Problems with the interpretation of epidemiological data in heartwater: a study on 23 farms.

In an epidemiological study undertaken on 23 farms where heartwater occurs endemically, it was found that on an overall average, antibodies to Cowdria ruminantium were detected in 64.3% of the cattle, 6 adult Amblyomma hebraeum ticks were counted per animal and 7.0% of ticks were infected with the heartwater agent. It was found that the seropositivity of the animals was determined largely by the tick loads to which they were subjected and that the influence of the tick C. ruminantium infection rate was less evident. There was no parallel between the prevalence of heartwater on the farms and the immune status of the animals. In general, higher tick counts were recorded in herds where strategic tick control is practised than on farms with a total tick control programme. The method of tick control did not, however, appear to influence the immune status of the cattle, the tick infection rate, or the prevalence of heartwater.