Comparison of efficacy of American and African Amblyomma ticks as vectors of heartwater (Cowdria ruminantium) infection by molecular analyses and transmission trials

The ability of Amblyomma americanum, Amblyomma cajennense, Amblyomma macula tum, and Amblyomma variegatum to acquire and transmit Cowdria ruminantium i nfection was investigated. Uninfected nymphs were fed on clinically reactin g C. ruminantium-infected sheep and then analyzed for infection by specific DNA detection assays and by tick transmission trials. By polymerase chain reaction (PCR), the mean infection prevalence of A. maculatum ticks (50.7%) was similar to that of A. variegatum, Elevage strain (43.5%; P = 0.83) and Petit Bourg strain (45.9%; P = 0.26) ticks. Though Amblyomma hebraeum were not tested by PCR, by DNA probe their infection prevalence was 94%. In con trast, A. americanum and A. cajennense ticks demonstrated very low suscepti bility to C.,ruminantium, and the prevalence of infection by PCR was approx imately 1%. The higher susceptibility of A. maculatum and A. variegatum to C. ruminantium correlated with superior Vector efficiency, depicted by simi lar prepatent periods and severity of disease transmissions to sheep. Ambly omma americanum and A. cajennense failed to transmit infection, confirming that low susceptibility to C. ruminantium correlates with the poor vector s tatus of these species. These results highlight the importance of A. macula tum as a potential vector that is likely to play a major role in the establ ishment and maintenance of heartwater, if the disease were to be introduced to the U.S.A., Central, and South America.