Investigations into the vector competency of arthropods for two Ehrlichias: Ehrlichia risticii and Cowdria rumantium

Three studies relating to the vector competency of several species of ticks and Simulium spp. (blackflies) for Ehrlichia risticii, causative agent of Potomac horse fever (PHF) and Amblyomma variegatum for Cowdria ruminantium, causative agent of heartwater, are described.

Dermacentor variabilis, Rhipicephalus sanguineus, Amblyomma americanum and Ixodes scapularis ticks were investigated for their ability to acquire and transmit PHF. Larval and nymphal ticks were exposed to E. risticii by feeding on mice inoculated with the organism. Molted exposed ticks were then allowed to feed on susceptible ponies or mice and were examined by light and electron microscopy. No evidence of transmission, either clinically or by seroconversion in mice or ponies was observed.

Blackflies (Simulium spp.) were trapped in an area endemic for PHF and inoculated into mice in an attempt to demonstrate I. risticii. No evidence of seroconversion by mice to E. risticii was observed.

Two laboratory colonies of Amblyomma variegatum ticks were investigated for their ability to acquire and transmit C. ruminantium and was febrile. Nymphs from both laboratory groups were simultaneously fed on a goat that had been infected with C. ruminantium and was febrile. Engorged nymphs from both groups were replete from feeding on three consecutive days. Nymphs from both groups were then incubated under identical conditions until molting.