Members of the genus Theileria are small protozoal parasites that infect erythrocytes and lymphocytes of wild and domestic ruminants. The organism has been most frequently described in Africa and India but is also observed in North American wildlife and cattle. The susceptibility of the host species and pathogenicity of the particular Theileria species are the determining factors of the severity of the infection. Many Theileria species cause only benign infections but severe, fatal infections are well described. The most significant of these is East Coast Fever (Theileria parva and Theileria annulata), a frequently fatal disease of domestic cattle in East Africa.2 A fatal Theileria infection of sable antelope (Hippotragus niger) calves has also been described in South Africa.3 Recent identification of a non-pathogenic Theileria in Mhorr gazelles (Gazella dama mhorr) at the San Diego Zoo has presented interesting questions concerning common concepts of transmission of the organism and concerns about interstate and international transfer of this species.
The Mhorr gazelle is an endangered species, considered to be extinct in its native Morocco and Western Sahara. The North American Mhorr gazelle population descends from animals captured in Spanish Sahara in the late 1970s and transferred to Spain. Most European stock are considered to be from this same collection. This initial population was brought to the San Diego Zoo and its descendants have been shipped to zoos throughout North America.
Historically, a small erythrocyte parasite has been frequently observed in peripheral blood smears obtained as part of a neonatal examination of 1-day-old Mhorr gazelles. No clinical disease has been seen in any Mhorr gazelles at the San Diego Zoo that could be attributed to this organism. Specific identification of the parasite was recently made possible due to advances in diagnostic technology. Due to concerns about this parasite and eventual release of Mhorr gazelles in their native range, blood samples from a 1-day-old and a 1-yr-old Mhorr gazelle were sent to the University of California-Davis for identification and molecular characterization.
Small, rod or ring-shaped piroplasms were identified in Giemsa-stained blood smears. Parasite DNA was extracted from each sample and the 18s ssrRNA gene was sequenced for identification of the organism. A similarity search of the entire GenBank database revealed that the parasite was a member of the genus Theileria. The two isolates were shown to be extremely similar if not identical to each other. The isolate from the older gazelle was most similar (96% similarity) to the Theileria sp.-Thung song isolate, a benign bovine species, and Theileria parva isolate (96% similarity). The isolate was not homologous with other piroplasm species originating in California wildlife. This suggests that this isolate did not originate in North America and may have been naturally occurring at the time of capture.
Theileria infection in all mammals has been traditionally considered to require a tick vector. Identification of this organism considered to require tick transmission in this group of animals was of interest due to the management of this species at the San Diego Zoo and the age at which the Theileria are observed. Observation of this parasite in 1-day-old calves suggests that tick transmission is unlikely due to the short period of time available to become parasitemic. There is also no recent history of tick infestation in any animals in the San Diego Zoo collection. This is attributed to the low-humidity, semi-desert climate and hoofstock exhibits that are typically free of vegetation. The extremely young age of some of the infected animals and lack of a tick vector are very suggestive of transplacental transmission of the organism.
Review of 224 CBCs obtained from 69 Mhorr gazelles over the past 5 yr revealed 27 individuals with the organism observed on at least one peripheral blood smear (37% of sampled population). Of these 27 infected individuals, 14 (51%) did not demonstrate the parasite at the time of the neonatal exam but were observed to be infected at subsequent examinations. No gazelles over 2 yr of age were observed to have the parasite.
Obvious concerns exist about interstate and international transportation of this species due to the presence of this organism. It is presumed that other Mhorr gazelles in North America and Europe may be infected as most descend from a single founder group. There are no known reports of Theileria-like disease in these captive Mhorr gazelles. Cross-species infection, especially to domestic cattle must also be considered as a possibility. The variety of management situations, including mixed-species exhibits, in which this species is kept throughout the world would appear to give the organism the opportunity to have infected other species. The absence of the specific (and as yet unknown) tick vector for this Theileria isolate in North America may prevent transmission of the organism to other species. Most Theileria have a very limited number of species of ticks which can successfully act as a vector.1
The U.S. Department of Agriculture has been informed of the presence of this Theileria isolate and has not expressed concern at this time. Individual state veterinarians may be more interested in this organism and could be more resistant to importation of Mhorr gazelles into their state. As of this time, animals have been shipped to a pre-existing herd in Oregon with the approval of the state veterinarian after extensive discussions of the preliminary findings. Further investigations should be pursued before release of these animals into their native habitat. These investigations will include a survey of other captive Mhorr gazelle populations for the presence of the organism. Ideally, transmission studies in domestic cattle and surveillance of domestic livestock in the country of origin should also be performed.
The authors wish to thank the Veterinary Services and Clinical Pathology Laboratory staff for their assistance in sample collection and diagnostic support.
1. Blood, D.C. and O.M. Radostits. 1989. Veterinary Medicine, 7th ed. Baillière Tindall, Philadelphia, Pennsylvania. 1010–1012.
2. George, L.W.1996. Cerebral Theileriosis. In: Smith, B.P. (ed.). Large Animal Internal Medicine, 2nd ed. Mosby-Year Book, Inc., St. Louis, Missouri. 1052–1053.
3. Stoltsz, W.H. and M.T. Dunsterville. 1992. In vitro establishment and cultivation of a Cytauxzoon sp.(Theileria sp.) from a sable antelope (Hippotragus niger, Harris 1938). Proceedings of the Parasitological Society of South Africa, 1992.