W. Garrett Hunt and Michael T. Brady
The Epstein-Barr virus (EBV) is recognized as the major cause of heterophil-positive and heterophil-negative infectious mononucleosis. Manifestations of EBV infection are varied and range from asymptomatic infection to fulminant lymphoproliferative disease. The virus is associated with a number of malignancies, including African Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin disease, and a spectrum of posttransplant lymphoproliferative diseases. The specific role of EBV in each tumor is now defined, in a number of circumstances, to the level of specific cell type and receptors, intra-cellular pathways, gene expression, and cytokine production.
EPIDEMIOLOGY
It is important to recognize that acute primary Epstein-Barr virus (EBV) infection is not synonymous with infectious mononucleosis. Most EBV infections acquired at any age, but particularly during childhood, are asymptomatic. Seroepidemiologic studies demonstrate that from 20% to 100% of children worldwide have antibodies to EBV by 6 years of age.9 In contrast, in the United States, only 40% to 50% of adolescents are seropositive,9-11 with higher socioeconomic groups being less likely to have evidence of prior infection. Seropositivity increases with age in all populations, so that almost all adults have serologic evidence of past EBV infection. Seroconversion is particularly high in college, where 10% to 15% of susceptible persons become infected each year. This group of EBV-naive adolescents in industrialized countries is susceptible to develop EBV-associated IM, much more common in the United States and Western Europe than in unindustrialized countries.
EBV is excreted in oropharyngeal secretions and is transmitted by contact with saliva via kissing or other mucosal contact with contaminated objects.12 Healthy seropositive individuals intermittently shed EBV into their oropharynx. Blood products or transplanted tissues can transmit EBV and are particularly problematic for seronegative immunocompromised transplant recipients. There is no evidence of urinary or fecal excretion. Transplacental transmission appears to be rare. Shedding of virus appears to be more frequent in immunosuppressed individuals, 60% of whom may excrete EBV at any one time. Because virus shedding is of a low titer in even immunocompromised patients, standard precautions are adequate for isolation of patients with acute or past EBV infections.12
The epidemiology of infectious mononucleosis is closely related to the age of primary EBV infection. In the United States, the incidence of infectious mononucleosis is approximately 50 per 100,000 persons per year, but in individuals 15 to 25 years old, the incidence doubles.13 Those areas where children are infected at an early age have the lowest incidence of the disease. Among susceptible adolescents and young adults, studies measuring both apparent and inapparent EBV infections indicate a clinical-to-subclinical ratio of 1:2 to 1:3.
PATHOPHYSIOLOGY
Epstein-Barr virus (EBV) is a member of the family Herpesviridae (gamma herpesvirus), which contains linear double-stranded DNA surrounded by a protein capsid with 162 cap-somers in an icosahedral arrangement.1 The nucleocapsid is covered by a lipid-containing envelope derived from the nuclear membrane of the host cell. EBV causes lytic infection of human oropharyngeal and salivary cells, and latent infection of human and primate B lymphocytes and epithelium of the nasopharynx.2 This virus has been long recognized to be lymphotropic for B lymphocytes and to infect both oropharyngeal epithelial cells and myocytes, but it has also been recognized that it infects T lymphocytes, natural killer (NK) cells,3,4 and monocytes, which may serve as an early site for viral replication.5
Infection of lymphocytes with EBV can transform them into continuously growing lymphoblastoid cell lines containing circular genome as a plasmid. Once infected, transformed lymphoblastoid cells rarely continue to produce infectious virus in vitro, although EBV-induced antigens can be detected in the cells. The appearance of new antigens on the cell surface of EBV-infected cells is believed to be responsible for the cellular immune response to the virus and for the pathogenesis of the disease produced.6-8 The EBV receptor on epithelial cells and B lymphocytes is the CD21 molecule (formerly CR2), which is also the receptor for the C3d fragment of the third component of complement. The virus elicits both humoral and cellular immune responses.
Epstein-Barr virus (EBV) acquired by ingestion appears to first infect oropharyngeal epithelial cells. Subsequently, the virus infects susceptible B lymphocytes within the lymphoid tissue of the pharynx. During a 30- to 50-day incubation period, virus actively replicates and disseminates throughout the entire lymphoreticular system.14
Cell-mediated immune function is essential in the control of and recovery from EBV infection. In EBV infectious mononucleosis, the initial infection of B lymphocytes is followed by an extensive proliferation of suppressor cytoxic T lymphocytes (CD8+ positive). Associated with the increase in these cytotoxic and suppressor cytotoxic T cells is a concomitant decrease in the number of T-helper/inducer cells (CD4+-positive lymphocytes), resulting in an inversion of the CD4:CD8 ratio. Fatal infectious mononucleosis and lymphoproliferative disorders, in which B-cell lymphomas develop, have been identified in adults and children with such cell-mediated immune defects, and, in particular, in their natural killer (NK) cell activity. Groups identified as developing this progressive B-immunoblastic disorder include kidney, heart, and bone marrow transplant recipients and individuals with X-linked lymphoproliferative syndrome, severe combined immunodeficiency syndrome, AIDS, ataxia-telangiectasia, and certain autoimmune diseases.
