic substances cross the endothelial layer through receptor-mediated endocytosis or through highly specific, saturable transport systems. Respiratory gases and lipophilic chemicals passively penetrate the layer of tight junctions readily. The cerebral vessel endothelial cells also possess second-messenger molecules that may regulate transmembrane permeability through receptor binding (55). Substances produced during infection or chemicals secreted by cells, such as histamine and interleukins, change the permeability of the blood–brain barrier, thus modulating entry of viruses and immune cells into the CNS. Astrocytes are metabolically important support cells of mononuclear macrophage origin that surround cerebral capillaries, induce tight junctions, and may regulate immune cell entry (1).
The brain is an immunologically “privileged” site into which immune cells do not readily enter. Increasingly, scientists are discovering that immune cells reside in and circulate through the Virchow-Robin space, a lymphatic channel lining the perivascular space in the brain (1) (Fig. 4.5). Moreover, many of the fixed glial support cells and pericytes surrounding the vessels in the CNS can transform to monocyte/macrophage antigen-presenting cells. The circulating lymphocytes act as surveillance cells, detecting small amounts of antigen presented by the macrophages in the perivascular space and initiating the immune response either within the Virchow-Robin space or peripherally at the lymph node. During periods of infection, immune cells readily enter the CNS and fill the Virchow-Robin space (1) (Fig. 4.6). The perivascular space provides a staging area where lymphocytes interact chemically and differentiate prior to entering the neuropil. Cells in the perivascular space as well as cerebral capillary endothelial cells are capable of regulating T-helper cell subsets in vitro and may influence the expression of the immune response, dictating which cells enter the CNS. Different viruses may activate characteristic lymphocyte subsets for entry into the parenchyma. In some cases, the immune response is instrumental in the pathogenesis of CNS damage (1,51,56,57).
VIRAL REPLICATION IN THE CENTRAL NERVOUS SYSTEM
The fundamental principles of viral replication and cell-to-cell spread provide a framework for examining the pathogenesis and clinical repercussions of neurologic infections. The clinical manifestations and the severity of illness reflect the location and extent of viral replication in the CNS. Once virus accesses the CNS, it must introduce its genome and transport proteins into the cytoplasm or the nucleus of the mammalian cell. Once the viral genome has been uncoated, transcription and translation proceed in a predictable and organized cascade of gene expression, culminating in the replication of the viral genome. Translation of late viral genes produces structural proteins essential for the construction of the next generation of viruses. Viral genomic material is packaged with structural proteins and exits the cell (1) (Fig. 4.7). Viruses exploit essential cell activities such as protein synthesis, intracellular transport, and cellular communication to enter the cell and replicate their genome. As in other biologic systems, both divergent and convergent evolution has resulted in an array of mechanisms for successful viral reproduction. As a result, numerous strategies exist for viral entry, gene expression, replication, assembly, and egress (1). The relative speed and efficiency with which the virus replicates determine the progression of infection.
Attachment and Entry
Attachment is an essential first step in viral infection. Multiple copies of proteins line the surface of a virus. These capsid or envelope proteins create high-affinity bonds with host receptors and initiate viral infection or a host cell response (58,59). Classically, this response involves viral entry but may include a change in cellular metabolism or generation of immune responses by the cell. Temperature, pH, receptor affinity, and the concentration of viral and host receptors influence the host–viral receptor interaction similar to a receptor-ligand reaction. The cell receptor consists of proteins, lipids, and/or oligosaccharides. Receptor binding provides close contact between virus and cell, facilitating but not ensuring viral entry into the cell. Some viruses require specific chemical or proteolytic conditions before entering the cell (60–62). For example, Semliki Forest virus requires the presence of cholesterol in the cell membrane as well as a pH change in the endosome for entry (63). The presence of one type of receptor for cell entry does not preclude other mechanisms of entry into a host cell.
Viral entry into the cell is essential. Although receptors have been identified, alternative entry mechanisms are being identified for viruses (64). Studies determining the structure of viral glycoproteins and host receptor interactions as well as experiments using viral recombinants and cell lines expressing cellular receptors provide two methods used to characterize viral entry. Viruses can bind nonspecifically to the cell surface; however, these nonspecific interactions do not produce a biologic response. Viruses frequently target essential and/or tightly conserved host receptor domains. Some viruses appear to interact with neurotransmitter receptors in the CNS. Experimental data indicate that rabies virus binds to acetylcholine receptors on mouse myocytes (1). Reovirus 3 binds to the β-adrenergic receptor. Viruses also bind to immunologic proteins on the surface of cells. Poliovirus, HSV, and measles virus bind to receptors in the immunoglobulin superfamily (65,66). Hormone and cytokine receptors provide additional targets for viral cell entry. Viruses can have more than one mechanism for entering a cell or different receptors for different cell types (1). The number and distribution of receptors help determine viral tissue tropism and the extent of viral CNS disease. Receptor prevalence is not the only determinant of viral tissue tropism (1). Transgenic mice, for example, develop poliovirus infection only in limited tissue sites despite the widespread expression of the receptor. Some viruses require the presence of certain genes and transactivating factors to infect a cell. While a cell may contain a certain receptor, a permissive environment for viral replication may not exist. The tissue, in such a case, is resistant to infection (67).
