Sweet itch is a condition that recurs annually during certain periods and is accompanied by dermatitis (an inflammation of the skin). This inflammatory reaction is caused by an allergy to the saliva of Culicoides, a type of mosquito which is active mainly at sunrise and sunset during warmer months (March to October).

This reaction causes a constant itch at the dock of the tail and around the mane, head, and ears, which the horse then tries to relieve by rubbing. This can lead to skin injuries and in turn to secondary bacterial infections.

Not all horses are allergic to this mosquito, so reactions will vary from horse to horse. Breeds like Icelandics, Shires, Welsh Ponies and Friesians seem to be more prone while some blood lines within a breed may be more prone than others. This means that the degree of allergic reaction is a genetic factor.