A longitudinal study of infection with genotypes of Theileria orientalis in calves and introduced cattle at Dorrigo, New South Wales, and the effect on weight gains.

Background: In regions with endemic Theileria orientalis, recovered cattle are carriers, leaving newborn calves and introduced stock as the most susceptible groups to develop clinical disease after tick infestation with parasite transmission.

Methods: To gain information on the kinetics of infection and development and effects of theileriosis caused by virulent ikeda and chitose genotypes and the "benign" buffeli genotype of T.orientalis, this study sampled a total of 134 calves from 3 farms in Dorrigo, Australia, a region with multiple genotypes of the parasite. In addition, 30 introduced beef weaners were bled and weighed for 6 months after arrival.

Results: In both cohorts, parasitaemia with the ikeda and chitose (genotypes 1 & 2) of T.orientalis was detectable by PCR within 3-4 weeks in >95 % of the groups, with maximal gene copies generated around 5-7 weeks after birth or introduction, before declining. In contrast, parasitaemias of T.orientalis buffeli (genotype 3), increased slowly, with around 80 % of the population testing positive by PCR after 4 months. The parasitaemias of the buffeli genotype did not exceed 40,000 gene copies /ul and were similar to those exhibited by ikeda and chitose genotypes following the "first wave of parasitaemia" as cattle entered the carrier state. In the 30 introduced weaners, the early infection with T.orientalis ikeda and chitose also caused significant reductions in packed cell volume (PCV) and incurred an estimated 20Kg loss in weight gain; the latter had not been recovered by 6 months after introduction.

Conclusions: The results support previous findings in other endemic regions of theileriosis, and imply that amelioration of the early high levels of parasitosis by the virulent genotypes could reduce the initial impact of theileriosis on production, which appears much less affected once animals enter the carrier state.