One morning you notice a few small lumps on your horse's neck. By afternoon, they've grown and multiplied, spreading across his shoulders and along his sides. He has a blooming case of hives--but what brought it on?
Hives are a common allergic response in horses, just as they are in people. While a mild case of hives or other allergic response, such as itchy or irritated skin, may appear mysteriously and disappear just as quietly on its own, it's worth your while to puzzle out what caused it. Allergic reactions can make a horse miserable, and they often become more serious with repeated exposure to the substance, or allergen, that triggers them.
In this article, two experts--veterinary dermatologist Stephen White, DVM, of the University of California Davis, and Sarah Gardner, DVM, an associate professor in equine medicine at North Carolina State University, discuss common allergic reactions.
Inside and Out
In an allergic reaction, your horse's immune system perceives a threat from something harmless and goes to Level Red, mounting a response that's entirely out of line. Instead of wiping out microorganisms, the reaction damages his own body tissues. Moreover, he becomes hypersensitive to the specific allergen and beefs up defenses against it--so the next time he's exposed to it, the reaction is likely to be quicker and stronger.
Many allergic reactions produce a cascade of chemical events that triggers the release of the chemical histamine, which produces swelling and other signs of inflammation. The skin is a prime site for this, but swelling also can concentrate around the eyes and muzzle, or it can be internal.
Signs can appear anywhere from a few minutes to hours after exposure to the allergen. You may see:
- Hives. These soft swellings, which indent under finger pressure, are probably the most common reaction. They usually start small, perhaps half an inch across, but can grow larger. Sometimes they run together, forming big raised welts. The neck, chest, shoulders and sides are common locations. Oddly, hives are not as itchy for horses as they are for humans.
- Pruritis. This is a general term for the sensation of "itch" in the skin. Sweet itch, also known as summer eczema, is the best-known example. It's an allergic reaction to the bites of Culicoides, the tiny midges many people call "no-see-ums". Depending on the species, these insects typically bite your horse along the crest of his neck, around his dock or on his belly. The area becomes covered with an oozing, crusty rash that itches intensely, causing him to rub and scratch. Continued scratching can allow a secondary infection to take hold.
- Respiratory signs. See "Cough, Wheeze . Could That Be Heaves" below for more about respiratory reactions, which erupt when internal swelling narrows breathing passages. Allergies also have been suspected in some cases of headshaking, a condition in which your horse constantly tosses or shakes his head during exercise. However, current thinking holds that other factors probably account for most of these cases.
- Anaphylaxis. This extreme, systemic allergic reaction is most likely to occur after repeated exposure to something to which your horse has become hypersensitive, and it usually appears quickly. His blood pressure drops suddenly, and he struggles to breathe. He may go into shock and die if the situation isn't immediately reversed. That's done with quick administration of epinephrine (Adrenalin) to stimulate the horse's body and raise his blood pressure, and a corticosteroid such as dexamethasone to reduce internal swelling and open his airways.
Mild allergic reactions usually clear up on their own without treatment--sometimes within hours. Call your veterinarian at once if your horse seems distressed, has difficulty breathing or has severe swelling around his muzzle, which could limit his breathing. Even if it doesn't seem like an emergency, call if you don't see improvement in 24 hours or if a reaction recurs.
Your veterinarian can prescribe several medications to calm an allergic response. Dexamethasone or other corticosteroids are effective for treating severe reactions. If your horse is only moderately itchy or has hives, antihistamines can be useful.
Insect bites are probably the most common cause of skin symptoms such as hives and pruritis, says Dr. White. Certain proteins in the saliva of Culicoides, found worldwide, are known to trigger an allergic reaction in horses. But many other biting or stinging insects can trigger such reactions. However, in these cases, it's not always clear whether a limited reaction (such as local swelling) results from a true allergic response or simple irritation from the bites.
Environmental allergens include pollen, mold and dust that your horse inhales. When a horse develops hives or other skin symptoms because of one of these substances, he's said to have an atopic dermatitis. These allergens vary from location to location depending on what's growing, climate and other factors. Some common allergens include molds and pollen from cocklebur and ragweed. Surprisingly, horses can be allergic to pollen from Bermuda grass, which grows widely in the southern half of the United States and is sometimes used for pasture.