Human Granulocytic Ehrlichiosis in Massachusetts

Two new tick-borne zoonoses have recently emerged as threats to public health in North America. Both are caused by infection with Ehrlichia species, obligate intracellular bacteria that localize within the phagosomes of leukocytes. The spectrum of illness varies from mild to severe; about one third of patients require hospitalization. Persons exposed to ticks may present with a “spotless spotted fever” or a disease similar to Lyme disease that does not feature erythema and is accompanied by fever, chills, severe headache, myalgia, malaise, and nausea. A maculopapular or petechial rash, however, may be present [1, 2]. Laboratory findings include thrombocytopenia, leukopenia, and abnormal hepatic function test results. Treatment with tetracyclines induces defervescence within 48 hours.

Ehrlichia first attracted attention in 1935 as pathogens of dogs [3] and shortly thereafter as potential zoonotic agents. An infection similar to mononucleosis was reported in western Japan [4], and the ehrlichial cause of Sennetsu fever was subsequently confirmed by subinoculating mice with patient blood and observing the characteristic inclusions (morulae) within mononuclear cells. The index case of human ehrlichiosis in North America was discovered in a tick-exposed Arkansas resident in 1986 [5]. Morulae were seen within the patient's monocytes, and his serum reacted against antigens of E. canis, the agent of tropical canine pancytopenia. In 1990, a new species, closely related to E. canis, was cultivated from a febrile Arkansas patient and designated E. chaffeensis [6, 7]. About 400 illnesses caused by this agent have since been confirmed in 30 states. Nine of these cases were fatal.

Since 1990, morulae have been discovered in the granulocytes of 12 febrile patients from northern Minnesota and Wisconsin [8]. The serum of these patients did not react with E. chaffeensis antigen. In addition, an analysis of sequences obtained from a polymerase …