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Background  Bovine theileriosis, caused by the haemoprotozoan  Theileria orientalis , is an emerging disease in East Asia and Australasia. Previous studies have demonstrated transplacental transmission of various  Theileria  spp. but molecular confirmation of transplacental transmission of  T. orientalis  has never been confirmed in the field. In this study, cow-calf (< 48 h old) pairs were sampled across 3 herds; opportunistic samples from aborted foetuses or stillborn calves were also examined. Molecular (multiplex qPCR) and serological (ELISA) methods were used to determine infection prevalence and the presence of anti- Theileria  antibodies in each herd. In addition, pregnant heifers and foetal calves were sampled at abattoir and tested for the presence of  T. orientalis  by qPCR.    Results  The qPCR results indicated that, even though there was a high prevalence of  T. orientalis  infection in cows, the rate of transplacental transmission to their calves was low, with only one newborn calf from one herd and one foetus from the abattoir testing positive for  T. orientalis  DNA. Five aborted foetuses and stillborn calves, 3 of which were derived from a herd experiencing a high number of clinical theileriosis cases at the time of sampling, all tested negative for  T. orientalis  by qPCR. This suggests that  in utero  infection of calves with  T. orientalis  may not be a major driver of abortions during theileriosis outbreaks. Temporal monitoring of 20 calves born to  T. orientalis -positive mothers indicated that  T. orientalis  was detectable in most calves between 10 and 27 days  post-partum , consistent with prior field studies on adult cattle introduced to  Theileria -affected herds. There was a positive correlation between the ELISA ratio of newborn calves and their mothers within 48 h of calving; however, maternal antibodies were only detectable in some calves and only for 4–4.5 weeks  post-partum . All calves displayed high parasite loads peaking at 4–8 weeks  post-partum , with only some calves subsequently mounting a detectable adaptive antibody response.    Conclusions  These findings indicate transplacental transmission of  T. orientalis  appears to play only a minor role in persistence of  T. orientalis  infection in the field; however calves are highly susceptible to developing high level  T. orientalis  infections at 4–8 weeks of age regardless of whether maternal antibodies are present  post-partum .    Electronic supplementary material  The online version of this article (doi:10.1186/s13071-017-2166-9) contains supplementary material, which is available to authorized users.

parasites and vectors

Transplacental transmission of Theileria orientalis occurs at a low rate in field-affected cattle: infection in utero does not appear to be a major cause of abortion