An outbreak of acute seasonal allergic dermatitis is described in sheep from the municipalities of Sao Vicente do Sul and Jaguari, Rio Grande do Sul, Brazil. Affected sheep were crossbreeds of different categories. Skin lesions were observed mainly in the head (ears and around the eyes, muzzle, and lips), mammary gland, distal portions of the limbs and ventral abdomen. Two lesion patterns were found; all acute pattern showed marked erythema associated with numerous papules, pustules, epidermal colarettes, and crust; in a chronic pattern the same anatomical sites had lichenified, ulcerated, exudative, crusted, and alopecic areas, which were occasionally hemorrhagic. Microscopically, sheep that were recently affected had perivascular eosinophilic dermatitis; in the skin of more chronically affected sheep there was superficial lymphocytic and histioplasmocytic perivascular dermatitis.
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