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Research Project: Development of Detection and Control Strategies for Bovine Babesiosis and Equine Piroplasmosis

Location: Animal Disease Research

Title: Equid infective Theileria cluster in distinct 18S rRNA gene clades comprising multiple taxa with unusually broad mammalian host ranges

item BISHOP, RICHARD - Washington State University
item Kappmeyer, Lowell
item ONZERE, CYNTHIA - Washington State University
item ODONGO, DAVID - International Livestock Research Institute (ILRI) - Kenya
item GITHAKA, NAFTALY - International Livestock Research Institute (ILRI) - Kenya
item SEARS, KELLY - Washington State University
item KNOWLES, DONALD - Washington State University
item Fry, Lindsay

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2020
Publication Date: 5/19/2020
Citation: Bishop, R.P., Kappmeyer, L.S., Onzere, C.K., Odongo, D.O., Githaka, N., Sears, K.P., Knowles, D.P., Fry, L.M. 2020. Equid infective Theileria cluster in distinct 18S rRNA gene clades comprising multiple taxa with unusually broad mammalian host ranges. Parasites & Vectors. 13. Article 261.

Interpretive Summary: Complex single-celled parasites of the genus Theileria infect blood cells of horses (equines) and other mammals. These parasites, which are transmitted by ticks to the mammalian host, are natural (endemic) to tropical and semi-tropical parts of the world. The impact to horses through infection with these parasites results in significant loss of health and even death, leading to economic loss. Countries for which the parasites are not endemic, including the United States, screen animals for infection prior to permitting import. There is no vaccine available for equine Theileria infections. This article reviews recent work that substantiates wide genetic diversity between types or isolates of equine Theileria. The primary method of measuring the diversity is a DNA gene called the 18S ribosomal DNA gene. The variation in parasite types can be sorted in to 5 groups or clades. Recent work has shown the genetic basis for one clade of parasites being undetectable by diagnostic tools developed against a second clade. Further study is needed to determine if parasites of other clades are likewise undetectable, allowing inadvertent import of infected horses in to non-endemic countries. A further observation is that there is also variation in the species of mammals that the equine Theileria parasites are able to infect, beyond horses. Animals related to horses such as various zebra species, other African wildlife species, camels, and even occasionally dogs may be infected.

Technical Abstract: Equine theileriosis, a tick-borne disease caused by the hemoprotozoan parasites Theileria equi and Theileria haneyi, affects equids throughout tropical and subtropical regions of the world. It is a significant regulatory concern in non-endemic countries, where testing for equine theileriosis is required prior to horse import to prevent parasite entry. Within endemic areas, infection causes significant morbidity and mortality, leading to economic losses. No vaccine for equine theileriosis is available, and current drug treatment protocols are inconsistent and associated with significant side effects. Recent work has revealed substantial genetic variability among equine theileriosis organisms, and analysis of ribosomal DNA from affected animals around the world indicates that the organisms can be grouped into five distinct clades. As these diverse parasites are capable of infecting a wide range of both tick and mammalian hosts, movement of different equine Theileria sp. parasites between endemic countries, and eventually into non-endemic countries, is a significant concern. Furthermore, the substantial genetic variability of these organisms will likely render currently utilized importation diagnostic tests unable to detect all equine Theileria sp. To this end, more complete characterization of these diverse parasites is critical to the continued global control of equine theileriosis. This review discusses current knowledge of equine Theileria sp. in this context, and highlights new opportunities and challenges for workers in this field.