Significance of Theileria orientalis types in individual affected beef herds in New South Wales based on clinical, smear and PCR findings.

Cattle within seven NSW herds with a history or risk of clinical Theileria orientalis disease associated with introductions of cattle were examined clinically and by haematological and PCR testing at sequential bleeds or at single sampling of different risk subgroups. The T. orientalis Ikeda type was detected in all herds and Chitose type was detected in six. Pale and jaundiced mucosal surfaces were associated with clinically affected groups of cattle, and herds containing cattle with ≥ 1% theilerias in erythrocytes were associated with high prevalence of Ikeda type, with or without Chitose type. In clinically normal cattle within these Ikeda-affected herds, over half of the smear negative animals were detected as infected with Ikeda type, while 90% of smear positive cases were positive for Ikeda type. Infection with Ikeda and Chitose organisms was detected in calves as young as 1-2 weeks, rapidly increased in prevalence within one month and was maintained until 4.5 months of age. In these calves Ikeda prevalence increased at a faster rate than the other MPSP types, particularly Buffeli which is generally considered to be avirulent, and suggests either an increased growth rate or rate of transmission of the Ikeda type or failure of the host immune system to clear this type. Particularly high T. orientalis prevalence rates were detected (in blood samples from a single time point) in adults that had been in direct contact with weaner cattle introduced from coastal areas; however, the lack of direct contact with affected cattle did not prevent infection with Ikeda type in some cases. Spread within previously naïve herds was variable, and results also depended on the sampling time point. In contrast, groups in which infection was already established gave repeatedly similar results at multiple samplings taken at one month intervals. Our results confirm that a large reservoir of infected but clinically normal animals exists within T. orientalis-affected cattle herds and PCR testing of EDTA bloods is more sensitive for detecting subclinical infection than blood smear examination. Direct contact with weaner cattle introduced from coastal areas appears to be a major risk factor for T. orientalis infection in adult cattle. Frequent sampling may be used to monitor spread of T. orientalis within newly affected herds, but may be unrewarding once a high prevalence is established.