Haemoparasite infection kinetics and the population structure of Theileria parva on a single farm in Uganda.

The development of sensitive PCR-based species-specific diagnostics and parasite genotyping methods offer the opportunity to provide important and detailed information on the infection dynamics of tick-borne disease. In this study we have exploited such tools to investigate the infection kinetics and parasite diversity within Theileria parva in a single farm in Uganda. Initial analysis of a sample of cattle showed high levels of infection with three Theileria species and Ehrlichia bovis, with most animals being infected with more than one pathogen. To study the infection dynamics, newborn calves were sampled longitudinally and it was shown that all animals became infected with T. parva, T. mutans, T. velifera and E. bovis with the average time to first infection being 53, 74, 116 and 109 days, respectively. However, the majority of these calves cleared the infections with T. parva and E. bovis but remained infected with the other two species of Theileria. In order to investigate the diversity of infecting genotypes of T. parva, samples from six calves were genotyped with a single mini-satellite marker at time points over a nine-month period. Each animal was infected with multiple different sets of genotypes and these were lost over different periods of time, implying that immunity is induced against particular infecting strains. To undertake a higher resolution analysis of parasite genotypes, samples from 30 calves were genotyped with a full panel of 12 micro- and mini-satellite markers but, due to the presence of mixed infections, only 16 samples could be used to generate parasite multi-locus genotypes (MLGs). A high degree of diversity of T. parva was seen on the farm, although some MLGs occurred more than once. Similarity analysis demonstrated a level of sub-structuring and the T. parva population was found to be in linkage disequilibrium. The basis for this high diversity coupled with apparent sub-structuring is discussed in relation to the possible causes.