Studies on allergic dermatitis (Queensland Itch) of the horse: the origin and significance of histamine in the blood and its distribution in the tissues.

Abstract

Studies were made of the distribution of histamine in the blood of susceptible horses and those not susceptible to allergic dermatitis. In the non-susceptible horses there was no difference in total histamine in the circulating blood during summer and winter. In the susceptible horses there was a rise in histamine content of whole blood in the summer. Analysis of the components of the blood showed that increase of histamine in the plasma is considerably greater than its increase in the white cell layer. Histamine, taken up by the eosinophils, is regarded as being bound to the cells and inactive, whereas plasma histamine remains free and is responsible for the cellular reactions in the skin and the accompanying irritation characteristic of the early stages of the disease. Plasma histamine is rapidly detoxified. The probable method of detoxification is discussed. The examination of body tissues obtained during the summer from a limited number of susceptible and non-susceptible horses showed higher values for histamine in the liver and in the skin of susceptible animals, but the differences did not reach statistical significance. It would appear, however, that, a t least with the skin, a more extensive series of observations would probably have demonstrated significance, as an increase in the concentration of histamine occurred in the skin of susceptible animals after intradermal injection of specific antigen.