Transmission of Theileria orientalis in cattle

Summary

​Theileria are blood-borne intracellular protozoal parasites belonging to the phylum Apicomplexa. Previously considered a benign parasite in Australia, outbreaks of clinical disease resulting from Theileria orientalis genotypes have been reported in Australia since 2006. Since this time, outbreaks have become widespread in south-eastern Australia, resulting in significant adverse impacts on local dairy and beef industries.

This project was the first investigation into the possible biological and mechanical vectors involved in the rapid spread of the parasite. Findings implicate the bush tick, Haemaphysalis longicornis, as the most likely biological vector, and identifies by PCR, Theilerial DNA in pools of mosquitoes from endemic areas and lice from infected calves.

Further work on active transfer from ticks was confounded by lack of suitable vectors and histopathology for sporozoites in the salivary glands from engorged, PCR-positive ticks was equivocal. Biting arthropods could be involved in mechanical (horizontal) transmission as inoculation of just 100µl of infected blood resulted in successful infection of recipient calves. Although mechanical transfer of infected blood did not result in clinical Theileriosis, calves remained positive for 15 months, with obvious implications for husbandry procedures and disease epidemiology. Transplacental and colostral transfer were also investigated, but were not demonstrated in a limited initial study.​ ​