IXODES PACIFICUS (ACARI, IXODIDAE) AS A VECTOR OF EHRLICHIA-EQUI (RICKETTSIALES, EHRLICHIEAE)

Ehrlichia equi, a rickettsia described from horses in California 30 yr ago, causes equine granulocytic ehrlichiosis throughout the Americas and possibly Europe. Here, we report experimental transmission of E. e qui from infected to susceptible horses through bites of western black legged ticks, Ixodes pacificus (Cooley & Kohls). In preliminary field studies, only I. pacificus consistently infested horses and vegetation at 3 locations with contemporary cases of equine ehrlichosis, and in particular, I. pacificus was the only species found attached to all of the infected horses. Exposure to bites of ticks in the genus Ixodes p oses previously unrecognized and serious health risks to humans and an imals.