Herd-level occurrence and spread of Theileria equi and its associated ticks on an Austrian horse farm

Background: Equine piroplasmosis (EP) is a tick-borne disease caused by the blood parasites Theileria equi, Theileria haneyi, and Babesia caballi transmitted primarily by hard ticks (Ixodidae). The clinical courses of EP range from asymptomatic to severe, and chronic infections for several years support the transmission among hosts. Methods: Over a period of two years, blood and fecal samples were taken from horses (n = 18) on a single farm in eastern Austria, starting with the first confirmed case of T. equi infection. Hard ticks (n = 1525 of Ixodes ricinus, Dermacentor reticulatus and Haemaphysalis concinna) were collected by flagging on the horses' pastures. For detection of infections with piroplasms, PCR on the 18S rRNA gene with subsequent sequence analysis and antibody detection with a commercially available cELISA were performed. Results: Initially, two horses showed clinical signs and tested positive for T. equi. Of the remaining 16 horses, 12 were positive for T. equi by PCR and serology at the first sampling timepoint without clinical symptoms. In the second year, 12/14 positive horses remained positive, two were unavailable. Additionally, two horses negative in the first year were then positive for T. equi by PCR and serology, leaving only one horse negative which had had no contact to the others and was kept on a separate pasture. None of the fecal samples was positive for piroplasms. Six out of 363 ticks flagged on three different pastures in the first year (four H. concinna, one I. ricinus, one D. reticulatus) and eight out of 196 D. reticulatus adults removed from an infected positive horse in the first year were T. equi-positive by PCR. The resulting sequences all belonged to T. equi genotype E. Conclusions: Here we report the occurrence and spread of T. equi on a horse farm in Austria, a country not considered endemic for EP but with sporadic cases documented in recent years.Due to the occurrence of T. equi on the farm and likelihood of lifelong infections, this pathogen will presumably remain in this herd and in Austria and might spread further due to the presence of putative vector ticks. Owners and veterinarians should be made aware of the risk of infection and the occurrence of clinical symptoms. Horses should be tested prior to import from endemic areas. Further studies are needed to identify risk areas, clarify the vector competence of the found tick species and develop strategies to mitigate the risk of endemisation.