Christopher R. Tainter
Zoonoses are diseases transmitted between vertebrate animals and humans. They remain common and often underestimated in prevalence in North America.
Infection may occur by direct contact, ingestion of contaminated water or food, inhalation, or through arthropod vectors.
Most zoonoses in the United States have the highest incidence in the spring and summer. These diseases are easily mistaken for other nonspecific self-limited diseases, and many patients at risk fail to volunteer their exposure history (eg, cannot recall a tick bite). Rabies and West Nile virus are discussed separately.
ROCKY MOUNTAIN SPOTTED FEVER
EPIDEMIOLOGY
Transmission of Rocky Mountain spotted fever (RMSF) to humans via tick bite occurs primarily between April and September, with the highest incidence in the mid-Atlantic states (although cases have been reported in most of the continental United States). Two-thirds of all cases are reported in children <15 years old.
PATHOPHYSIOLOGY
RMSF is caused by Rickettsia rickettsii, a pleomorphic, obligate intracellular organism, carried by Dermacentor ticks. Deer, rodents, horses, cattle, cats, and dogs are the usual animal reservoir hosts.
CLINICAL FEATURES
RMSF is classically defined by a triad of fever, rash, and history of tick exposure. Unfortunately, only about 50% of afflicted patients can recall a tick bite, and rash may be absent in up to 20% (“spotless RMSF,” which is usually seen in African Americans, in the elderly, and in severe, fatal cases).
The incubation period following a tick bite is usually 4 to 10 days and is followed by onset of nonspecific symptoms such as fever, malaise, severe headache, myalgias, nausea and vomiting, diarrhea, anorexia, abdominal pain, and photophobia.
Additional signs and symptoms may include lymphadenopathy, hepatosplenomegaly, conjunctivitis, confusion, meningismus, renal or respiratory failure, and myocarditis.
A rash usually begins 2 to 4 days after the onset of fever. It is generally maculopapular, typically begins on the extremities around the wrists and ankles (often involving the palms and soles), and spreads centripetally to the trunk, usually sparing the face (it may become petechial or purpuric later).
DIAGNOSIS AND DIFFERENTIAL
Lab findings are usually nonspecific, but the combination of neutropenia, thrombocytopenia, hyponatremia, and elevated liver function tests (LFTs) suggests RMSF.
The mortality rate for RMSF is between 5% and 10%. The clinical diagnosis must be presumed in order to start therapy early, since serology to confirm a rise in antibody titer is not reliably positive until 6 to 10 days after onset of symptoms (diagnosis may also be confirmed by skin rash biopsy with immunofluorescent testing).
Differential diagnosis includes viral illness, ehrlichiosis, disseminated gonorrhea, meningococcemia, secondary syphilis, scarlet fever, leptospirosis, typhoid, gastroenteritis, pneumonia, and toxic shock syndrome.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Treatment for adults includes doxycycline 100 milligrams PO or IV twice daily for 5 to 10 days. Alternatives include tetracycline 500 milligrams PO four times daily, or chloramphenicol 12.5 to 18.75 milligrams/kg/dose IV every 6 hours.
Treatment for children <45 kg includes doxycycline 2.2 milligrams/kg twice daily. Tetracycline and IV chloramphenicol are alternatives, although chloramphenicol is contraindicated in children less than 2 years old.
Doxycycline is the first-line therapy for rickettsial diseases recommended in all ages by the Centers for Disease Control and Prevention and the American Academy of Pediatrics.
LYME DISEASE
EPIDEMIOLOGY
Lyme disease remains the most common vector-borne zoonosis in the United States, and is most prevalent in the north-central, northeastern, and mid-Atlantic states, but has been reported in all 48 contiguous states.
PATHOPHYSIOLOGY
Borrelia burgdorferi, a spirochete, is the responsible organism and is transmitted to humans by Ixodes species (black-legged) ticks, with rabbits, rodents, and deer serving as host reservoir animals. The overall risk of contracting Lyme disease after a deer tick bite is relatively low, about 3% in highly endemic areas, and proportional to the length of tick attachment. There is almost no risk when the duration of attachment is less than 24 hours.
CLINICAL FEATURES
Lyme disease is divided into three distinct stages, but not all patients suffer all stages; stages may overlap and remissions between stages may occur.
