INHIBITION OF ANTIGEN-INDUCED CUTANEOUS RESPONSES OF PONIES WITH INSECT HYPERSENSITIVITY BY THE HISTAMINE-1 RECEPTOR ANTAGONIST CHLORPHENIRAMINE

A whole-body extract of Culicoides impunctatus induced a biphasic incr ease in oedema formation in ponies with insect hypersensitivity, with maxima after one and eight hours. The Culicoides antigen did not induc e similar responses in ponies with no previous history of the disease. In insect-hypersensitive ponies the local administration of chlorphen iramine (12 mu g) completely inhibited oedema formation in response to histamine (0.04 mu g) and to Culicoides antigen (0.5 mu g) at one hou r, and the response to Culicoides antigen at eight hours was inhibited by 63 per cent. Chlorpheniramine also partially inhibited the accumul ation of eosinophils and neutrophils induced by Culicoides antigen aft er two hours.