More recently, EBV has been demonstrated to infect T cells or NK cells to cause unique systemic lymphoproliferative diseases such as EBV-associated hemophagocytic lymphohistiocytosis (EBV-HLH), as well as chronic active EBV (CAEBV) infection, clinical features of which are distinct from the tumor-forming diseases outlined previously.3,4 Systemic EBV-related lymphoproliferative disease (LPD) is now classified into B-cell LPD and T/NK-cell LPD. The former causes fulminant infectious mononucleosis, whereas the latter causes EBVHLH and CAEBV.
CLINICAL MANIFESTATIONS
Infectious Mononucleosis
The incubation period of infectious mononucleosis syndrome is 30 to 50 days. The clinical syndrome of infectious mononucleosis is usually preceded by a 3- to 5-day prodrome of malaise, fatigue, headache, nausea, or abdominal pain.15 Over the next 7 to 20 days, sore throat and fever gradually increase. The triad of fever, sore throat, and posterior cervical adenopathy occurs in more than 80% of patients. Sore throat is often accompanied by evidence of moderate-to-severe pharyngitis, with marked tonsillar enlargement that may be covered with shaggy gray or white exudates. Fine petechiae may cover the uvula and soft palate during the initial week of illness. Throat cultures are positive for group A β-hemolytic streptococci in about 30% of patients, which may confuse the correct diagnosis of Epstein-Barr virus (EBV) mononucleosis.2,15,16 Fever is present in 85% to 95% of patients, from 39°C (102°F) up to 40.5°C (105°F), and on average lasts 10 days, but may persist for weeks.2
Adenopathy most often involves only the bilateral posterior cervical nodes but can involve any nodes. The nodes are affected singly or in groups (not necessarily symmetrically) and may be very large or small (the size of grapes); they are most often firm, discrete, and moderately tender to palpation. Splenomegaly, with the spleen palpable 2 to 3 cm below the costal margin beyond the neonatal period, occurs in about 50% of infections.17 Rupture is rare but can be a potentially fatal complication. Hepatomegaly occurs in 10% to 30% of patients, but less than 5% of patients develop jaundice. Serum aspartate aminotransferase (AST) and serum lactate dehydrogenase (LDH) are mildly elevated in the majority of patients and may persist for weeks to months. Chronic liver disease, however, does not typically result.
Other clinical findings include bilateral supraorbital edema and rashes. A blanching, erythematous, maculopapular exanthema occurs in about 5% to 15% of patients, but as many as 80% develop this rash if treated with ampicillin or other β-lactam antibiotics (Fig. 311-1). The same rash may occur with cytomegalovirus (CMV)-associated mononucleosis and so does not differentiate CMV- from EBV-associated mononucleosis. Urticarial, bullous, hemorrhagic, and scarlatiniform rashes, as well as the Gianotti-Crosti syndrome, are also associated with infectious mononucleosis.
Neurologic complications include aseptic meningitis, encephalitis, optic neuritis, GuillainBarré syndrome, transverse myelitis, Bell palsy, and, in numerous more recent epidemiologic studies, multiple sclerosis following EBV-associated infectious mononucleosis.18 An autoimmune hemolytic anemia occurs in 0.5% to 3% of infectious mononucleosis patients and is usually mediated by antibodies against the “i” antigen. A mild thrombocytopenia below 140,000 platelets/μL, but with absence of profound thrombocytopenia or bleeding, occurs in approximately 50% of patients. Granulocytopenia, or thrombocytopenia may occur during the acute illness or in the immediate recovery period. Respiratory and cardiac complications include interstitial pneumonia, laryngeal obstruction, pharyngeal edema, myocarditis, and pericarditis.
FIGURE 311-1. Morbilliform rash on the lower extremities in a patient with infectious mononucleosis following treatment with a penicillin derivative. (Source: From Wolff K, Goldsmith LA, Katz SI, et al, eds. Fitzpatrick’s Dermatology in General Medicine. 7th ed. http://www.accessmedicine.com. Copyright © The McGraw-Hill Companies. All rights reserved.)
EBV Infection in Young Children
Infection with EBV usually does not present in children younger than 4 years of age with the symptoms and signs of typical infectious mononucleosis as described for adolescents in developed countries above. When infected, younger children are more likely to exhibit rashes and hepatosplenomegaly. Failure to thrive, otitis media, abdominal pain, and recurrent pharyngitis are also more common in young children. Involvement of the hematopoietic system or the central nervous system, or the occurrence of prolonged fever, may be the primary or only manifestation of acute EBV infection in this population. In children and adults, EBV infection may be followed by persistent pharyngitis, lymphadenopathy, fever, headaches, arthralgia, fatigue, and psychoneurosis.
