Date of this Version
In April 1986, a medical intern scanning the peripheral blood smear of a severely ill man with an unexplained illness observed peculiar intracytoplasmic inclusions in several of the patient’s monocytes. The patient described multiple tick bites sustained approximately 2 weeks earlier during a visit to a rural area in northern Arkansas, and a presumptive diagnosis of Rocky Mountain spotted fever had been made (104, 174). Clinicians and scientists subsequently identified these inclusions as clusters of bacteria belonging to the genus Ehrlichia, previously known in the United States solely as veterinary pathogens (174). Within the next 5 years, the organism was isolated in cell culture, characterized by molecular techniques, and formally named Ehrlichia chaffeensis (9, 73). During this interval, surveillance efforts identified several hundred cases of moderate to severe, and occasionally fatal, ehrlichiosis in patients with unexplained illnesses following tick exposures (97, 106, 107, 125, 233, 263). These findings indicated that ehrlichiosis was a widespread and significant public health problem of increasing but undefined magnitude.