Bolo disease: a specific, localised skin disease of woolled sheep

Bolo disease is limited to Merino and Döhne merino sheep in the Stutterheim and Cathcart districts of the eastern Cape Province. It occurs under natural grazing conditions regardless of the season of the year and the condition of the natural grazing. Ewes and wethers are most frequently affected. Skin lesions are well-defined, and the corresponding fleece has dark-grey to almost black spots, patches or bands varying in number, size and distribution between individual sheep. The wool in the affected areas is visibly shorter, less dense and tender, and the tips of the staples are spiky. In freshly-shorn sheep, the affected areas appear chalky white. Chronic and superimposed acute lesions are present in the same specimen histologically. Skin lesions include superficial and follicular hyperkeratosis, acanthosis, and sebaceous gland hyperplasia and hypertrophy. These changes are accompanied by dilatation of some of the follicles in the midshaft area, and collapse of the subepidermal tissue with only a few remaining collagen fibres separating the follicles and the sebaceous glands from the thickened epidermis. Corynebacterium spp. is the most consistent bacterium isolated from the lesions. Lesions produced by suspensions of this organism simulated both clinical signs and histopathological findings of the natural condition.