Association of Ixodes pacificus (Acari : Ixodidae) with the spatial and temporal distribution of equine granulocytic ehrlichiosis in California

This study was conducted to determine if the biology of certain ticks assoc iated with horses regulates the spatial and temporal distribution of equine granulocytic ehrlichiosis (EGE) in California north of Monterey County. We compared the spatial and temporal distribution of EGE cases with the seaso ns of activity and life histories of ticks that infest horses. Spatially, c ases collected fr om equine veterinarians clustered around each other in a manner different from the way in which control cities of practice were dist ributed with foci limited to the Sierra Nevada and coastal foothills. Cases also clustered seasonally; most were diagnosed between November and April. The spatial and temporal pattern of EGE cases closely parallels the well-c haracterized life history and distribution of Ixodes pacificus Cooley & Koh ls, but not other ticks commonly associated with horses. Building on previo us studies, there is compelling evidence that this tick has the vectorial c apacity to transmit Ehrlichia equi to horses. Based on the life history and distribution of I. pacificus in relation to EGE cases, we reason that this tick is the only biologically plausible vector of E. equi in California, a nd provide evidence for a tightly linked association between I. pacificus a nd the epidemiology of EGE.