Studies on the allergic dermatitis (Queensland Itch) of the horse : the aetiology of the disease.

Abstract

Studies are reported on the aetiology of allergic dermatitis of horses in Australia. This disease is considered identical with a dermatitis of horses reported from India, the Philippine Islands, and the United States of America, of which the skin-infesting microfilariae of Onchocerca reticdata are claimed to be the cause. These larval helminths could not, however, be incriminated in allergic dermatitis in Australia, which proved to be due to the development of a hypersensitivity to the bites of a sandfly, Culicoides robertsi. Observations on blood histamine levels revealed that in non-susceptible horses the level was constant throughout the day and also throughout the year, whereas in susceptible animals in the summer the levels fluctuated widely during the day, reaching a maximum value at 6-9 p.m. This period coincides with the period of maximum activity of C. robertsi. Susceptible horses stabled in an insectary from 4 p.m. to 7 a.m. during the summer months did not develop the disease. The geographical and seasonal distributions of the disease are also similar to the known geographical and seasonal distributions of C. robertsi.Intradermal skin tests with antigen prepared from biting insects gave consistently positive results in susceptible animals only with C. robertsi antigen. The pathological and blood histamine pictures following the use of antigen from this insect were identical with those seen in naturally occurring cases of the disease and also following experimental biting and engorgement by this insect. Evidence was obtained of the presence in susceptible animals of a thermolabile, skin-sensitizing antibody which was capable of passive transfer. An immune antibody was demonstrated in susceptible animals, after a course of treatment with "Hapamine", by means of the precipitin test and also by the in-vitro neutralization of histamine liberated in the blood of untreated, susceptible animals. The absence of immune antibodies in non-susceptible horses is regarded as evidence that their freedom from the disease is due to an inherent immunity and not to an acquired immunity or to desensitization.