HME was first described in 1987 and, to date, approximately 400 cases of HME have been diagnosed in 30 states, most of them from south central or southeastern states. The three cases in California described above are the first known cases of ehrlichiosis acquired in this State. Both HME and HGE are transmitted to humans through the bite of an infected tick and the majority of patients reported tick exposure or tick bite in the three weeks before onset of their illness. Animal reservoirs of both HME and HE have not been fully identified, but white-tailed deer are potential reservoirs for E. chaffeensis in the southern region. In California, Ixodes pacificus is the potential tick vector for HGE, Whereas the vectors for HME are unknown.
Older persons seem to be at higher risk for severe disease.
Patients may occasionally present with cough, confusion, or a maculopapular rash. Laboratory abnormalities commonly include elevated liver enzymes, leukopenia, and thrombocytopenia. Anemia may also be noted. Whereas a rash is present in the majority of patients with RMSF, it is not commonly present in ehrlichiosis. Chloramphenicol has been used effectively in some cases but invitro data showed that E. chaffeensis is not susceptible to this drug.