Q.When I purchased my Thoroughbred gelding, his owner supplied some photos from earlier this year. In one, it looked like he had welts on his skin. A friend suggested he might have hives, which I’m not familiar with. Could you tell me about hives in horses? Are certain breeds more likely to get hives? What kind of treatment should he have?
—Nicole, via e-mail
A.Hives (also called urticaria) are roughly circular, steep-walled, plaquelike wheals that are relatively common in the horse. The lesions usually occur on the head and neck but may occur on the entire body and legs. Hives is not a disease but a cutaneous reaction pattern that is caused by fluid leaking from local dermal vessels in response to inflammatory mediators released by mast cells.
Most urticarial lesions are cool to the touch and pit or “dimple” (due to the edema) when finger pressure is applied. Occasionally the lesions are pruritic (itchy) and may leak tissue fluid on the surface of the lesions. Self trauma from scratching may contribute to the severity of skin lesions and secondary infections may occur. Rarely urticaria may appear as large coalescing lesions, circular “targetlike” lesions in gyrate patterns, or as parallel linear lesions. When lesions of the head and neck coalesce the entire head may appear swollen and breathing may be compromised.
Urticaria occurs in all breeds of horses although some researchers believe Thoroughbreds and Arabians are more commonly affected.
What causes hives?
Allergic reactions are the most common cause of urticaria. In the summer months urticaria often appears as a response to insect bites, particularly in horses with insect bite hypersensitivity (or IBH). Because of the plethora of substances that can serve as triggers for urticaria the cause is often not determined.
Substances that induce urticaria may be ingested (feed, supplements, any oral medication, etc.), may be medications given by injection (such as vaccines, phenylbutazone, or penicillin), or may be due to contact with the skin (bedding, tack or blankets, or topically applied substances such as fly sprays or certain shampoos). Contaminants in hay or feed such as mold or other antigens that are inhaled may also cause urticaria.
More rarely physical conditions such as cold, heat, stress, and sunlight, with or without added exercise, may induce urticaria.
How are hives treated?
Many cases of urticaria resolve in 24-72 hours. Other cases may resolve only to reoccur, while others will persist. For acute cases (seen by your veterinarian in the first 24 hours) a single administration of a short-acting corticosteroid will usually resolve the lesions.
However, the ideal therapy is avoidance or elimination of the triggering factors. Thus, a review of all aspects of management is warranted. Simple steps include changing paddocks and bedding, eliminate supplements (at least for a trial period), and reviewing any recently added medications with your veterinarian. This type of investigation can be prolonged and often still does not result in identification of the cause.
If a topical substance is suspected and or the horse exhibits pruritus and skin abrasions from self trauma, bathing with a medicated shampoo to reduce pruritus and control secondary infections may be helpful.
Recurrent or persistent urticaria may be treated with a more prolonged administration of low doses of steroids, however if the horse is prone to laminitis this may not be advisable.
How can I prevent urticaria?
Atopic horses and some horses with IBH may benefit from allergen-specific immune therapy (ASIT, or allergy “shots”) after an intradermal skin test has been performed to identify suspect antigens. Usually the ASIT must be combined with management changes and sometimes low dose steroids to completely control the occurrence of lesions.
Antihistamines can be helpful in prevention of lesions but are not helpful in the treatment of existing lesions. Hydroxyzine has been the most effective for me, however horses react differently to antihistamines and more than one kind may need to be tried before an effective therapy is found. If your horse is used in competition, medications for controlling urticaria may not be permitted. Check with your veterinarian.
Should urticaria reoccur or become persistent, working with your veterinarian is very important to determine the cause and design a treatment and prevention strategy for your horse.