Enveloped viruses have different mechanisms than nonenveloped viruses for cell entry. Once the virus binds to the host cell receptor domain, the virus can enter the cell by direct fusion or receptor-mediated endocytosis. The receptor-bound virus frequently becomes encased in a clathrin-coated pit during endocytosis. Other modes of endocytosis exist, and virus has been found in uncoated vesicles (1,68). Fusion proteins contain hydrophobic regions and initiate the union of viral and cell membranes in some enveloped viral infections. Nonenveloped viral entry is more enigmatic. Conformational changes or proteolytic cleavage may expose hydrophobic regions of capsid proteins, enabling the protein capsid to fuse with or embed in the cell membrane. The capsid then opens and releases the viral genome into the cytoplasm. Endocytosis may provide the pH change or enzymes necessary for virus–cell fusion and ensures that the cell is metabolically viable. Furthermore, it has been suggested that endocytosis delivers the viral genome to the proper intracellular location from which replication occurs. Viral fusion, in most cases, occurs before the endosome fuses with the lysosome.
Replication and Egress
Viral replication begins after uncoating and delivery of the genome to a satisfactory intracellular location (68). Viruses replicating in the nucleus often contain nuclear targeting signals and can use existing cellular mechanisms to enter the nucleus (1). Alternatively, viruses that replicate in the cytoplasm uncoat and deliver the genome to the perinuclear area (68). The production of positive stranded viral messenger RNA (mRNA) and the subsequent translation of gene products provide a unifying pathway for viral infections (Fig. 4.8). Viruses use either host enzymes or specialized viral polymerases carried with or encoded by the viral genome.
Host protein synthesis decreases at the start of viral protein synthesis in most viral infections. Viruses have unique mechanisms for inhibiting protein synthesis and can interfere with the translation, transport, ribosomal binding, or stability of host transcripts (1). With some viruses, premade proteins and synthesized viral gene products decrease the transcription of host mRNA (69–71). Cellular transport of mRNA out of the nucleus is inhibited late in adenovirus infections. Viral mRNA copies can outnumber host mRNA or can be more efficiently transcribed, thus restricting access to ribosomes (1). For example, poliovirus inactivates the host cap-binding protein. This alters the cell’s ability to modify transcripts and results in less efficient translation of host proteins (1). Degradation of host mRNA is another mechanism used by some HSV to inhibit host protein synthesis (71). Some viral gene factors act as repressors and inhibit host mRNA export (70).
Viral protein translation occurs in a stereotyped progression. Early gene expression regulates the transcription and translation of the remaining viral genome, inhibits host protein and nucleic acid synthesis, and codes for enzymes necessary for viral nucleic acid replication (1). After viral nucleic acid replication, late viral genes are selectively expressed and transcribe templates for capsid and structural proteins necessary for virion assembly. Proteins synthesized from viral transcripts can undergo posttranslational modification and glycosylation (72). Viruses contain regulatory proteins and promoter sequences that control the differential expression of transcripts. Proteolytic modifications are made to the structural protein following attachment of fatty acids and oligosaccharides in the Golgi apparatus (1). In some cases, these proteolytic changes are necessary for producing infectious progeny. TBE virus, for example, requires cleavage of a membrane-bound precursor protein (prM). The proteolytic change produces a small, membrane-bound protein (M protein) that protects another membrane-bound protein from conformational changes in the acidic secretory pathway. Viruses that contain the uncleaved prM moiety lack fusion capability and are noninfectious (1).
Replication of the viral genome involves the synthesis of full-length, complementary genomic transcripts that act as templates for replication of the viral genome. The efficiency and fidelity of genomic replication influence the likelihood of disease. Defects in the viral genome cause abortive replication or result in conditionally defective viruses that multiply only in the presence of cells or viruses carrying complementary genes. The newly synthesized progeny genomes are transported to capsid structures, where they enter viral capsid shells. Enveloped viruses bud from the cell membrane, whereas nonenveloped viruses exit the cell by lysis (1).
Viral Spread in the Central Nervous System
Viral disease of the CNS requires cell-to-cell spread of the virus. The densely packed neuropil provides a unique environment with limited extracellular space for viral dispersion. Viruses can spread through the CNS in four prototypical ways: (a) sequential cellular infection, (b) movement in the extracellular space, (c) neuronal axoplasmic transport, or (d) transit via migrating lymphocytes or glial cells. Viruses may spread within the neural tissue using more than one mechanism. Few viruses infect the CNS by contiguous cell-to-cell spread. Sindbis virus provides one example of a virus that spreads from ependymal cells directly to glial and neuronal cells in experimentally infected mice (1). Viruses exhibit cell tropism, frequently infecting one cell type more readily than another. For example, HSV-1 infects neurons early during encephalitis but is not present in glial cells until late in the infection. Herpesvirus spreads in the nervous system via axoplasmic transport in neurons (73,74). Electron microscopy has demonstrated togaviruses within extracellular space in the CNS. Some viruses enter the CNS through a Trojan horse mechanism via leukocytes (1,75).
