A study of sarcoid tumours in Cape Mountain zebra (Equus zebra zebra)

Abstract

The Cape Mountain Zebra (CMZ) has been described as one of the most vulnerable mammals in the Republic of South Africa with current populations existing in isolated units. In recent years, South African conservation officials have noted the appearance of tumour like growths, similar to equine sarcoids, in some of these populations. The possibility that the existing populations, numbering around 1 500 animals in total, arose from a very small gene pool is very real, considering that in the early 1970’s there were only 35 breeding animals in the Mountain Zebra National Park. A genetic component to the susceptibility to sarcoid tumour development has been identified in horses. Researchers have found an association between susceptibility to sarcoid and certain heritable cell-surface proteins called major histocompatibility antigens. Studies have reported the possibility of high levels of inbreeding in CMZ in the Bontebok National Park (BNP). The non-territorial social organisation of mountain zebras allows the population to increase to a high density within a relatively short period of time. These observations, coupled with the fact that there are no confirmed reports of sarcoids in the relatively outbred Burchell’s zebra population in the Kruger National Park and the Hartmann’s mountain zebra populations in Namibia, strongly support the hypothesis that a correlation exists between inbreeding in zebra populations and expression of sarcoids. The small number of CMZ in BNP with an apparent high prevalence of suspected sarcoid tumours made this an ideal population to capture, identify and to study the skin tumour. The entire population of CMZ in the BNP was observed and sampled. A total of 15 CMZ was present in the Park at the time of the study. The individual animals were darted from a helicopter using M99, Azaperone and Hyalase after which a ground team moved in and covered their eyes and ears. Each animal was thoroughly examined for the presence of any tumours; the distribution was recorded on a body chart and sizes and appearance were recorded. Anatomical location on the body was divided into head and neck, ventral abdomen and limbs. Biopsies were taken from all of the zebras with sarcoid-like growths. The sarcoids were either surgically excised or a representative biopsy was taken by means of 6mm biopsy punches. The samples were collected in 10% buffered formalin for histological examination. The prevalence of sarcoid in CMZ in BNP was 53%. Of the affected animals, 50% had multiple tumours. The main predilection sites were on the trunk, followed by the limbs and then the head and neck. The severity of the lesions in one stallion was so extreme that it warranted euthanasia. The verrucous type, followed by much lower percentages of the fibroblastic and nodular types, dominated the clinical appearance. Lastly, the sarcoids examined showed either all or some of the typical epidermal and dermal histological features of equine sarcoid. A total of 7 of the affected CMZ were treated using four different methods. The sarcoids were surgically excised (n = 2), treated with intra-lesional 5-fluorouracil (n = 2), autogenous vaccine (n = 2) or autogenous vaccine combined with 5-fluorouracil (n =1). The zebras were immobilised for examination 18 months later. One animal that had been treated with intralesional 5-fluorouracil only had large numbers of verrucous and fibroblastic sarcoids. Her condition was so severe that she had to be euthanased. No signs of sarcoids could be found anywhere on the remaining 6 treated CMZ. This study confirmed that the growths in the CMZ in BNP population are indeed sarcoids and that many of them exhibit an aggressive nature. Sarcoid tumours is a disease that is considered multifactorial in aetiology and therefore other parameters such as immune status of tumour-affected populations and associated environmental variables warrant further investigation.