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.
Contact allergens can include almost anything that comes in contact with your horse's skin, including bedding, pasture plants, shampoo, coat conditioners, fly sprays, lotion or similar products. These products don't cause problems for most horses, but an occasional horse will react to one ingredient or another.
Food allergies are extremely rare in horses, says Dr. White. Your horse is more apt to be allergic to dust in his alfalfa hay than to the hay itself.
Drug reactions can produce symptoms ranging from hives to anaphylaxis. You may have heard of reactions to such drugs as acepromazine, penicillin and phenylbutazone, but it's not clear that these drugs are especially dangerous--they're just widely used. Reactions to vaccines are fairly common, too.
These aren't the only possibilities. For instance, Dr. White says, hives sometimes herald the onset of a case of ringworm, a common skin disease caused by a fungus. The horse first has an allergic reaction to chemical substances that the ringworm fungi produce. The classic signs of this disease--hair loss in circular patches, with raised, scaly skin--soon follow.
Heredity may be a factor in some cases. There have been reports, for example, of a family of Arabians that all had hives due to atopic dermatitis, and Icelandic horses are unusually sensitive to Culicoides (especially those horses imported from Iceland, where there are no Culicoides). Breeders sometimes report cases where a horse shows the same allergies as its sire or dam. But in most cases it's not clear what role, if any, heredity plays.
Follow the Clues
It may take some detective work to determine which of many possible factors causes your horse's symptoms. For a true allergy, suspicion falls first and heaviest on anything new in your horse's life--a new medication or a new grooming product. If you try a different shampoo on your horse and he breaks out in hives, obviously you'd suspect a contact allergy.
A combination of clues may point to insect allergies. If your horse erupts in crusty pustules and rubs his mane out in the spring, Culicoides should top the list of suspects. If the problem clears up when you keep the midges off your horse (more on this in a minute), they were probably the cause. But you wouldn't suspect insect allergy in the dead of winter in most parts of the United States because insects aren't active then.
Environmental allergens can be especially hard to pin down. Often the source isn't obvious--pollen can be carried nine or 10 miles on the wind. Keeping records of when and where your horse's reaction appears can help narrow down the range of causes. For example, if he's sprinkled with hives after a day in the pasture in September (ragweed season), then that plant's pollen is a strong suspect.
You can get a clearer picture of what's causing an allergy with intradermal (skin) tests. These tests are usually done by veterinary dermatologists at large clinics. The dermatology service at the UC Davis veterinary clinic keeps a bank of about 70 allergens for testing.
If you opt for testing, here's what to expect: Your horse is sedated, and an area of skin (usually on the neck) is shaved. Then small amounts of different allergens are injected into the skin, just far enough apart so that any reactions won't overlap. The injection sites are evaluated at intervals--typically 15 to 30 minutes, four to six hours and 24 hours--and the reactions are scored. The cost varies with the clinic and the number of allergens tested, but it generally runs to several hundred dollars.
Even a "normal" horse may show some reactions, so test results have to be put in context. The answer has to fit the history (that is, the substance was something that your horse was or could have been exposed to) and the physical signs. The final check is to see whether eliminating it (if that's possible) solves the problem.
There are tests for Culicoides allergies, but the results aren't always clear. Tests are useful mostly for atopic dermatitis (when a horse develops hives because of an environmental allergen such as pollen, mold and dust), Dr. White says.
There are also blood tests for atopic allergies. They're less accurate than skin tests, but they're more practical in the field. They're also useful if a horse can't be sedated or clipped, or if he's on medications that could obscure the results of skin tests.
Blood tests available for food allergies are extraordinarily inaccurate, Dr. White says. The best way to track down a food allergy is through a process of elimination. Change your horse's hay and withdraw all grain, supplements and treats; if the problem doesn't go away, food is not a likely cause. If it clears up, then add back the extras one at a time to see if any cause the problem to recur.