Host Defense and Immunopathogenesis
Intrinsic and systemic antiviral defenses limit viral replication and infection (44,51,76). Viral replication can activate enzymatic pathways that degrade viral nucleic acid transcripts. Other cells undergo apoptosis, creating a nonviable environment for the virus (1). Interferon-mediated intrinsic antiviral pathways within cells can retard viral penetration, uncoating, transcription, translation, and assembly, representing an important factor of host resistance to viral infection (1,77–79). Interferons—type I (interferon-α and interferon-β), type II (interferon-γ), and type III (interferon-λ)—are secreted by distinct cells, bind to different receptors, and represent evolutionarily distinct molecules that limit viral replication (44,80–84). Interferons activate a cascade of enzymes and kinases that inhibit protein synthesis at different steps in the synthetic pathway. Interferons also modify the binding properties, electrostatic charge, and receptor expression (major histocompatibility complex [MHC] antigen, βσ2-microglobulin, and Fc receptor) of cellular membranes, further restricting viral access and replication (1). These cytokines can enhance or suppress expression of immune cell subsets (82,85). Although interferon can protect host cells from viral infection, some pathogens have developed resistance. Furthermore, the inflammatory response in some cases causes damage to tissue and constitutes a pathogenic mechanism for viral disease (57).
The presence of viral envelope proteins in the host cell membrane elicits an immunologic response. The host immune response targets and destroys the infected CNS cells, thus limiting spread of the virus but potentially compounding disease. For example, rabies virus causes metabolic derangement in the neuron, usurps the cellular metabolic machinery, and inhibits the synthesis of cellular proteins (1). The actual pathogenic cause of neuronal cell death is not known but may involve the synthesis of toxic metabolites by rabies virus. The immune response changes the character of disease and the pathologic findings. In paralytic or dumb rabies, patients have disease limited to the brainstem and demonstrate reduced B-cell, interleukin, and cellular activity in response to rabies antigens (86,87). Patients with furious or classic rabies generate brisk, late intracranial immune responses to rabies antigens. An experimental model, involving immunosuppression, demonstrates that this late immune response compounds CNS damage in infected animals (1).
In postinfectious encephalitis, the immune response is misdirected against the brain itself. There is no evidence of direct viral damage or viral antigens in the CNS (1). Viral antigens can share homology with host proteins, and the ensuing immune reaction can damage normal host tissue resembling virally infected cells (7,88). Immune deregulation may cause immune-mediated demyelination. For example, most patients with (post-Semple) rabies vaccine encephalitis have antibodies against myelin basic protein. Forty-seven percent of people with postinfectious measles encephalitis have lymphocytes directed against myelin basic protein, as compared with a 15% rate in nonencephalitic patients with measles (1). The pathogenic mechanism of postinfectious encephalitis is not fully understood.
HIV infection is associated with a variety of CNS diseases. Patients can develop a leukoencephalopathy with diffuse gliosis and loss of the cerebral white matter in addition to the opportunistic infections and neoplasms associated with the disease (51,56,89,90). Pathologic specimens show a multifocal accumulation of giant cells with focal cerebral necrosis. PCR in tissue samples demonstrates large amounts of HIV nucleic acids in multinucleated giant cells. The viral structural and/or regulatory proteins may be toxic to the CNS tissue (91). Alternatively, macrophages and T lymphocytes may damage the brain by aberrant secretion of interleukin and tumor necrosis factor (1).
Transmissible Spongiform Encephalopathies
The TSEs produce clinical changes related to CNS dysfunction similar to the encephalitides (1). Unlike encephalitis, the TSEs are slowly progressing noninflammatory CNS diseases with long incubation periods involving the accumulation of an abnormal form of a normal glycoprotein, the prion protein (PrP) (92). Sporadic CJD occurs between the ages of 50 and 70 years and is characterized by dementia, tremors, and more rarely abnormal movements and ataxia. Unlike sporadic CJD, vCJD disease affects young adults and adolescents and produces cerebellar ataxia and sensory involvement (dysesthesias) with florid amyloid plaques detected in the brain on autopsy (5). Neurologic deterioration progresses relentlessly in the case of vCJD and most patients die less than a year after onset of their neurologic manifestations.
These encephalopathies differ in mode of transmission. Although most of the TSEs are experimentally transmissible by direct inoculation in the CNS, this mode rarely occurs except for iatrogenic transmissions (1). The scrapie agent spreads by contact and lateral transmission. There is no evidence for lateral transmission in the case of BSE or vCJD, and all cases appear to have occurred following parenteral or ingestion of affected materials. The transmissible agents remain infectious after treatments that would normally inactivate viruses or nucleic acids (detergent formalin, ionizing radiation, nucleases). Most of the experimental work on TSEs has involved analysis of the scrapie agent. The current working model is that posttranslational alteration of the normally α-helical form of the PrP protein results in a protease resistant β-pleated sheet structure that accumulates in neurons, leading to progressive dysfunction, cell death, and subsequent astrocytosis. In studies on the scrapie agent, gastrointestinal tract involvement with infection of abdominal lymph nodes occurs first, followed by brain involvement a year or more later. Experimental subcutaneous inoculation in mice and goats also lead to local lymph node involvement followed by splenic spread and then CNS involvement. The mode of transmission to the CNS (direct vs. hematogenous) or the infectivity of body fluids at different stages of infection is not known at this time.
