Average daily gains and response to artificial insemination in beef replacement heifers with persistent subclinical Theileria orientalis Ikeda genotype infection

Abstract

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. Clinical signs of theileriosis from T. orientalis Ikeda include anemia, icterus, ill-thrift, and death. The acute phase of the infection has been associated with a decrease in milk production in dairy cows, 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 and these have been associated with negative effects on reproduction in dairy cows and decreased mean daily gain in suckling beef calves. Prior to 2017, non-pathogenic genotypes of T. orientalis were present in the United States. In late 2017, T. orientalis Ikeda/genotype 2 was detected in beef cattle from multiple counties in Virginia and was associated with anemia, weakness, and death. As of 2020, T. orientalis Ikeda has been identified in beef cattle in 22 of 95 Virginia counties and 4 of 55 West Virginia counties. Beef production through primarily cow/calf operations is the second largest agricultural commodity in Virginia. After detection of T. orientalis Ikeda in a subpopulation of spring breeding beef heifers at a replacement breeding farm, we thoroughly investigated the fall breeding heifers. The objective of this study was to determine if persistent subclinical infection with T. orientalis Ikeda affected the average daily gain (ADG) from arriving on farm to pregnancy verification or conception rates to artificial insemination (AI) in beef replacement heifers.