Seroconversion to Cowdria ruminantium of Malawi zebu calves, reared under different tick control strategies.

The seroconversion by indirect ELISA to Cowdria ruminantium over the first year of life of sixty-six Malawi zebu calves born into groups which were dipped 17 times per year was compared to seroconversion of 32 calves born into non-dipped groups. Amblyomma variegatum tick counts and clinical disease in each group of cattle were monitored throughout the study period. No cases of heartwater were seen in either group of calves over the first 22 months of life. Only one case of heartwater was observed, in an 8 year old cow, in the 1,800 intensively monitored cattle over the same period. By 12 months of age almost all undipped calves had seroconverted and 50% of seroconversions were attributed to nymphal challenge. In contrast, only 41% of calves had become seropositive by 12 months of age in the dipped groups. The dipping regime used therefore significantly decreased seroconversion rates to C. ruminantium in these calves. 73% of calves had detectable levels of maternal antibodies to C. ruminantium in the first 4 weeks of life. Antibody levels in each of the calves in dipped groups had waned to below the cut off point for the ELISA by 8-12 weeks. Seroconversion did not occur in the first 8-12 weeks of life in dipped herds. The indirect ELISA test results were not significantly different in the proportion positive in single tests at 12 months of age, or by cumulative test results of the previous 9 months, and therefore the test may be of value as a test of herd immunity.(ABSTRACT TRUNCATED AT 250 WORDS)