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.
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