Sweet itch

Sweet itch is a medical condition in equines caused by an allergic response to the bites of Culicoides midges.

[2] A hypersensitivity reaction to specific allergens (protein molecules causing an extreme immune response in sensitised individuals) in the saliva of Culicoides midges.

There are multiple allergens involved,[3] although some works claim that the larger proteins (of molecular weight 65kDa) are the most important.

[5] The hypersensitivity response is mediated by IgE, an antibody produced by the horse's immune system which binds the allergens, causing a cascade production of histamine and cytokines which make the horse's skin inflamed and itchy.

Of these, histamine appears the most important in the initial phase of reaction.