Stage I is characterized by the erythema chronicum migrans (ECM) skin lesion, which occurs in 60% to 80% of cases. It consists of an annular, erythematous skin plaque with central clearing (see Fig. 99-1), which develops 2 to 20 days after a tick bite at the inoculation site, as a result of a vasculitis. It resolves after 3 to 4 weeks, and may recur during the second stage.
Stage II corresponds to dissemination of the spirochete, resulting in multiple secondary annular skin lesions (ECM), fever, adenopathy, splenomegaly, and flulike constitutional symptoms. Neurologic symptoms may occur during stage II, most often cranial neuritis (especially uni- or bilateral facial nerve palsy), but may also include headache, neck stiffness, cerebellar ataxia, or encephalitis. Asymmetric oligoarticular arthritis (usually in large joints, especially knees) may develop as well. Cardiac abnormalities occur in approximately 8% of patients and typically present as first-, second-, or third-degree atrioventricular nodal heart block or myocarditis.
Stage III represents chronic persistent infection, and occurs years after the initial infection. It may include chronic intermittent migratory arthritis, myocarditis, encephalopathy, and axonal polyneuropathy.
FIG. 99-1. Erythema chronicum migrans. This pathognomonic eruption of Lyme disease forms at the site of the tick bite. The initial papule forms into a slowly enlarging oval area of erythema while clearing centrally. Reproduced with permission from Knoop K, Stack L, Storrow A: Atlas of Emergency Medicine, 3rd ed. © 2010 McGraw-Hill, New York. (Photo contributed by David Effron, MD.)
DIAGNOSIS AND DIFFERENTIAL
Diagnosis is made clinically, and may be confirmed by polymerase chain reaction (PCR), polyvalent fluorescence immunoassay, or Western immunoblot testing.
The differential diagnosis depends on clinical manifestation of the disease stage, and may include cellulitis, erythema multiforme, tinea corporis, viral/bacterial meningitis/encephalitis, rheumatic fever, septic arthritis, endocarditis, and other inflammatory/autoimmune and viral syndromes.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
The treatment of choice for early Lyme disease is oral doxycycline 100 milligrams PO twice daily for 14 to 21 days for primary stage infection (28 days if treating secondary stage, and 28 to 60 days for tertiary stage). Acceptable alternatives include amoxicillin, cefuro-xime, ceftriaxone, or erythromycin.
A single dose of doxycycline 200 milligrams given within 72 hours of the deer tick bite can prevent Lyme disease, but this is not routinely suggested due to the low transmission rate, the potential to depress the immune response to the disease, and the high incidence of gastrointestinal side effects from high-dose oral doxycycline.
EHRLICHIOSIS
EPIDEMIOLOGY
A zoonotic disease with two clinical subtypes (human granulocytic and human monocytic). The human monocytic form (Ehrlichia chaffeensis) predominates in the United States.
PATHOPHYSIOLGY
Caused by Ehrlichia species, small gram-negative coccobacilli that infect circulating leukocytes. Transmission occurs viabite or exposure to Amblyomma americanum (lone star tick) and Dermacentor variabilis. The major animal reservoir in North America is the white-tailed deer in the southeastern United States.
CLINICAL FEATURES
Symptoms usually develop within 10 to 14 days of tick bite, and may include fever, headache, malaise, nausea, vomiting, diarrhea, abdominal pain, and arthralgias.
More serious complications of renal or respiratory failure and encephalitis occur in a minority of patients.
The acute phase of illness lasts less than 4 weeks, with most patients recovering and proceeding to a convalescent phase.
DIAGNOSIS AND DIFFERENTIAL
Diagnosis is made clinically, but can be confirmed by a rise in antibody titer between the acute and convalescent phase.
Laboratory findings may include leukocytopenia, thrombocytopenia, and elevated serum hepatic enzymes.
The differential diagnosis includes cholecystitis/cholangitis, Lyme disease, babesiosis, malaria, meningitis, RMSF, and typhoid.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
The treatment of choice for both adults and children is doxycycline 100 milligrams PO twice daily for 7 to 14 days (for children, 2.2 milligrams/kg PO twice daily).
COLORADO TICK FEVER
EPIDEMIOLOGY
Colorado tick fever is endemic to the western mountainous regions of the United States, at elevations above 4000 ft. Only about 300 cases are reported annually.
