Serologic evidence of rickettsialpox (Rickettsia akari) infection among intravenous drug users in inner-city Baltimore, Maryland

We tested single serum samples from 631 intravenous (IV) drug users from inner-city Baltimore, Maryland for serologic evidence of exposure to spotted fever group rickettsiae. A total of 102 (16%) individuals had titers greater than or equal to 64 to Rickettsia rickettsii by an indirect immunofluorescence assay. Confirmation that infection was caused by R. akari was obtained by cross-adsorption studies on a subset of serum samples that consistently resulted in higher titers to R. akari than to R. rickettsii. Current IV drug use, increased frequency of injection, and shooting gallery use were significant risk factors for presence of group-specific antibodies reactive with R. rickettsii. There was a significant inverse association with the presence of antibodies reactive to R. rickettsii and antibodies reactive to the human immunodeficiency virus. This study suggests that IV drug users are at an increased risk for R, akari infections. Clinicians should be aware of rickettsialpox, as well as other zoonotic diseases of the urban environment, when treating IV drug users for any acute febrile illness of undetermined etiology.