Hives is a symptom rather than a specific disease. Of all the domestic species horses are the most likely to develop hives. Although there is no sex predilection the average age of onset is 1-10 years and the Arabian and Thoroughbred breeds are predisposed.
Hives is a consequence to mast cell, and less commonly, basophil degranulation. When mast cells degranulate they release a range of chemicals including histamine, heparin, and cytokines. These cause local inflammation and increased permeability of small blood cells. The primary skin lesion is an oedematous wheal. Some cases are pruritic.
There are many underlying triggers or causes of hives. The most common category are immune based hypersensitivity reactions, classically type I hypersensitivities although type III reactions may contribute. Drug eruptions are very common with numerous antibiotics (penicillin, tetracyclines, sulphonamides), anti-inflammatories (phenylbutazone, flunixin), tranquilisers (acepromazine), and de-wormers (ivermectin, moxidectin) all implicated. Insect bites can also trigger urticaria, particularly biting midges (Culicoides species), stable flies and horse flies. Other immune based wheal triggers are food allergy, contact urticaria, and atopic dermatitis.
Other non-immunologic causes include exercise induced urticaria, cold urticaria, and pressure urticaria (dermatographism).
Treatment is largely symptomatic. These include oral prednisolone, injectable dexamethasone, topical corticosteroids, anti-histamines, and pentoxifylline.