The TSEs are currently only diagnosed by histologic examination, characteristic electroencephalography (EEG), magnetic resonance imaging (MRI) changes, and the clinical context. Most laboratory tests are of little value in the diagnosis. CSF examination shows normal values or slightly elevated protein levels. The EEG in classic CJD reveals generalized slowing early in the disease, punctuated by biphasic or triphasic peaks late in the disease with the onset of myoclonus. MRI changes late in the illness reveal global atrophy with hyperintense signal from the basal ganglia (5). Fluid-attenuated inversion recovery (FLAIR) MRI provides greater sensitivity and demonstrates signal intensity changes in the cortex that are undetectable by T2-weighted spin-echo MRI. Histopathologic examination of the brain using a specific antibody to the PrP-res protein confirms the disease. In addition, evidence of gliosis, neuronal loss, and spongiform changes support the diagnosis. In cases of vCJD, characteristic amyloid plaques (so-called florid plaques) microscopically define the disease. The florid plaques are not seen in other TSEs and consist of flower-like amyloid deposits surrounded by vacuolar halos. The detection of PrP-res in the tonsillar tissue by immunohistochemical staining is also strongly supportive of vCJD diagnosis (5).
Clinical Correlates to Disease
Patients with encephalitis have clinical and laboratory evidence of parenchymal disease. Some viruses (rabies, B virus) produce encephalitis without significant meningeal involvement; however, most patients with encephalitis have concomitant meningitis (1). Most patients also have a prodromal illness with myalgias, fever, and anorexia reflecting the systemic viremia. Neurologic symptoms can range from fever, headache, and subtle neurologic deficits or change in level of consciousness to severe disease with seizures, behavioral changes, focal neurologic deficits, and coma (93,94). Clinical manifestations reflect the location and degree of parenchymal involvement and differ based on viral etiology. For example, HSE infects the inferomedial frontal area of the cortex, resulting in focal seizures, personality changes, and aphasia. These symptoms reflect the neuroanatomic location of infection with inflammation near the internal capsule, limbic, and Broca regions (1). Paresthesias near the location of the animal bite and change in behavior correlate temporally with the axoplasmic transport of rabies and the viral infection of the brainstem and hippocampal region (94). Rabies has a predilection for the limbic system, producing personality changes. The damage spares cortical regions during this phase, allowing humans to vacillate between periods of calm, normal activity and short episodes of rage and disorientation (1). Alternatively, Japanese encephalitis virus initially produces a systemic illness with fever, malaise, and anorexia, followed by photophobia, vomiting, headache, and changes in brainstem function. Brainstem encephalitis leads to difficulty with autonomic functions with increased risk for cardiac and respiratory instability, reflecting infection of brainstem nuclei (1,95–97). Other patients have evidence of multifocal CNS disease involving the basal ganglia, thalamus, and lower cortex and develop tremors, dystonia, and parkinsonian symptoms.
Seizures are frequent during encephalitis. For example, approximately 40% of patients with HSE develop seizures (1). EEG patterns include focal slowing, spiking, and paroxysmal lateralizing epileptiform discharges. The cellular mechanisms for seizures are incompletely understood. This may result from dysfunction of the smaller, inhibitory, γ-aminobutyric acid (GABA)–secreting neurons. Although the seizures encountered in patients with HSE could be directly attributed to cellular destruction, an alternative hypothesis for epileptogenesis in HSE centers on the uptake of virus in the long projections of neurons. This uptake causes perturbations in the cellular machinery necessary for the retention of acetylcholine within the nerve terminal. As a result, the excitatory neurotransmitter could leak from the cell and ultimately trigger a seizure focus. In addition to this mechanism, suboptimal uptake of acetylcholine by malfunctioning presynaptic and postsynaptic terminals can result in a relative excess of the neurotransmitter and abnormal electric discharges. An excess of acetylcholine could also result from the decreased synthesis of degradatory enzymes (such as acetylcholinesterase) as viral replication proceeds. Finally, chronic seizure foci are known to be hypermetabolic during interictal periods. The first stage of viral cellular infection is the inhibition of the cell’s homeostatic mechanisms. The crippled cell, unable to maintain homeostasis, may be predisposed to disordered electric discharges (1).
Encephalitis, unlike meningitis, has higher mortality and complication rates. Case-fatality rates differ based on the viral etiology and host factors. For example, within the arthropod-borne viral encephalitides, St. Louis encephalitis virus has an overall case mortality rate of 10%. The mortality rate is only 2% in children but increases to 20% in the elderly (1). Similarly, WNV meningoencephalitis produces greater mortality rates in the elderly than in younger adults (98,99). Other viruses like western equine and eastern equine encephalitis produce higher mortality and morbidity in children than in adults (1).
The age, immune status, and viral etiology also influence the clinical manifestations of viral meningitis (51,100). Patients with enterovirus meningitis often present with nonspecific symptoms such as fever (38° to 40°C) of 3 to 5 days duration, malaise, and headache (8,101). Approximately 50% of patients have nausea or vomiting. Although nuchal rigidity and photophobia are the hallmark sign and symptom for meningitis, 33% of patients with viral meningitis have no evidence of meningismus. Fewer than 10% of children younger than 2 years develop signs of meningeal irritation. Most of these children with meningitis present with fever and irritability. Children may also present with seizures secondary to fever, electrolyte disturbances, or the infection itself (1). The clinician must have a high index of suspicion for meningitis especially in younger patients. In the immunocompromised host, enterovirus infection is both a diagnostic quandary and a potentially life-threatening disease. Immunocompromised patients frequently do not mount a brisk immune cell response, and therefore CSF analysis may underrepresent the extent of CNS involvement.
