An outbreak of equine sarcoid in a population of Cape mountain zebra (Equus zebra zebra) - a retrospective study

Abstract

Equine sarcoid was diagnosed in the Cape mountain zebra (CMZ) population at the Gariep Nature Reserve (GNR) located in the southern Free State Province of South Africa in 1996. The course of the disease outbreak over the period from 1996 to 2003 is retrospectively described from data gathered during that time. In total, data from 39 affected animals was gathered during the study period. The average population size during the outbreak was 69 individuals. The initial prevalence was 9.4% in 1996. When ompared to the neighbouring domestic horse population, where no cases of equine sarcoid had been noted, the CMZ population showed a high prevalence of sarcoid for reasons unknown at the time. To mimic dynamics in a natural ecosystem with predators, it was decided to remove sarcoid-affected zebra from the population during 1996 and 1997. No sarcoid cases were seen in 1998 and 1999. After thoroughly examining the population in 2000, seven new sarcoid cases were found. Given the endangered status of the CMZ, no further affected animals were culled and a decision was made to study the disease more intensively, with emphasis on epidemiology, aetiology, clinical appearance and pathology, and treatment options as well as to investigate the genetic status of the population and the possibility of a genetic predisposition to the development of equine sarcoid. Prevalence of sarcoid cases in the Gariep CMZ population increased to 24.7% in 2002. Incidence varied between 4.65-17.6% during the study period with higher incidence rates recorded towards the second half of the study period. No sexual predilection was established. Sarcoids were not seen in animals younger than three years of age. Of the affected individuals, 64.1% had a single lesion and no animal had more than four lesions. Sarcoids were mostly of the fibroblastic (57%), verrucose (16%), and nodular (10%) types or a combination of these. The majority of lesions in males occurred in the inguinal area (55.17%), whereas the majority of lesions in females occurred on the head and neck (41.38%). Because treatment trials were conducted in a number of affected individuals, there were not many untreated control animals in which to study the rate of growth of the tumours, but the average annual increase in lesion size in untreated animals was found to be as much as 260%, becoming so large as to mechanically impede movement. During the study period, known sarcoid-related mortalities numbered four, while nine animals were euthanased for humane reasons, and ten other animals having been identified once with sarcoid were not seen again and presumed dead.