Clinicopathological study of an outbreak of squamous cell carcinoma in sheep.

This paper describes the clinical and histopathological features of an outbreak of cutaneous squamous cell carcinomas in a flock of Berrichon du Cher sheep in south-western Spain. The outbreak was unusual because of the high prevalence recorded (15 to 18 per cent over the last two years) and because the majority of the lesions involved the eyelids. Adult and old sheep were the most commonly affected, and the largest numbers of cases were recorded in summer and autumn. The initial lesions were non-specific and consisted of erythema, hyperkeratosis, actinic keratosis, periocular dermatitis and conjunctivitis. The tumours grew slowly but progressively over a period of one to two years, with frequent complications by bacterial infections or secondary myiasis. Histopathological examination of the tumours revealed well-differentiated squamous cell carcinomas, surrounded by a moderate to abundant infiltrate of lymphocytes, plasma cells and macrophages. The overlying epidermis was frequently ulcerated and inflamed. The possible relationships between the high prevalence of the condition and the poorly pigmented skin of this breed, the extensive farming system employed and the exposure of the animals to ultraviolet radiation were analysed.