Symptoms of meningitis (nuchal rigidity, headache, and photophobia) occur in approximately 11% of men and 36% of women with primary HSV-2 genital infection (1,102–104). Examples exist of recurrent HSV-2 meningitis (with or without genital lesions), although cases associated with primary infection are more common (105,106). HSV meningitis may spread to the CSF by neuronal spread along the sacral nerves. Alternatively, the virus may reach the CSF by hematogenous spread, as the virus has been cultured from the blood buffy coat layer. VZV, cytomegalovirus, Epstein-Barr virus (EBV), and parainfluenza virus have all been cultured or detected by PCR from the CSF of patients with meningitis (1). The three herpesvirus infections occur more frequently in immunocompromised patients and rarely produce isolated meningitis. Instead, these infections usually progress and involve the parenchyma.
CONCLUSION
Clinical symptoms produced by a disease have a pathophysiologic basis. An understanding of the pathogenesis of viral CNS disease provides the physician with a framework for studying related neurologic diseases. Moreover, the pathogenic mechanism of a viral disease provides clues toward the development of antiviral medications and strategies for the prevention of viral CNS infections. Improved diagnostic techniques are essential for advancing both research and therapy of viral neurologic infections. Application of viral PCR and other molecular diagnostic techniques have already changed some of the fundamental concepts of viral infection. Basic research in neurosciences and infectious diseases will result in a better understanding of the host–virus interaction in the CNS. These advances have the potential for improving the care of patients with neurologic diseases.
References
1. Cassady KA, Whitley RJ. Pathogenesis and pathophysiology of viral central nervous system infections. In: Scheld WM, Whitley R, Marra MC, eds. Infections of the Central Nervous System. 3rd ed. Philadelphia: Lippincott-Raven; 2004:57–74.
2. Whitley RJ, Cobbs CG, Alford CA Jr, et al. Diseases that mimic herpes simplex encephalitis. Diagnosis, presentation, and outcome. NIAD Collaborative Antiviral Study Group. JAMA. 1989;262(2):234–239.
3. Bale JF Jr. Human herpesviruses and neurological disorders of childhood. Semin Pediatr Neurol. 1999;6(4):278–287.
4. Rotbart H. Enteroviral infections of the central nervous system. Clin Infect Dis. 1995;20:971–981.
5. Whitley RJ, Macdonald N, Ascher DM. Technical report: transmissible spongiform encephalopathies: a review for pediatricians. Pediatrics. 2000;106(5):1160–1165.
6. Cassady KA, Whitley RJ. Viral central nervous system infections. In: Richman DD, Whitley RJ, Hayden FG, eds. Clinical Virology. 3rd ed. Washington, DC: ASM Press; 2009:29–44.
7. Stuve O, Zamvil SS. Pathogenesis, diagnosis, and treatment of acute disseminated encephalomyelitis. Curr Opin Neurol. 1999;12(4):395–401.
8. Lee BE, Davies HD. Aseptic meningitis. Curr Opin Infect Dis. 2007;20(3): 272–277.
9. Domingues RB, Lakeman FD, Mayo MS, et al. Application of competitive PCR to cerebrospinal fluid samples from patients with herpes simplex encephalitis. J Clin Microbiol. 1998;36(8):2229–2234.
10. Glaser CA, Gilliam S, Schnurr D, et al. In search of encephalitis etiologies: diagnostic challenges in the California Encephalitis Project, 1998–2000. Clin Infect Dis. 2003;36(6):731–742.
11. Antona D, Leveque N, Chomel JJ, et al. Surveillance of enteroviruses in France, 2000–2004. Eur J Clin Microbiol Infect Dis. 2007;26(6):403–412.
12. Hammer SM, Connolly KJ. Viral aseptic meningitis in the United States: clinical features, viral etiologies, and differential diagnosis. Curr Clin Top Infect Dis. 1992;12:1–25.
13. Sferra TJ, Pacini DL. Simultaneous recovery of bacterial and viral pathogens from cerebrospinal fluid. Pediatr Infect Dis J. 1988;7(8):552–556.
14. Basmaci R, Mariani P, Delacroix G, et al. Enteroviral meningitis does not exclude concurrent bacterial meningitis. J Clin Microbiol. 2011;49(9): 3442–3443.
15. Kharfan-Dabaja MA, Ayala E, Greene J, et al. Two cases of progressive multifocal leukoencephalopathy after allogeneic hematopoietic cell transplantation and a review of the literature. Bone Marrow Transplant. 2007;39(2):101–107.
16. Leveque N, Galambrun C, Najioullah F, et al. Two cases of varicella zoster virus meningitis found in pediatric patients after bone marrow transplantation despite valaciclovir prophylaxis and without skin lesions. J Med Virol. 2006;78(4):514–516.
17. Sejvar JJ. The evolving epidemiology of viral encephalitis. Curr Opin Neurol. 2006;19(4):350–357.
18. Centers for Disease Control and Prevention. West Nile virus update—United States, January 1—August 14, 2007. MMWR Morb Mortal Wkly Rep. 2007;56(32):821–822.
19. LaBeaud AD, Kile JR, Kippes C, et al. Exposure to West Nile virus during the 2002 epidemic in Cuyahoga County, Ohio: a comparison of pediatric and adult behaviors. Public Health Rep. 2007;122(3):356–361.
20. Rios J, Hacker CS, Hailey CA, et al. Demographic and spatial analysis of West Nile virus and St. Louis encephalitis in Houston, Texas. J Am Mosq Control Assoc. 2006;22(2):254–263.
21. Whitley RJ, Kimberlin DW. Herpes simplex encephalitis: children and adolescents. Semin Pediatr Infect Dis. 2005;16(1):17–23.
