Attempted Ehrlichia risticii transmission with Dermacentor variabilis (Acari: Ixodidae).

Larval Dermacentor variabilis (Say) (n = 327) were fed on Balb/C mice inoculated with Ehrlichia risticii, the etiologic agent of equine monocytic ehrlichiosis (Potomac horse fever). All mice displayed clinical signs of E. risticii infection at the time of feeding. After molting, resulting nymphs (n = 74) were fed on susceptible mice. No clinical signs were observed, and the mice remained seronegative for 6 wk after feeding.