| Summary: | Bovine anaemia caused by <i>Theileria orientalis</i> group (BATOG) causes significant production and economic losses in Australia’s cattle industry. The pathogenic <i>T. orientalis</i> genotypes reported in Australian cattle are type 1 (Chitose) and type 2 (Ikeda). The present study aimed to determine the prevalence and distribution of <i>T. orientalis</i> genotypes in adult lactating cows in Western Australia (WA) dairy herds. A total of 100 whole blood samples from lactating cows from 10 farms were obtained and screened for <i>T. orientalis</i> using polymerase chain reaction (PCR). Sanger sequencing was subsequently used to characterise <i>T. orientalis</i> genotypes isolated from positive samples. A total of thirteen cows (13%; 95% CI: 7.1–21.2%) were positive for <i>T. orientalis</i>, and six out of ten farms (60%; 95% CI: 26.2–87.8%) housed at least one <i>T. orientalis</i>-positive cow. The distribution of <i>T. orientalis</i> was found to be wide and dense in the South west region of WA and the southern coast of WA. The predominant <i>T. orientalis</i> genotype identified was Ikeda (n = 11, 11%; 95% CI: 5.6–18.8%), while the Buffeli genotype was identified in WA for the first time, albeit at a low prevalence (n = 1, 1%; 95% CI: 0.0–5.4%). This study has provided useful epidemiological evidence on the prevalence and distribution of <i>T. orientalis</i> in adult lactating dairy cows in WA dairy farms, and on the importance of conducting widespread surveillance programs for the understanding of BATOG in WA.
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