Other EBV-Related Disorders
Oral hairy leukoplakia of the tongue is a benign EBV-associated lesion commonly found in HIV-infected persons, and it is associated with few or no symptoms. The X-linked lymphoproliferative syndrome (Duncan disease or Purtilo syndrome) manifests as fatal EBV infection in males in particular families or sporadically in girls and boys with no family history.19 In these individuals, EBV infection is usually fatal and is associated with either a lymphoproliferative response, such as fatal mononucleosis, lymphoma, hemophagocytic syndrome, and B-cell immunoblastic sarcoma (seen in 75% of cases), or with an aproliferative response such as agammaglobulinemia, aplastic anemia, agranulocytosis, recurrent bacterial infections, and late malignancies (seen in 25% of cases). Alternatively, although an immunocompromised status is suspected in the development of EBV-associated hemophagocytic lymphohistiocytosis (EBV-HLH), the exact characteristics of host vulnerability are largely unknown. Alternatively, the majority of EBV-HLH cases occur in apparently immunocompetent children. EBV is also associated with a number of malignant disorders, including nasopharyngeal carcinoma, Burkitt lymphoma, leiomyosarcoma in immunocompromised patients, and, to a lesser extent, Hodgkin lymphoma.14
DIAGNOSIS
The diagnosis of Epstein-Barr virus (EBV)-associated infectious mononucleosis (IM) in an immunocompetent child or adolescent is based on clinical manifestations, characteristic blood abnormalities, and positive heterophil or EBV antibodies. By the second week of infection, the relative and absolute numbers of lymphocytes increase, with at least 10% to 20% atypical cells. Early in the disease, the atypical cells, or Downey cells, are both B and T lymphocytes (Fig. 311-2). By early convalescence, the majority of the atypical lymphocytes are cytotoxic suppressor cytotoxic T cells (CD8+). The total leukocyte count is usually 10,000 to 20,000 cells/μL, but may be as high as 50,000 cells/μL. The leukocyte abnormalities may persist for 4 to 8 weeks.
FIGURE 311-2. Buffy coat from a patient with infectious mononucleosis. Two large reactive lymphocytes. (Source: From Lichtman MA, Beutler E, Kipps TJ, et al, eds. Williams Hematology. 7th ed. New York: McGraw-Hill; 2006.)
The heterophil test detects the presence of antibodies induced by the virus that are directed against EBV-specific antigens, as well as to nonspecific heterophil antigens (polyclonal B-cell stimulation), which cross-react with sheep and horse red-cell agglutinins. Heterophil antibodies are present in as many as 90% of EBV-associated IM in children older than 4 years. The titer reported is the highest serum dilution at which sheep or horse erythrocytes agglutinate after serum absorption with guinea pig kidney cells. Such absorption decreases interference (false positives) caused by Forssman antibodies and by antibodies that are associated with serum sickness. The antibodies detected by the Paul Bunnell or monospot tests agglutinate antibodies to equine and ovine erythrocytes and are directed to no known EBV antigens. These IgM antibodies usually appear during the first or, more commonly, the second or third week of illness and become undetectable by 1 year in about 25% of individuals, when using the horse red-cell agglutination test, and in 70% when using sheep red-cell agglutinins. A number of rapid spot kits for detecting heterophil antibodies (using equine or ovine erythrocytes) are now available commercially. The correlation between results obtained by the spot and slide tests and the classic tube heterophil test is usually excellent. False-positive monospot tests have been reported in patients with lymphoma, pancreatitis, mumps, or hepatitis. False-negative tests occur most frequently in children younger than 4 years or in older children tested in the first 2 weeks of illness. Following infection with EBV, only 5% to 10% of children younger than 2 years have positive heterophil antibodies, in contrast to as many as 50% of children between 2 and 4 years of age.
In older children, the most common cause of heterophile-negative infectious mononucleosis is still the EBV infection. CMV is the second most common cause of heterophil-negative mononucleosis (Chapter 310). Streptococcus pyogenes pharyngitis, hepatitis A and B, acute toxoplasmosis, rubella, and enteroviral infection may each result in fever, lymphadenopathy, malaise, atypical lymphocytosis, and rash. Throat cultures are positive for group A β-hemolytic streptococci in about 30% of patients, which may confuse the correct diagnosis of EBV. Hematologic abnormalities seen in infectious mononucleosis such as lymphadenopathy, and splenomegaly associated with infectious mononucleosis may suggest the possibility of lymphoma or acute lymphocytic leukemia. The heterophil test may be helpful in differentiation. Bone marrow should be examined in any individual who has a lymphoproliferative disease without evidence of infectious mononucleosis.
