The epidemiology of squamous cell carcinoma of the perineal region of sheep: Abattoir and flock studies

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Abstract
The roles of age, tail length and the Mules operation in the epidemiology of squamous cell carcinoma in ewes were studied. The prevalence of the disease in adult sheep slaughtered at 2 abattoirs was 0.08%. No cases occurred in lambs. Of all adult sheep condemnations before slaughter, 38.5% were due to squamous cell carcinoma. Of these sheep, 73.3% had a single lesion of the vulva (62.9%), tail (6.6%), or anus (3.8%), while 26.7% had lesions at more than one site. The number of lesions was significantly greater (p < 0.01) in sheep with a radical Mules operation than in those with a modified Mules operation or not mulesed. Tails were significantly shorter (p < 0.01) in affected than unaffected sheep. Small keratinised outgrowths on the skin of the tail and perineum were considered on histological grounds to be precursors of squamous cell carcinomas. On one affected farm 4% of ewes were culled in one year for gross lesions of squamous cell carcinomas. A further 3.5% of sheep with gross lesions and 25.3% with precursor lesions remained in the flock, undetected by the farmer. Gross lesions were not observed in ewes under 3 years of age, whereas precursors occurred in all age groups, including one-year-old ewes. The prevalence of lesions increased with age, from 0.43% in 5-year-old ewes to 5.14% in 6-year-old and 41% in 7-year-old ewes. Discriminant analysis indicated that age of ewe, tail length and the width of the healed Mules operation were important determinants of squamous cell carcinoma. In all the sheep studied the style of mulesing was consistently radical, with a mean healed width of 12.3 ± 2.6 cm. Tails were amputated much shorter than the traditionally recommended length of level with the tip of the vulva. The presence of gross and precursor lesions was associated with shorter tails and a radical Mules operation. It is suggested that the prevalence of the disease may be reduced by the adoption of a less severe mulesing technique and leaving tails longer.