In Australia, outbreaks of clinical theileriosis caused by Theileria orientalis have been largely associated with the Ikeda genotype which can occur as a sole infection, or more commonly, as a mixture of genotypes. The most prevalent genotype, Chitose, frequently co-occurs with type Ikeda, however the role of this genotype in clinical disease has not been clearly established. Furthermore, the dynamics of individual genotypes in field infection of cattle have not been examined. In this study we developed quantitative PCR (qPCR) and genotyping methods to examine the role of the Chitose genotype in clinical disease and to investigate the temporal dynamics of T. orientalis Ikeda, Chitose and Buffeli genotypes in naïve animals introduced to a T. orientalis-endemic area. Analysis of the major piroplasm surface protein (MPSP) genes of Chitose isolates revealed the presence of two distinct phylogenetic clusters, Chitose A and Chitose B. A genotyping assay aimed at determining Chitose A/B allele frequency revealed that the Chitose A phylogenetic cluster is strongly associated with clinical disease but nearly always co-occurs with the Ikeda genotype. qPCR revealed that the Chitose genotype (particularly Chitose A), undergoes temporal switching in conjunction with the Ikeda genotype and contributes substantially to the overall parasite burden. The benign Buffeli genotype can also undergo temporal switching but levels of this genotype appear to remain low relative to the Ikeda and Chitose types. Interplay between vector and host immunological factors is presumed to be critical to the population dynamics observed in this study. Genotypic switching likely contributes to the persistence of T. orientalis in the host.

Background Theileria orientalis infection causes a clinical syndrome in cattle characterised by weakness, reluctance to walk, anaemia, jaundice and death in peri‐parturient cows and young calves, referred to as bovine anaemia caused by Theileria orientalis group (BATOG). Abortions in pregnant cows are also reported. Pallor, pyrexia and elevated heart and respiratory rates are typical findings on physical examination. Case Report A syndrome of abortions, lethargy, inappetence, jaundice and deaths in beef cattle on two separate properties and a separate cluster of three properties within 15 km west of the town of Denmark in Western Australia was associated with the presence of severe regenerative anaemia and the presence of Theileria orientalis Ikeda genotype in blood samples taken from affected cattle and their cohorts. A diagnosis of bovine anaemia caused by the T. orientalis group was based on consistent clinical, haematological, biochemical and PCR findings. Conventional PCR testing detected only the T. orientalis Ikeda type. On the two properties where it was investigated, quantitative PCR testing for parasite load was suggestive of recent infections. Sequencing of T. orientalis major piroplasm surface protein gene PCR products demonstrated that they were identical to those from similar bovine cases in New South Wales. Conclusion The clinical history of affected cattle and the biochemical, haematological and PCR findings were consistent with bovine anaemia caused by the T. orientalis Ikeda genotype. This clinical syndrome had not been recognised in Western Australia before this series of cases.