PATHOPHYSIOLOGY
An acute viral illness caused by an RNA virus of the genus Coltivirus, transmitted to humans primarily via Dermacentor andersoni (wood tick). Animal reservoirs are deer, marmots, and porcupines.
CLINICAL FEATURES
Symptoms begin suddenly 3 to 6 days following tick bite and include fever, chills, headache, myalgias, and photophobia. There may be a macular or petechial rash. Complications are rare.
DIAGNOSIS AND DIFFERENTIAL
Diagnosis is based on clinical findings and geography.
Differential diagnosis includes meningitis (bacterial or viral), and other tick-borne illnesses (especially RMSF).
EMERGENCY DEPARTMENT CARE AND DISPOSITION
No specific therapy exists, and supportive care is usually sufficient.
TULAREMIA
EPIDEMIOLOGY
Tularemia has been widely reported in the continental United States, with the highest incidence in Arkansas, Missouri, and Oklahoma. Incidence is highest in early winter (adults) and early summer (children).
PATHOPHYSIOLOGY
Tularemia is caused by Francisella tularensis, a nonmotile, gram-negative coccobacillus carried by Dermacentor and Amblyomma ticks. The principal animal reservoirs are rabbits, hares, cats, and deer.
Transmission occurs via arthropod bite or direct innoculation from an infected host.
CLINICAL FEATURES
There are several distinct clinical syndromes, depending on the route of inoculation: ulceroglandular, glandular, typhoidal, pneumonic, and oropharyngeal.
The ulceroglandular form is the most common, characterized by an ulcer at the site of the tick bite, and painful regional adenopathy. Glandular tularemia consists of tender regional adenopathy without a skin lesion.
Typhoidal tularemia (any form of transmission) is associated with fever, chills, cephalgia, and abdominal pain. Ocular-oropharyngeal and pneumonic forms are the result of deposition of the bacterium in the eyes, or inhalation.
DIAGNOSIS AND DIFFERENTIAL
Clinical diagnosis can be confirmed by culture and enzyme-linked immunosorbent assay (ELISA). Other laboratory findings are nonspecific.
The multiple clinical variations of tularemia lead to a broad differential diagnosis that should include pyogenic bacterial infection, syphilis, anthrax, plague, Q fever, psittacosis, typhoid, brucellosis, and rickettsial infection.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Treatment is with streptomycin 1 gram IM twice daily or IM or IV (pediatric dose is 15 milligrams/kg IM twice daily, max 2 grams/d), or gentamicin 5 milligrams/kg IM or IV once daily. Other alternatives include doxycycline, chloramphenicol, and ciprofloxacin. Therapy is given for 10 to 14 days.
HANTAVIRUS
EPIDEMIOLOGY
Hantavirus infection is a viral zoonosis identified in 1977. In North America the etiologic agent is the Sin Nombre virus (a member of Bunyaviridae family), and to date at least 10 distinct serotypes have been identified, each with a specific rodent vector, geographic distribution, and clinical manifestation.
PATHOPHYSIOLOGY
In the southwestern United States, Peromyscus maniculatus (deer mouse) is the primary vector, with transmission to humans accomplished via inhalation of dried particulate feces, contact with urine, or by rodent bite.
CLINICAL FEATURES
Worldwide the majority of hantavirus serotypes have a predilection for the kidney, with a clinical presentation of acute renal failure, thrombocytopenia, ocular abnormalities, and flulike symptoms.
In the United States, the most common presentation is hantavirus pulmonary syndrome: a flulike prodrome for 3 to 4 days, followed by pulmonary edema, hypoxia, hypotension, tachycardia, and metabolic acidosis. The presence of dizziness, nausea and vomiting, and thrombocytopenia and the absence of a cough may help with clinical diagnosis.
DIAGNOSIS AND DIFFERENTIAL
Diagnosis relies on clinical features and a history of exposure, but may be confirmed by an immunofluorescent or immunoblot assay. Differential diagnosis includes bacterial pneumonia, acute respiratory distress syndrome (ARDS), and influenza pneumonia.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Hantavirus pulmonary syndrome has reported a mortality rate of 50% to 70%. Treatment is primarily with supportive care (especially oxygenation and ventilation) and possibly inhaled ribavirin.
ANTHRAX
EPIDEMIOLOGY
Anthrax infection is very rare in North America, but remains a concern in part because of its potential use as an agent of biological warfare or terrorism.