22. Wonham MJ, Lewis MA, Renclawowicz J, et al. Transmission assumptions generate conflicting predictions in host-vector disease models: a case study in West Nile virus. Ecol Lett. 2006;9(6):706–725.
23. Solomon T, Winter PM. Neurovirulence and host factors in flavivirus encephalitis—evidence from clinical epidemiology. Arch Virol Suppl. 2004;(18):161–170.
24. Liu W, Clemens JD, Yang JY, et al. Immunization against Japanese encephalitis in China: a policy analysis. Vaccine. 2006;24(24):5178–5182.
25. Solomon T. Control of Japanese encephalitis—within our grasp? N Engl J Med. 2006;355(9):869–871.
26. Kabilan L, Rajendran R, Arunachalam N, et al. Japanese encephalitis in India: an overview. Indian J Pediatr. 2004;71(7):609–615.
27. Vaughn DW, Hoke CH Jr. The epidemiology of Japanese encephalitis: prospects for prevention. Epidemiol Rev. 1992;14:197–221.
28. Roehr B. Texas records worst outbreak of West Nile virus on record. BMJ. 2012;345:e6019.
29. Manning SE, Rupprecht CE, Fishbein D, et al. Human rabies prevention—United States, 2008: recommendations of the Advisory Committee on Immunization Practices. MMWR Recomm Rep. 2008;57(RR–3):1–28.
30. Pape WJ, Fitzsimmons TD, Hoffman RE. Risk for rabies transmission from encounters with bats, Colorado, 1977–1996. Emerg Infect Dis. 1999;5(3):433–437.
31. Centers for Disease Control and Prevention. Human rabies—California, Georgia, Minnesota, New York, and Wisconsin, 2000. MMWR Morb Mortal Wkly Rep. 2000;49(49):1111–1115.
32. Parrish JB, Yeh EA. Acuted disseminated encephalomyelitis. Adv Exp Med Biol. 2012;724:1–14.
33. McNabb SJ, Jajosky RA, Hall-Baker PA, et al. Summary of notifiable diseases—United States, 2005. MMWR Morb Mortal Wkly Rep. 2007;54(53):1–92.
34. Dalmau J, Gleichman AJ, Hughes EG, et al. Anti-NMDA-receptor encephalitis: case series and analysis of the effects of antibodies. Lancet Neurol. 2008;7(12):1091–1098.
35. Gleichman AJ, Spruce LA, Dalmau J, et al. Anti-NMDA receptor encephalitis antibody binding is dependent on amino acid identity of a small region within the GluN1 amino terminal domain. J Neurosci. 2012;32(32):11082-11094.
36. Kimberlin DW. Herpes simplex virus infections of the newborn. Semin Perinatol. 2007;31(1):19–25.
37. Abzug MJ, Keyserling HL, Lee ML, et al. Neonatal enterovirus infection: virology, serology, and effects of intravenous immune globulin. Clin Infect Dis. 1995;20(5):1201–1206.
38. Rios M, Zhang MJ, Grinev A, et al. Monocytes-macrophages are a potential target in human infection with West Nile virus through blood transfusion. Transfusion. 2006;46(4):659–667.
39. Johnston LJ, Halliday GM, King NJ. Langerhans cells migrate to local lymph nodes following cutaneous infection with an arbovirus. J Invest Dermatol. 2000;114(3):560–568.
40. Colman PM. Influenza virus neuraminidase: structure, antibodies, and inhibitors. Protein Sci. 1994;3(10):1687–1696.
41. Colman PM, Varghese JN, Laver WG. Structure of the catalytic and antigenic sites in influenza virus neuraminidase. Nature. 1983;303(5912):41–44.
42. Sharpe AH, Fields BN. Pathogenesis of viral infections. Basic concepts derived from the reovirus model. N Engl J Med. 1985;312(8):486–497.
43. Spindler KR, Hsu TH. Viral disruption of the blood-brain barrier. Trends Microbiol. 2012;20(6):282–290.
44. Lancaster KZ, Pfeiffer JK. Limited trafficking of a neurotropic virus through inefficient retrograde axonal transport and the type I interferon response. PLoS Pathog. 2010;6(3):e1000791.
45. Mrak RE, Young L. Rabies encephalitis in humans: pathology, pathogenesis and pathophysiology. J Neuropathol Exp Neurol. 1994;53(1):1–10.
46. Chou J, Kern ER, Whitley RJ, et al. Mapping of herpes simplex virus-1 neurovirulence to gamma 134.5, a gene nonessential for growth in culture. Science. 1990;250(4985):1262–1266.
47. Bolovan CA, Sawtell NM, Thompson RL. ICP34.5 mutants of herpes simplex virus type 1 strain 17syn+ are attenuated for neurovirulence in mice and for replication in confluent primary mouse embryo cell cultures. J Virol. 1994;68(1):48–55.
48. Shah AC, Parker JN, Shimamura M, et al. Spontaneous and engineered compensatory HSV mutants that counteract the host antiviral PKR response. Viruses. 2009;1(3):510–522.
49. Jouanguy E, Zhang SY, Chapgier A, et al. Human primary immunodeficiencies of type I interferons. Biochimie. 2007;89(6–7):878–883.
50. Kunze U, Asokliene L, Bektimirov T, et al. Tick-borne encephalitis in childhood—consensus 2004. Wien Med Wochenschr. 2004;154(9–10):242–245.
