STUDIES ON ALLERGIC DERMATITIS (“QUEENSLAND ITCH”) OF THE HORSE

Abstract

Summary.Studies have been made on a skin disease of horses in Australia, commonly known as “Queensland Itch” and referred to by the author as allergic dermatitis. It appears to be similar to conditions described by other workers in horses and mules in France, India, Philippine Islands and the United States of America.The disease is prevalent during the summer and recedes with the advent of cold weather to recur the following summer. Only certain horses are susceptible but animals of all ages, breeds, colour and sex may be affected. Ani‐ mals show great irritation and itchiness and will bite affected areas or rub against fences and trees.Lesions are confined almost exclusively to the dorsal surfaces and appear initially as discrete papules which later exhibit matting of the hair. These crusts eventually fall off and leave hairless areas which, in severe cases, become confluent.Microscopic examination of early lesions shows oedema, engorgement of capillaries and eosinophilic infiltration of the dermis. In older lesions the oedema is absent and lymphoid cells are present with the eosinophils. Hyperkeratosis is seen in the more advanced cases and in chronic cases fibrosis and hypertrophy of epidermal tissue are well in evidence. In many animals trauma masks the earlier tissue changes.Haematological examinations of susceptible animals revealed very little departure from normal. The white blood cells, eosinophils and platelets were, however, significantly higher in susceptible animals during the summer months.