Endemic infection of cattle with multiple genotypes of Theileria orientalis on the Northern Tablelands of New South Wales despite limited presence of ticks

Abstract

Bovine theileriosis, caused by the Theileria orientalis complex, causes a mild persistent infection with the severity dependent on the infecting genotype and host exposure status. Clinical theileriosis was first detected on the Northern Tablelands of NSW in 2009 and a high prevalence of infection in cattle reported in 2013. However, the parasite was not genotyped and likely vectors not investigated. In response to ongoing clinical cases, here we identify the Theileria genotypes present in the region and potential vectors. Genotype-specific multiplex qPCR of 90 blood samples from eight farms revealed a 100 % prevalence of T. orientalis in individual cattle with concurrent infection with all three genotypes present in 73 % of cases. The prevalence of the pathogenic genotype (Ikeda) differed significantly between farms; however, the level of parasitemia was not affected by genotype or associated with clinical disease. Parasitaemia levels were higher in heifers than cows. Questing tick collection on six of the farms between November 2017 and May 2019 yielded 358 questing ticks from one farm, all of which were morphologically identified as Haemaphysalis bancrofti. Larvae accounted for 59 % of the ticks followed by nymphs (34 %) and adults (7%). Theileria was detected only in nymphs with Ikeda and Buffeli genotypes each being detected in one of four pools of ticks. The high prevalence of co-infection with three genotypes of T. orientalis indicates that they are now endemic in the region and confirms the lack of cross-protection between genotypes. This is the first detection of T. orientalis in questing H. bancrofti ticks: indicating that it may be a vector for T. orientalis in this region. However, the high prevalence of bovine infection is at odds with the absence of captured ticks or history of tick infestation on five of the six farms raising the possibility that other vectors or transmission pathways play key roles.