THE RELATIONSHIP BETWEEN TICK (AMBLYOMMA-HEBRAEUM) INFESTATION AND IMMUNITY TO HEARTWATER (COWDRIA-RUMINANTIUM INFECTION) IN CALVES IN ZIMBABWE

The occurrence of endemic stability for heartwater ( Cowdria ruminanti um infection) is a controversial issue, because the means by which you ng cattle and other neonatal ruminants become infected and acquire imm unity to the disease have never been adequately explained. We conducte d a study in a heartwater-endemic area in southeastern Zimbabwe to inv estigate the relationships between calf immunity to C. ruminatium, inf estations of the tick vector Amblyomma hebraeum and dam, colostral and calf antibody titres to C, ruminantium. Two groups of cows (tick-infe sted and acaricide-treated) were artificially inseminated and then mon itored through pregnancy and lactation by means of tick counts and ser um antibody titres. The calves born to the cows in each treatment grou p were similarly monitored from birth until after weaning, when they w ere challenged with a heartwater stabilate (Ball-3 vaccine), Colostrum was collected from the cows on the days that the calves were born. Se rum and colostrum samples were screened for Cowdria-specific antibodie s using an indirect fluorescent antibody test. The cows and calves in the acaricide-treated group remained essentially tick-free for the dur ation of the study. The cows in the tick-infested group were continuou sly infested with A, hebraeum, but their calves only became infested b etween 12 and 31 weeks after birth. Cowdria-specific antibodies were d etected in the sera of the cows and calves and in the colostrum of bot h treatment groups, but the titres were consistently higher in the tic k-infested group than in the acaricide-treated group, Antibodies, prob ably of maternal origin since the precolostral serum titres were negat ive, were present in the calves of both groups for at least 8 weeks af ter birth. Between Weeks 24 and 52 postpartum, the antibody titres of the tick-infested group of calves were significantly correlated with t he numbers of A. hebraeum present on the animals. The antibody titres of the calves in the acaricide-treated group increased considerably fo llowing challenge with heartwater stabilate, 60 weeks after birth. No clinical heartwater was detected in either group of calves following i noculation of stabilate, and it was concluded that the calves in both groups were immune to the, disease. In the acaricide-treated group, im munity did not correlate with exposure to tick-transmitted infections. Hence, we concluded that the calves in both groups had probably been infected by vertical transmission around the time of birth. Such early infections are likely to have been controlled by maternal antibodies or by an unidentified immune mechanism. The probable occurrence of ver tical transmission, together with the absence of Amblyomma ticks from calves in the first months of life (even without acaricide treatment), has lead us to believe that the dams' immune status does influence the documented age-related resistance to heartwater and casts doubt on th e validity of the widely held view that the age-related resistance to heartwater is totally independent of the immune status of the dam.