51. Kaushik DK, Gupta M, Basu A. Microglial response to viral challenges: every silver lining comes with a cloud. Front Biosci. 2011;16:2187–2205.
52. Shrestha B, Diamond MS. Role of CD8+ T cells in control of West Nile virus infection. J Virol. 2004;78(15):8312–8321.
53. Abzug MJ, Levin MJ, Rotbart HA. Profile of enterovirus disease in the first two weeks of life. Pediatr Infect Dis J. 1993;12(10):820–824.
54. Wyatt HV. Poliomyelitis and infantile paralysis: changes in host and virus. His Philos Life Sci. 1993;15(3):357–396.
55. Abbott NJ. Inflammatory mediators and modulation of blood-brain barrier permeability. Cell Mol Neurobiol. 2000;20(2):131–147.
56. Williams DW, Eugenin EA, Calderon TM, et al. Monocyte maturation, HIV susceptibility, and transmigration across the blood–brain barrier are critical in HIV neuropathogenesis. J Leukoc Biol. 2012;91(3):401–415.
57. Irani DN, Prow NA. Neuroprotective interventions targeting detrimental host immune responses protect mice from fatal alphavirus encephalitis. J Neuropathol Exp Neurol. 2007;66(6):533–544.
58. Daley JK, Gechman LA, Skipworth J, et al. Poliovirus replication and spread in primary neuron cultures. Virology. 2005;340(1):10–20.
59. Tucker PC, Lee SH, Bui N, et al. Amino acid changes in the Sindbis virus E2 glycoprotein that increase neurovirulence improve entry into neuroblastoma cells. J Virol. 1997;71(8):6106–6112.
60. Soulie C, Calvez V, Marcelin AG. Coreceptor usage in different reservoirs. Curr Opin HIV AIDS. 2012;7(5):450–455.
61. Chang A, Dutch RE. Paramyxovirus fusion and entry: multiple paths to a common end. Viruses. 2012;4(4):613–636.
62. Campadelli-Fiume G, Menotti L, Avitabile E, et al. Viral and cellular contributions to herpes simplex virus entry into the cell. Curr Opin Virol. 2012;2(1):28–36.
63. Fazakerley JK. Semliki Forest virus infection of laboratory mice: a model to study the pathogenesis of viral encephalitis. Arch Virol Suppl. 2004;(18):179–190.
64. Noyce RS, Bondre DG, Ha MN, et al. Tumor cell marker PVRL4 (nectin 4) is an epithelial cell receptor for measles virus. PLoS Pathog. 2011;7(8): e1002240.
65. Navaratnarajah CK, Miest TS, Carfi A, et al. Targeted entry of enveloped viruses: measles and herpes simplex virus I. Curr Opin Virol. 2012;2(1):43–49.
66. Gromeier M, Bossert B, Arita M, et al. Dual stem loops within the poliovirus internal ribosomal entry site control neurovirulence. J Virol. 1999;73(2):958–964.
67. Kauder S, Kan S, Racaniello VR. Age-dependent poliovirus replication in the mouse central nervous system is determined by internal ribosome entry site-mediated translation. J Virol. 2006;80(6):2589–2595.
68. Marsh M, Helenius A. Virus entry: open sesame. Cell. 2006;124(4):729–740.
69. Pilipenko EV, Pestova TV, Kolupaeva VG, et al. A cell cycle-dependent protein serves as a template-specific translation initiation factor. Genes Dev. 2000;14(16):2028–2045.
70. Sandri-Goldin RM. The many roles of the regulatory protein ICP27 during herpes simplex virus infection. Front Biosci. 2008;13:5241–5256.
71. Dauber B, Pelletier J, Smiley JR. The herpes simplex virus 1 vhs protein enhances translation of viral true late mRNAs and virus production in a cell type-dependent manner. J Virol. 2011;85(11):5363–5373.
72. Hunt CL, Lennemann NJ, Maury W. Filovirus entry: a novelty in the viral fusion world. Viruses. 2012;4(2):258–275.
73. Mehta H, Muller J, Markovitz NS. Ultrastructural analysis of ICP34.5-herpes simplex virus 1 replication in mouse brain cells in vivo. J Virol. 2010;84(21):10982–10990.
74. Penfold ME, Armati PJ, Mikloska Z, et al. The interaction of human fetal neurons and epidermal cells in vitro. In Vitro Cell Dev Biol Anim. 1996;32(7):420–426.
75. Reuter JD, Gomez DL, Wilson JH, et al. Systemic immune deficiency necessary for cytomegalovirus invasion of the mature brain. J Virol. 2004; 78(3):1473–1487.
76. Klein RS, Lin E, Zhang B, et al. Neuronal CXCL10 directs CD8+ T-cell recruitment and control of West Nile virus encephalitis. J Virol. 2005;79(17):11457–14566.
77. Johnson KE, Knipe DM. Herpes simplex virus-1 infection causes the secretion of a type I interferon-antagonizing protein and inhibits signaling at or before Jak-1 activation. Virology. 2010;396(1):21–29.
78. Leib DA, Harrison TE, Laslo KM, et al. Interferons regulate the phenotype of wild-type and mutant herpes simplex viruses in vivo. J Exp Med. 1999;189(4):663–672.
79. Cassady KA. Human cytomegalovirus TRS1 and IRS1 gene products block the double-stranded-RNA-activated host protein shutoff response induced by herpes simplex virus type 1 infection. J Virol. 2005;79(14): 8707–8715.
