Theileria cervi-associated morbidity and mortality in a cohort of captive mule deer following relocation.

Theileria spp. are tick-transmitted apicomplexan parasites that infect a variety of mammalian species. Although several species of Theileria infect ungulates of North America, from a historical perspective, reports of clinical disease are relatively sporadic. Herein we report an outbreak of theileriosis in captive deer following transport from Illinois to Iowa, USA. Although the facility housed both whitetail and mule deer, only mule deer were affected clinically by lethargy, anorexia, and anemia. Post-mortem examination revealed pleural effusion, mucous membrane pallor, and splenomegaly. Microscopically, there was evidence of erythrocyte degradation in multiple organs. Evaluation of blood smears revealed intraerythrocytic piroplasm-like organisms. Collection of ticks from the affected premises yielded a mixed population of Dermacentor variabilis and Amblyomma americanum. Polymerase chain reaction and sequencing of DNA from ticks and affected deer was consistent with Theileria cervi. Although T. cervi appears to be an infrequent cause of disease in North American cervids it is possible that animals develop immunity to local variants which is ineffective upon transport to an area with other isolates. Our findings, in combination with previous reports, indicate that mule deer may be more susceptible to isolates of T. cervi in areas outside of mule deer's native range. T. cervi should be considered as a differential diagnosis for anemia in cervids, particularly those transported and introduced to new habitats or populations of ticks in North America.