The emergence of another tickborne infection in the 12-town area around Lyme, Connecticut: human granulocytic ehrlichiosis.

Human granulocytic ehrlichiosis (HGE) is an emerging tickborne infection, increasingly recognized in areas in which Lyme disease is endemic, but there are few data on the incidence of HGE. Prospective population-based surveillance was conducted in the 12-town area around Lyme, Connecticut, by means of both active and passive methods, from April through November of 1997, 1998, and 1999. Five hundred thirty-seven residents presenting to their primary care provider with an acute febrile illness suggestive of HGE were identified. Of these, 137 (26%) had laboratory evidence (by indirect fluorescent antibody staining or polymerase chain reaction) of HGE; 89 were confirmed cases, and 48 were probable cases. The incidence of confirmed HGE was 31 cases/100,000 in 1997, 51 cases/100,000 in 1998, and 24 cases/100,000 in 1999. A subset of sera was tested by use of immunoblot assays, and results were in agreement with indirect fluorescent antibody methods for 86% of samples analyzed. Thus, HGE is an important cause of morbidity and is now the second most common tickborne infection in southeastern Connecticut.