Haemaphysalis longicornis ticks are unable to transstadially transmit Theileria haneyi to horses

Reginaldo G Bastos ^1,2* Karen C. Poh ^1,2 Kennan Oyen ^1,2 Cynthia Onzere ² Lowell Kappmeyer ¹

• ¹ Animal Disease Research Unit, Agricultural Research Service (USDA), Pullman, United States
• ² Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, Washington, United States

The recent discovery of Theileria haneyi, a tick-borne hemoparasite that causes mild clinical signs of equine piroplasmosis, has added complexity to this reportable disease that affects equids globally. Knowledge gaps regarding competent tick vectors for transmitting T. haneyi and the recent emergence of Haemophysalis longicornis in the US prompted us to conduct this experiment to investigate the ability of H. longicornis to transstadially transmit T. haneyi to horses.Material and Methods H. longicornis larvae (0.5g) and nymphs (n=500) were applied on a splenectomized T. haneyi-infected horse for parasite acquisition. During tick feeding, parasitemia was monitored by nested PCR (nPCR) and blood smear. Acquisition ticks fed to repletion and were transferred to an incubator for molting. Concomitantly, red blood cells (RBC) were collected from the acquisition horse use for infection. Freshly molted nymphs (n=282) and adults (n=212), offsprings of the acquisition larvae and nymphs, respectively, were placed on two individual naïve spleen-intact horses for transstadial parasite transmission. One additional naïve horse was inoculated with 1 ml of RBC from the acquisition horse. After tick infestation or RBC inoculation, the transmission horses were monitored for 38 days for the presence of T. haneyi DNA in peripheral blood by nPCR and clinical signs of infection.The splenectomized acquisition horse developed canonical signs of acute T. haneyi infection during parasite tick acquisition. The percentage of parasitized RBC in the acquisition horse varied between 2.2%-8.1% during tick feeding. Out of a subset of 10 engorged larvae that fed on the acquisition horse, all ticks were nPCR positive for T. haneyi. However, only 4 out of 10 engorged nymphs that fed on the acquisition horse were T. haneyi PCR positive. We found no evidence for the presence of parasite DNA in the transmission ticks and horse blood or for T. haneyi clinical signs of infection in the transmission horses. In contrast, the horse inoculated with RBC from the acquisition horse tested nPCR positive for T. haneyi 15 days after inoculation, showed parasites in blood smears, and developed canonical clinical signs of acute infection.We demonstrate that H. longicornis ticks are unable to transstadially transmit T. haneyi to horses.