Serologic evidence for exposure to Rickettsia rickettsii in eastern Arizona and recent emergence of Rocky Mountain spotted fever in this region.

During 2002 through 2004, 15 patients with Rocky Mountain spotted fever (RMSF) were identified in a rural community in Arizona where the disease had not been previously reported. The outbreak was associated with Rickettsia rickettsii in an unexpected tick vector, the brown dog tick (Rhipicephalus sanguineus), which had not been previously associated with RMSF transmission in the United States. We investigated the extent of exposure to R. rickettsii in the local area through serologic evaluations of children and dogs in 2003-2004, and in canine sera from 1996. Antibodies to R. rickettsii at titers > or = 32 were detected in 10% of children and 70% of dogs in the outbreak community and 16% of children and 57% of dogs in a neighboring community. In comparison, only 5% of canine samples from 1996 had anti-R. rickettsii antibodies at titers > or = 32. These results suggest that exposures to RMSF have increased over the past 9 years, and that RMSF may now be endemic in this region.

Spotted fever group rickettsial infection in dogs from eastern Arizona: how long has it been there?

A serosurvey of free-roaming dogs for antibodies to spotted fever group rickettsiae was conducted using archival samples that had been collected in the White Mountain region of eastern Arizona during a plague study in 1996. Immunoglobulin G antibodies to Rickettsia rickettsii (5.1%) and to R. rhipicephali (3.6%) were demonstrated, and no cross-reactive samples were identified. This study indicates that R. rickettsii was likely present in the dog populations in this area prior to the recognition of human cases of Rocky Mountain spotted fever (RMSF). The role of dogs as short-term reservoirs and primary hosts for the vector tick, Rhipicephalus sanguineus, should receive closer attention.