Incremental effect of natural tick challenge on the infection and treatment method-induced immunity against T. parva in cattle under agro-pastoral systems in Northern Tanzania.

This study was conducted to assess the incremental effect of natural tick challenge on the infection and treatment method-induced immunity against T. parva under agro-pastoral systems in Simanjiro district, Northern Tanzania. T. parva specific antibody percent positivity and prevalence of T. parva parasites were studied in relation to duration post vaccination and proximity to Tarangire National park. A total of 381 cattle were included in this study, of which 127 were unvaccinated and 254 had been vaccinated at different time points between 2008 and 2014. Antibody percent positivity (PP) determined by the PIM-based T. parva ELISA and the prevalence of T. parva parasites detected by a nested PCR based on the p104 gene were used to compare vaccinated and unvaccinated cattle. Results showed that seroprevalence was significantly higher in vaccinated than unvaccinated cattle (OR 10.89, p = 0.0341). Only 1.6% (6/381) of all cattle were seronegative and 5/6 of these were unvaccinated. Prevalence of T. parva parasites was significantly higher in vaccinated (50.39%) than unvaccinated (19.69%) cattle (OR 2.03, p = 0.0144). While there was a positive association between PP and duration post vaccination but the latter was inversely associated with T. parva parasite prevalence. This study also showed that cattle which were closer to the park had higher antibody PP and T. parva prevalence. It is concluded that duration post vaccination as well as proximity from the wildlife in Tarangire National park together may exert an incremental effect on the outcome of ECF vaccination by influencing stronger antibody immunity of cattle and ability to withhold high T. parva infection pressure under constant field tick challenge. Further, the high seroprevalence in vaccinated and unvaccinated cattle suggests a likely state of endemic stability to T. parva in the study area.