The availability of sensitive and specific EBV antibody tests has enabled more accurate diagnosis of EBV infection. Antibodies detected by indirect immunofluorescence include IgG and IgM antibody to viral capsid antigen (IgG-VCA and IgM-VCA); antibodies to early antigens (EAs); and IgG antibody to Epstein-Barr virus nuclear antigen (EBNA). IgG antibodies to VCA are present in almost 100% of patients during the acute phase of infectious mononucleosis. Similarly, more than 95% of patients with infectious mononucleosis will have demonstrable IgM-VCA on presentation.13,21,22 Because IgM antibody usually lasts only 2 to 3 months, occasionally the antibody response may not be detected. Following recovery, IgG-VCA antibody remains detectable throughout life. Antibodies to early antigens are present in 70% to 80% of patients with acute IM. Because these antibodies have a relatively similar time course to that of IgM-VCA, they may not often add to the clinical interpretation of acute, subacute, or past EBV infection. That being said, EA may sometimes be useful because these antibodies may persist for 3 to 6 months after infectious mononucleosis (as opposed to 2-3 months of IgM-VCA) and may be very elevated in patients with Burkitt lymphoma or chronic active EBV (CAEBV). IgG antibodies to EBNA appear late in the course of infectious mononucleosis and remain detectable for life. Therefore, when antibody to EBNA is absent in the presence of other EBV-specific antibodies, recent infection is likely. Table 311-1 and Figure 311-3 summarize these patterns.
Table 311-1. Antibody Patterns Associated with EBV Infection Status
Qualitative and quantitative EBV polymerase chain reaction of whole blood or plasma may be used to aid in the diagnosis of EBV infection and disease in patients with primary or acquired immune deficiency or in patients with systemic, severe EBV-associated diseases such as overwhelming IM, hemophagocytic syndrome, or chronic active EBV infection. DNA-DNA hybridization and testing for EBV RNA (electron beam electro-reflectance [EBER]) can aid in detection of EBV in histopathologic samples.
TREATMENT
Currently, there is no specific antiviral therapy for Epstein-Barr virus (EBV) infection. Symptomatic treatment with antipyretics may be helpful. Although forced rest is neither helpful nor indicated, contact sports or other activities that may result in abdominal trauma should be avoided while the spleen is enlarged (usually 1-3 months). In some instances, vigorous examination of the abdomen can result in splenic rupture. Caution should be used when examining the abdomen of a child with suspected EBV-associated infectious mononucleosis. Corticosteroids are not indicated for most patients but may be used for potentially life-threatening complications, including airway obstruction, neurologic complications, fulminant hepatitis, myocarditis, pericarditis, thrombocytopenic purpura, or hemolytic anemia. A short course of prednisone, 1 to 2 mg/kg/day given the first day in divided doses and rapidly tapered over 7 to 10 days, is usually sufficient, although longer courses may be necessary to treat hemolytic anemia and certain neurologic complications.
Specific antiviral therapy has generally not been beneficial in treating EBV infections. If group A β-hemolytic streptococci are isolated from the throat, then appropriate antibiotic therapy is indicated. Ampicillin and other beta-lactam antibiotics should be avoided because of the high frequency of rashes associated with their use during infectious mononucleosis. Acyclovir treatment of patients with infectious mononucleosis results in interruption of viral shedding in the throat, but has little clinical effect. Similarly, prophylactic interferon-α decreases the incidence of EBV shedding by kidney transplant recipients but is not widely used for prophylaxis or treatment.
FIGURE 311-3. Typical course of serum antibody titers in infectious mononucleosis. EA, early antigen (diffuse pattern); EBNA, Epstein-Barr virus nuclear antigen; EBV, Epstein-Barr virus; VCA, viral capsid antigen.
Lesions of oral hairy leukoplakia respond to oral or intravenous acyclovir, but they frequently recur in patients with HIV infection after treatment is discontinued. Aggressive, successful antiretroviral therapy frequently causes remission of lesions of oral hairy leukoplakia without specific therapy of EBV. Occasional remissions of polyclonal and monoclonal tumors have been described in persons with lymphoproliferative disorders treated with interferon-α and intravenous gamma globulin.
Acyclovir is not helpful because it is active only on the lytic phase of EBV and not on the latent phase of EBV occurring in the lymphoproliferative conditions. Except for the rare occurrence of splenic rupture, severe central nervous system complications, severe hematologic problems, untreated respiratory compromise, or specific immunologic defects, the prognosis for the patient infected with EBV is excellent. Complete recovery is to be expected. During convalescence, some patients experience marked fatigue, which occasionally persists for months after the acute infection.