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HME is transmitted to humans by the bite of the Lone Star tick, Amblyomma
americanum. The wood tick, Dermacentor variabilis, has also been implicated.
Reported infections are concentrated in the the south central states of Arkansas,
Missouri, Oklahoma, and eastern Texas, and in North Carolina and Virginia.
The clinical courses of infections by either organism are quite similar. Typically
the first manifestation is an acute febrile illness associated with headache and
myalgia. Approximately 75 percent of patients have a history of tick exposure.
Laboratory studies usually disclose leukopenia and thrombocytopenia, sometimes
anemia, and elevated hepatic aminotransferases. A nonspecific rash occurs in
approximately one-third of the patients with HME, but is less common in patients
with HGE.
The diagnosis is aided by finding organisms in peripheral blood smears, but
typical morules are found in only about 80 percent of serologically confirmed
infections. Infections are most often diagnosed or confirmed by
immunofluorescence assay (IFA), although polymerase chain reaction (PCR)
assays are being used more commonly.
Most patients have a mild illness that rapidly responds to doxycycline (100 mg
twice a day). Defervesence usually occurs in twenty-four to forty-eight hours.
However, a significant number of individuals, mostly older patients, have life-
threatening complications that include adult respiratory distress syndrome
(ARDS), renal failure, neurologic disorders, and disseminated intravascular
coagulation (DIC). Case-fatality ratios as high as 10 percent for HGE and 5
percent for HME have been reported, but these appear high.
Reference: McQuiston JH, Paddock CD, et al: The human ehrlichioses in the
United States. Emerging Infectious Diseases
.
www.cdc.gov/ncidod/eid/vol5no5/mcquiston.htm
(last accessed May 8, 2005).
E. Babesiosis: Babesiosis is an infection by intraerythrocytic Babesia parasites. In
the Northeastern United States it has the same distribution as Lyme disease. The
main etiologic agent is B. microti, and it is transmitted by the same tick, Ixodes
scapularis, and has the same principal animal reservoir, the white-footed mouse,
peromyscus leucopus. A second pattern includes cases reported from California,
Georgia, and Washington, but the etiologic agents have not been speciated.
Clinical signs of infection vary considerably; many patients remain asymptomatic.
Symptoms are similar to malaria, also an intraerythrocytic parasite, except the
periodicity of malaria is not seen. Symptoms usually appear one to four weeks
following a tick bite and consist of the gradual onset of malaise, anorexia, and
fatigue. Within a week or so, fever that ranges from 37.8
° to 40.3°C (100° to
104
°F) drenching sweats, and myalgia develop. As with malaria, nausea,
vomiting, headache, shaking chills, hemoglobinuria, altered mental status,
disseminated intravascular coagulation, anemia with dyserythropoiesis,
hypotension, respiratory distress, and renal insufficiency are common.
Severe and fatal cases of human babesiosis occur most commonly in elderly
patients, patients who have had a splenectomy, and patients who are
