| Zoonosis Update: |
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Ehrlichiosis |
| Author(s): |
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James G. Olson, PhD |
| Source: |
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From the Viral and Rickettsial Zoonoses Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention (CDC), Mailstop G13, 1600 Clifton Road,
Atlanta, GA 30333 |
| Date: |
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Feb 15, 1989; revised 1994 |
Q: What organisms cause human ehrlichiosis?
A: Human ehrlichial diseases are caused by Ehrlichia chaffeensis and other yet uncharacterized ehrlichiae. Because of the similarity between the clinical presentation of E canis infection in dogs and E chaffeensis infection in human beings and the serologic cross-reaction between the 2 species, E canis was considered the etiologic agent of human ehrlichiosis in North America until E chaffeensis was isolated and characterized by molecular means in 1991.1,2 Another Ehrlichia species has been implicated in human disease in Wisconsin and Minnesota. This disease is called human granulocytic ehrlichiosis to distinguish it from the other form that infects principally human monocytes. Although the etiologic agent has not been isolated from patients, a genetic sequence significantly different from that of E chaffeensis and remarkably similar to that of E phagocytophila, has been detected in patients' blood by using polymerase chain reaction analysis.3,4
Q: What is the distribution of human ehrlichiosis in the United States?
A: A total of 320 cases of human monocytic ehrlichiosis caused by E chaffeensis from 27 states were confirmed by laboratory diagnosis between 1985 and 1993.5 States with the highest numbers of cases include Missouri, Oklahoma, Texas, Virginia, Arkansas, and Georgia, and are concentrated in west south-central, south Atlantic, and west north-central regions of the United States. Human granulocytic ehrlichiosis cases have only been reported from Wisconsin and Minnesota,3 but the distribution of cases is not known.
Q: What are the signs and symptoms of human ehrlichiosis?
A: Fever, malaise, and headache are the most commonly reported symptoms among ehrlichiosis patients. Similar to most rickettsial diseases, ehrlichiosis is associated with nonspecific signs and symptoms. Most patients (68%) report tick bite, but only 36% develop a rash. Laboratory findings include leukopenia and thrombocytopenia and high liver enzyme activities.5
Q: How is ehrlichiosis transmitted to human beings?
A: Epidemiologic evidence suggests that tick bite is the means of transmission of ehrlichiae to human beings. Amblyomma americanum, the lone star tick, has been shown to be infected with E chaffeensis6 and has a geographic and seasonal distribution compatible with the epidemiology of human monocytic ehrlichiosis.7 Further laboratory and field studies are required to completely define the tick vector. Human granulocytic ehrlichiosis occurs outside the geographic range of A americanum and is probably transmitted by another species of tick.
Q: Is ehrlichiosis transmitted directly from dogs to human beings?
A: The evidence strongly indicates that E chaffeenisis is not transmitted from dogs to human beings. Ehrlichia canis, the agent that causes canine ehrlichiosis, is not a human pathogen. It is not known whether the agent that causes human granulocytic ehrlichiosis infects dogs.
Q: How is ehrlichiosis diagnosed?
A: The diagnosis is suggested by the nonspecific signs and symptoms compatible with ehrlichiosis and a history of tick bite or likelihood of tick contact. Laboratory confirmation by indirect immunofluorescence testing8 depends on documenting a fourfold change in serum antibody titer between the acute and convalescent phases of illness. Laboratory confirmation may also be made by polymerase chain reaction analysis of anticoagulated (EDTA) blood, using Ehrlichia-specific primers.3,4,9
Q: What is the treatment for ehrlichiosis?
A: Appropriate antibiotic intervention is effective for management of human ehrlichiosis. The recommended antibiotics are tetracyclines, doxycycline, and chloramphenicol. Schedules and dosages are the same as those for the treatment of Rocky Mountain spotted fever.5 There have been 9 fatalities among the 320 confirmed cases of human ehrlichiosis.
Q: How can human ehrlichiosis and other tick-borne diseases be prevented?
A: Body checks for ticks and wearing clothing sprayed with tick repellents or into which repellent has been impregnated may prevent tick infestations.9 Be cautious to follow product guidelines and avoid spraying product on skin or in eyes.
Q: How can ehrlichiosis and other tick-borne diseases be prevented in dogs?
A: Prevention of ehrlichiosis in dogs depends on preventing tick infestations. Use of tick repellents and acaricides may reduce tick burden and the probability of ehrlichiosis and other tick-borne diseases as well. Pet owners should use these methods (eg, tick collars, dipping) on their pets, examine their dogs for ticks on a daily basis, and promptly remove attached ticks.
Discussion
Human ehrlichiosis is a potentially fatal illness that manifests itself in nonspecific clinical signs and symptoms and may be confused with Rocky Mountain spotted fever or Lyme disease. Clinicians should initiate appropriate antibiotic therapy while awaiting laboratory confirmation of the diagnosis. Delay in initiating specific therapy may prolong the illness and worsen the patient's prognosis. Essential to reducing morbidity attributable to human ehrlichiosis is that clinicians recognize the possibility of ehrlichiosis among patients with signs and symptoms compatible with a diagnosis of ehrlichiosis and who either report tick bite or have been exposed to tick infestation. The natural history of human ehrlichiosis is not completely understood. Further study is necessary to determine the tick vectors of both human forms of ehrlichiosis.
References
1. Dawson JE, Anderson BE, Fishbein DB, et al. Isolation and characterization of an Ehrlichia sp. from a patient diagnosed with human ehrlichiosis. J Clin Microbiol 1991; 29:2741-2745.
2. Anderson BE, Dawson JE, Jones DC, et al. Ehrlichia chaffeensis, a new species associated with human ehrlichiosis. J Clin Microbiol 1991; 29:2838-2842.
3. Chen SM, Dumler JS, Bakken JS, et al. Identification of a granulocytic Ehrlichia species as the etiologic agent of human disease. J Clin Microbiol 1994; 32:589-595.
4. Bakken JS, Dumler JS, Chen SM, et al. Human granulocytic ehrlichiosis in the upper midwest United States. JAMA 1994; 272:211-218.
5. Fishbein DB, Dawson JE, Robinson LE. Human ehrlichiosis in the United States, 1985-1990. Ann Intern Med 1994; 120:736-743.
6. Anderson BE, Sims KG, Olson JG, et al. Amblyomma americanum: A potential vector of ehrlichiosis. Am J Trop Med Hyg 1993; 49:239-244.
7. Olson JG. Human ehrlichiosis: status of an emerging disease, in Proceedings and Abstracts. First International Conference on Tick-borne Pathogens at the Host-vector Interface: An Agenda for Research. 1992. University of Minnesota, Saint Paul, 126-129.
8. Dawson JE, Fishbein DB, Eng TR, et al. Diagnosis of human ehrlichiosis with the indirect fluorescent antibody test: Kinetics and specificity. J Infect Dis 1990; 162:91-95.
9. Anderson BE, Sumner JW, Dawson JE, et al. Detection of the etiologic agent of human ehrlichiosis by polymerase chain reaction. J Clin Microbiol 1992; 30:775-780.
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