Effects of subclinical Theileria orientalis Ikeda genotype infection on average daily gain ratios and a satisfactory rating in the breeding soundness exam in bull test stations in Virginia

Theileria orientalis Ikeda genotype is a tick borne hemoprotozo­an that typically causes economic losses in dairy and beef cattle in Australia, New Zealand and Japan. Acute clinical infections from T. orientalis Ikeda include anemia, icterus, ill-thrift and death. The acute phase of the infection has been associated with decreased libido in dairy bulls, decreased live weight gain in beef bulls and increased mortality in naïve adults and calves. A sequela to acute infections within a herd is persistent sub­clinical infections, which have been associated with decreased mean daily gain in suckling beef calves. In late 2017, T. orienta­lis Ikeda was detected in beef cattle from multiple counties in Virginia and was associated with anemia, weakness, late term abortions and death. As of 2022, T. orientalis Ikeda has been identified in beef cattle in 31 of 95 Virginia counties. Beef pro­duction, typically in naturally bred cow-calf operations, is the second largest agricultural commodity in Virginia. Central bull testing programs for performance evaluation and marketing of beef bulls has existed for over 60 years in Virginia. T. orienta­lis Ikeda was first detected at the Southwest bull test station in 2020 when screened at conclusion of the test. The objective of this study was to determine if subclinical infection with T. ori­entalis Ikeda affected the average daily gain (ADG) ratios of all bulls on test and the achievement of a satisfactory rating of the breeding soundness exam (BSE) for senior bulls.