Insect bites can cause problems. Hypersensitivity associated with midges or gnat bites is a common problem, although allergic reactions to mosquito bites have also been reported. The most commonly affected areas include the back, ears, mane, and tail. Itching is a characteristic feature of these types of allergies, and horses will frequently rub their manes and tails until the hair is sparse in these areas. Initially, isolated bumps may appear, with larger hives developing, too.
Most horses affected with hives usually recover uneventfully.
If hives are a recurrent problem, identification of the source can probe frustrating. Your veterinarian can perform an intradermal skin test that may be helpful identifying the problem. If you suspect hives to be a result of a food allergy, change the grain and hay ration for at least two weeks. Then slowly reintroduce the original feed. If this stimulates the appearance of hives, you can assume that the feed is the cause of the allergy.