PATHOPHYSIOLOGY
Anthrax is an acute bacterial infection caused by Bacillus anthracis, an aerobic gram-positive rod that forms central oval spores.
In nature, the disease is most commonly seen in domestic herbivores (cattle, sheep, horses, and goats) and wild herbivores. Human infection can result from inhalation of spores, inoculation of broken skin, arthropod bite (fleas), or ingestion of inadequately cooked infected meat.
CLINICAL FEATURES
Inhalational anthrax is a mediastinitis without alveolar involvement, and therefore not a true pneumonia. Initially patients suffer a flulike illness that progresses over 3 to 4 days to respiratory failure, and is generally fatal.
Cutaneous anthrax (woolsorter’s disease) accounts for 95% of infections. Spores are deposited in a wound (usually on the hands or fingers), and in 1 to 5 days progresses to a pruritis macule.
The lesion then becomes an ulcer with multiple sero-sanguinous vesicles containing the bacilli, and are infectious. The ulcer eventually becomes a black eschar and falls off within 2 weeks.
DIAGNOSIS AND DIFFERENTIAL
Diagnosis may be established via Gram’s stain, direct fluorescent antibody stain, or culture of skin lesions or vesicular fluid.
The differential diagnosis depends on the type of exposure. For inhalational anthrax, it may include influenza, tuberculosis, and other causes of mediastinitis (bacterial, viral, parasitic, sarcoidosis). With cutaneous anthrax, warfarin necrosis, calciphylaxis, ischemic necrosis, tularemia, plague, spider/insect bite, mycobacterial infection, ecthyma gangrenosum, and aspergillosis/mucormycosis should be considered.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Treatment for both inhalational and cutaneous anthrax is ciprofloxacin 400 milligrams (10–15 milligrams/kg for children) IV every 12 hours or doxycycline 100 milligrams (2.2 milligrams/kg for children) IV every 12 hours, plus either clindamycin or rifampin. Extended treatment for adults is ciprofloxacin 500 milligrams PO every 12 hours or doxycycline 100 milligrams PO every 12 hours for 60 days.
A vaccine is available for high-risk populations (military personnel and laboratory technicians).
Prophylaxis for exposed individuals can be done with either ciprofloxacin 500 milligrams PO twice per day or doxycycline 100 milligrams PO twice per day for 60 days in combination with a three-dose vaccination course.
PLAGUE (YERSINIA)
EPIDEMIOLOGY
Plague is endemic to the United States, most often found in the Southwest, but may also be carried by cats and dogs.
PATHOPHYSIOLOGY
Yersinia pestis is a gram-negative bacillus of the Enterobacteriaceae family. Rock squirrels and ground rodents are the animal reservoir, and the rodent flea is the primary vector.
Transmission to humans occurs via the bite of a flea from an infected animal host or through handling or ingestion of infected rodents.
CLINICAL FEATURES
There are three clinical forms of human disease: bubonic or suppurative (most common), which may progress to the pneumonic or septicemic forms.
The incubation period ranges from 2 to 7 days following exposure. Frequently an eschar develops at the bite site, followed by a painful, sometimes suppurative bubo (enlarged regional lymph nodes), often at the groin. Associated symptoms may include fever, headache, malaise, abdominal pain, nausea and vomiting, and bloody diarrhea.
The pulmonary form is highly contagious and can be transmitted from person to person via aerosolized respiratory secretions (respiratory isolation is required). It is rapidly fatal if not aggressively treated.
DIAGNOSIS AND DIFFERENTIAL
Diagnosis must be made on clinical findings in a patient with possible contact with a vector or animal host. Blood culture or culture of suspected sites may reveal organisms, but treatment should be initiated in suspected cases without awaiting these results.
The differential diagnosis includes lymphogranuloma venereum, syphilis, staphylococcal or streptococcal lymphadenitis, other causes of pneumonia, or tularemia.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Therapy should begin immediately for any suspected case with gentamicin 2.0 milligrams/kg IV loading dose, then 1.7 milligrams/kg IV every 8 hours (2–2.5 milligrams/kg/dose for children), or streptomycin 15 milligrams/kg or 1 gram IV or IM every 12 hours. Therapy is continued for 10 to 14 days. Alternatives include doxycycline or ciprofloxacin.
For further reading in Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 7th ed., see Chapter 155, “Zoonotic Infections,” by John T. Meredith.