80. Okabayashi T, Kojima T, Masaki T, et al. Type-III interferon, not type-I, is the predominant interferon induced by respiratory viruses in nasal epithelial cells. Virus Res. 2011;160(1–2):360–366.
81. Pott J, Mahlakoiv T, Mordstein M, et al. IFN-lambda determines the intestinal epithelial antiviral host defense. Proc Natl Acad Sci U S A. 2011;108(19):7944–7949.
82. Bhowmick S, Duseja R, Das S, et al. Induction of IP-10 (CXCL10) in astrocytes following Japanese encephalitis. Neurosci Lett. 2007;414(1):45–50.
83. Wacher C, Muller M, Hofer MJ, et al. Coordinated regulation and widespread cellular expression of interferon-stimulated genes (ISG) ISG-49, ISG-54, and ISG-56 in the central nervous system after infection with distinct viruses. J Virol. 2007;81(2):860–871.
84. Holub M, Beran O, Lacinova Z, et al. Interferon-gamma and cortisol levels in cerebrospinal fluid and its relationship to the etiology of aseptic meningoencephalitis. Prague Med Rep. 2006;107(3):343–353.
85. Christensen JE, de Lemos C, Moos T, et al. CXCL10 is the key ligand for CXCR3 on CD8+ effector T cells involved in immune surveillance of the lymphocytic choriomeningitis virus-infected central nervous system. J Immunol. 2006;176(7):4235–4243.
86. Niu X, Wang H, Fu ZF. Role of chemokines in rabies pathogenesis and protection. Adv Virus Res. 2011;79:73–89.
87. Watson NF, Woo D, Doherty MJ, et al. Humoral immune responses after rabies infection. Arch Neurol. 2007;64(9):1355–1356.
88. Bennetto L, Scolding N. Inflammatory/post-infectious encephalomyelitis. J Neurol Neurosurg Psychiatry. 2004;75(suppl 1):i22–i28.
89. Stroup JS, Stephens JR, Bury J, et al. Cytomegalovirus encephalitis in an HIV-seropositive person. AIDS Read. 2007;17(3):133–134, 136.
90. Khetsuriani N, Holman RC, Lamonte-Fowlkes AC, et al. Trends in encephalitis-associated deaths in the United States. Epidemiol Infect. 2007;135(4):583–591.
91. Wiestler OD, Leib SL, Brustle O, et al. Neuropathology and pathogenesis of HIV encephalopathies. Acta Histochem Suppl. 1992;42:107–114.
92. Aguzzi A, Heikenwalder M, Polymenidou M. Insights into prion strains and neurotoxicity. Nat Rev Mol Cell Biol. 2007;8(7):552–561.
93. Sejvar JJ, Marfin AA. Manifestations of West Nile neuroinvasive disease. Rev Med Virol. 2006;16(4):209–224.
94. Laothamatas J, Sungkarat W, Hemachudha T. Neuroimaging in rabies. Adv Virus Res. 2011;79:309–327.
95. Jereb M, Lainscak M, Marin J, et al. Herpes simplex virus infection limited to the brainstem. Wien Klin Wochenschr. 2005;117(13–14):495–499.
96. Gelpi E, Preusser M, Garzuly F, et al. Visualization of Central European tick-borne encephalitis infection in fatal human cases. J Neuropathol Exp Neurol. 2005;64(6):506–512.
97. Ayukawa R, Fujimoto H, Ayabe M, et al. An unexpected outbreak of Japanese encephalitis in the Chugoku district of Japan, 2002. Jpn J Infect Dis. 2004;57(2):63–66.
98. Murray K, Baraniuk S, Resnick M, et al. Risk factors for encephalitis and death from West Nile virus infection. Epidemiol Infect. 2006;134(6): 1325–1332.
99. LaBeaud AD, Lisgaris MV, King CH, et al. Pediatric West Nile virus infection: neurologic disease presentations during the 2002 epidemic in Cuyahoga County, Ohio. Pediatr Infect Dis J. 2006;25(8):751–753.
100. Padate BP, Keidan J. Enteroviral meningoencephalitis in a patient with non-Hodgkin’s lymphoma treated previously with rituximab. Clin Lab Haematol. 2006;28(1):69–71.
101. Lee BE, Chawla R, Langley JM, et al. Paediatric Investigators Collaborative Network on Infections in Canada (PICNIC) study of aseptic meningitis. BMC Infect Dis. 2006;6:68.
102. Gorander S, Andersen O, Leiram B, et al. Multiphasic encephalomyelitis in a patient with recurrent herpes simplex type 2 meningitis. Scand J Infect Dis. 2006;38(10):942–925.
103. Kupila L, Vuorinen T, Vainionpaa R, et al. Etiology of aseptic meningitis and encephalitis in an adult population. Neurology. 2006;66(1):75–80.
104. Tyler KL. Herpes simplex virus infections of the central nervous system: encephalitis and meningitis, including Mollaret’s. Herpes. 2004;11(suppl 2): 57A–64A.
105. Kumar S, Kumar S, Kohlhoff SA. Recurrent HSV-2 meningitis in a 9-year-old girl. Scand J Infect Dis. 2006;38(6–7):570–572.
106. Bergstrom T, Vahlne A, Alestig K, et al. Primary and recurrent herpes simplex virus type 2-induced meningitis. J Infect Dis. 1990;162(2):322–330.