A report on clinical aspects and histopathology of sweet itch.

Sweet itch is an intensely pruritic dermatitis of horses recurring annually in Ireland from April to November. The tissue changes of sweet itch have similarities to immediate-type hypersensitivity reactions which occur in skin sensitised to the saliva of bloodsucking insects. There was subepidermal oedema, and marked eosinophilia; the blood vessels were tortuous and enlarged. Microfilaria were not found in serial sections of lesions of 5 affected horses. The histopathology of the immediate dermal remal reaction to the intradermal injection of Culicoides extract shows dermal vasodilation and eosinophil diapedesis. Seven horses challenged intradermally with whole extract of Culicoides, Stomoxys calcitrans, Tabanidae and saline, gave immediate reactions to Culicoides, 3 gave immediate reactions to Stomoxys calcitrans. Immediate and delayed reactions only occurred at the sites of challenge with Culicoides extract. Three normal ponies when challenged intradermally with extracts of biting flies, showed minimal dermal reactions.