Protection and Prevention
Knowing what triggered your horse's allergic reaction will help you protect him from repeat episodes. The most obvious way to do this is to banish the cause. Don't use medications or grooming products that trigger the response. If insects are the problem, apply a fly repellent containing the highly effective chemical permethrin during fly season. Keep horses that are sensitive to Culicoides in during early morning and evening hours when these insects are most active. Fine mesh screening can help keep the midges out of stalls, and fans can help by keeping air moving in the barn. (Culicoides are not strong fliers.)
Inhaled allergens are harder to eliminate. You can't filter pollen and mold spores out of the air. And try as you may to limit dust in your barn, you're not likely to make it dust-free. For skin reactions caused by pollen, mold spores or dust, hyposensitization--allergy injections--may be an option. In fact, the main purpose of doing skin tests is to develop shots custom-tailored to your horse's allergies. With repeated injections, he becomes less sensitive to the trigger.
Like skin tests, allergy shots are most helpful for atopic allergies. At UC Davis the success rate is about 70 percent with this technique. But the injections aren't right for every situation. They're expensive, and the program is lifelong.
Although protocols vary, a typical program might involve shots every other day for a month, then weekly. If your horse is doing well after six months of weekly treatments, you can then give shots every other week. This protocol can cost $1,500 to $2,000 a year. These shots are especially useful for show horses because the main medications used to control reactions are barred or restricted by the U.S. Equestrian Federation and other show organizations.
Cough, Wheeze . Could That Be Heaves?
A chronic cough, labored breathing, nasal discharge, exercise intolerance--those are classic signs of heaves, also known as broken wind, recurrent airway obstruction and chronic obstructive pulmonary disease (COPD). Allergies play a major role in heaves, which is a lot like human asthma, and affects about 12 percent of horses, says Sarah Gardner, DVM, PhD.
Allergies are also suspected in a related syndrome, inflammatory airway disease. IAD produces milder signs than heaves, but it tends to affect younger horses. Heaves typically doesn't show up until a horse is older than 6, but in some studies, Dr. Gardner says, 22 to 50 percent of young Thoroughbred and Standardbred racehorses showed signs of IAD.
Triggers: Barn dust is the most common culprit in allergic respiratory reactions. This is organic dust, full of mold spores, pollen and endotoxins produced by certain bacteria. In horses predisposed to heaves, the dust irritates the airways and triggers an allergic bronchitis. Some unfortunate horses (mostly in the Southeast) have "grass heaves" and develop seasonal signs while on pasture. Airborne pollen and mold are most likely responsible in these cases.
Find the cause by noting when and where your horse's symptoms appear. Generally, skin tests aren't very helpful in isolating more specific causes for these reactions, Dr. Gardner says. Similarly, it's not clear that allergy shots can help. More research is needed.
Management: Providing good air circulation in your barn and cutting down on dust--for example, by storing hay in a separate building and using low-dust bedding such as shredded paper--may reduce the symptoms. But those steps often aren't enough, Dr. Gardner says.
- If your horse reacts to dust, he'll do best living outside, with a run-in shed, full time.
- Replace his hay with a forage substitute. Give him a pelleted complete feed or a fermented hay product such as Horsehae (Tri-Forage, www.triforage.com).
- If your horse develops signs on pasture, he may do best in a clean, low-dust barn with turnout on a dry lot.
- If he reacts to both dust and pasture, the dry lot (with a run-in shed) should be his home during the season when his pasture allergies flare.
Medications: Drugs can help your horse breathe more easily while his lifestyle is adjusted. Dr. Gardner prescribes corticosteroids and, if necessary, a bronchodilator. She generally recommends inhaled drugs, which act locally rather than systemically. Her goal is always to get the horse off medication as soon as possible. But sometimes management alone can't control the symptoms. A horse with a pasture reaction may need corticosteroids through the growing season, for example.
Stephen White, DVM, is a board-certified veterinary dermatologist and a professor in the Department of Medicine and Epidemiology at the University of California Davis School of Veterinary Medicine. Sarah Gardner, DVM, PhD, is an associate professor at the College of Veterinary Medicine of North Carolina State University Raleigh, where she specializes in respiratory conditions.
This article is reprinted from the April 2007 issue of Practical Horseman.