Evidence of exposure to spotted fever group rickettsiae among Arizona dogs outside a previously documented outbreak area

Abstract

Since 2003, two communities in eastern Arizona have experienced a sustained outbreak of Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii, associated with transmission by Rhipicephalus sanguineus, the brown dog tick; 70 human cases, including eight deaths, were reported from these communities during 2003 through 2008. In both of the affected communities, antibodies to spotted fever group rickettsiae (SFGR) were present in dogs before the notice of the first human cases, suggesting that dogs may serve as useful sentinels for human risk of RMSF in this region. During 2005 and 2006, an exploratory serosurvey was conducted among stray and relinquished dogs presenting to animal control facilities in eastern Arizona located outside the area where human cases had been reported. Antibodies to SFGR were detected in 5.7% (14 of 247) dogs assessed outside the RMSF outbreak area. Animal shelters located in counties that either included or shared large borders with the outbreak area were significantly more likely to have seropositive dogs than facilities in more geographically separated counties (P = 0.01). In addition, stray dogs were significantly more likely to be antibody-positive than relinquished animals (P = 0.01), suggesting that control of stray dog populations should be considered as a means of limiting SFGR transmission in this region. The findings from this study may be extrapolated to suggest that the current risk for human RMSF infection may extend beyond the noted outbreak area. Heightened surveillance for human disease is needed in the region.