Effects of experimentally induced fleece-rot on the structure of the skin of Merino sheep.

Four medium- and six strong-wool Merino sheep were exposed to continuous simulated rain (4 x 5 mm/h) for periods ranging from 3 to 9 days. In response, the sheep developed characteristics typical of fleece-rot lesions such as exudative encrustation and bacterial discoloration of the fleece. Skin samples were taken periodically for histological examination from the trunk region of the sheep prior to, during, and after wetting. An inflammatory response to wetting occurred in the upper layers of the dermis and this caused the release of cells which began invading the epidermis after only 6 h of wetting. An associated increased in vascular permeability was observed macroscopically following intravenous injection of Pontamine Blue dye. The incidence and extent of the inflammation increased during subsequent wetting while epidermal acanthosis and hyperkeratosis became increasingly evident. Epidermal thickness returned to pretreatment levels by 13 days after cessation of wetting when inflammatory regions were absent from most of the sheep. Cellular lysis occurred in the suprabulbar and keratogenous regions of a small proportion of follicles (1-13%) during wetting, resulting in the interruption of fibre growth and keratinization in these follicles and the occlusion of follicle hair canals with inner root sheath material ('plugs') which persisted after wetting ceased. These morphological changes in the epidermis, follicles and fleece are considered to occur as a consequence of the initial rapid inflammatory response to wetting during the developing of fleece-rot lesions.