Hypersensitivity to a variety of foods, insects and environmental agents can cause your horse profound distress.
When your horse is fighting allergy symptoms, it can frustrate you just as much as your suffering equine. There could be a simple solution to the problem, but more than likely you’ll use several resources trying to figure out exactly what has created the woe.
Allergies are, essentially, the body taking offense to a normally harmless substance. The immune system goes into overdrive when it meets the cause of the problem, called a “trigger.” That encourages the white blood cells to produce antibodies to prepare the immune system for a confrontation with the offensive allergen.
Allergy-caused irritated skin causes itching and loss of hair.
There are no outward symptoms as the antibodies prepare for battle by attaching to mast cells that are found in the respiratory system, gastrointestinal tract and the skin. Additional exposure lets the battle begin as allergens combine with the antibodies and release chemicals the likes of histamine or leukotrienes, which produce the allergy symptoms.
The resulting allergy symptoms are generally a manifestation of inflammation, according to Karen E. Davison, PhD, Manager of Equine Technical Services for the Horse at Land O’Lakes Purina.
“The resulting allergy symptoms depend on where in the body the chemicals are released,” Davison says. “The most common symptoms associated with allergies in horses are skin irritations, such as hives, welts and itching [urticaria] or respiratory problems such as recurrent airway obstruction [RAO], which is very similar to asthma in people. Weepy eyes, nasal discharge or digestive upsets can also be symptoms of allergic reactions.”
Finger the Trigger
Davison explains there are many, many sources that create or trigger an allergic reaction, complete with symptoms. Two types of environmental triggers are those that come in direct contact with the animal and those that are airborne. One symptom is contact dermatitis, a skin irritation.
Not all shavings are made from the same type of tree, and can, therefore, cause an allergic reaction in horses.
There are almost 900 different bug killers that can be used in the United States. Thirty-seven belong to a class of insecticides known as organophosphate pesticides. Gander at your latest equine supply catalog under “fly repellant” and you might find all 37.
Exposure to heavy metals, dyes, bedding, topical medications, soaps, shampoos, blankets, wool and even a variety of products as seemingly harmless as Neatsfoot Oil may also cause problematic skin irritation. You’re probably ready to burn that mail order catalog by now.
Environmental triggers don’t all come out of your local farm supply store. Much like humans, horses can become hypersensitive to molds, dust, pollens and other airborne allergens. In fact, symptoms such as hives, runny eyes, nasal discharge and coughing may be more indicative of an inhaled allergen, according to Davison.
You might think shavings are just shavings when it comes to bedding a stall, right? But not all shavings are created equal. Bulk shavings can be dustier, like saw dust, with questionable origin as to what types of woods are included.
For example, black walnut trees, according to the Ontario Ministry of Agriculture Food and Rural Affairs, are highly toxic to horses, causing laminitis from the shavings, respiratory distress from pollen and can cause death in equines.
Yellow poplar, oak and red maple can affect liver and kidney function, respiratory and skin problems, while cedar often causes allergic skin reactions or hives. Pine shavings have been touted as one of the safest materials for most horses. So, knowledge about and management of feed and bedding is important for controlling contact and airborne triggers.
“Removing long stemmed hay and using a complete feed that is formulated to replace hay often helps alleviate these problems,” Davison says. “Other management options including immersing hay thoroughly in water before feeding, feeding in a trough at ground level, wetting stall bedding or changing the type of bedding and providing as much pasture time as possible will help minimize exposure to respirable dust and molds.”
Insects can really bug a lot of horses too. “Sweet itch” is a hypersensitivity to insects, such as culicoides.
“The little culicoides are technically a midge, if you want to know insect terminology,” Davison says with a laugh. “Usually in the spring and summer, horses become hypersensitive to the insects, often called no-see-ums. If a horse can get against a post or a tree, they rub and itch until they rub their hair off anywhere they can.”
Medications, vaccinations and dewormers pose only a rare chance of causing allergic reactions in equines. In a few cases, however, drug consequences have been fatal. Symptoms include localized swelling at the injection site and sometimes hives.
Anaphylactic shock may cause a horse to collapse and die immediately after being medicated. Penicillin can also result in immediate death, but is also rare.
While food allergies are blamed for a lot of symptoms, Davison suggests it is very hard to determine the actual numbers of horses affected by what they ingest. Unlike humans, who have more sophisticated testing, “we don’t have the true numbers, percentages, in horses.” But the number of people affected by food allergies are also low—only six to eight percent of children and two percent of adults, according to Davison.
Horses can have food allergies to common equine foods, and the best way to test for them is through an “elimination” diet.
“In dealing with horses and horse owners over the years, we’ve had instances where it sure appeared that there may be a food allergy,” Davison says. “Very rarely does the change in diet make a difference. But because it occasionally does, it is something to look at. In the vast majority, it usually is not the food. It is usually something environmental.”
Davison explains that the most reliable way to diagnose a food allergy is an elimination diet. The main problem with the elimination diet is finding a menu that contains none of the suspect foods, yet still meets the horse’s nutrient demands. If the list of allergens is too long, it may not be possible.
“A horse that has previously been on a good plane of nutrition can be fed a hay-only diet for one to four weeks to see if the symptoms resolve,” Davison says. “If they don’t improve, then the symptoms weren’t caused by a food allergy, and you have to look for other causes. If symptoms do resolve, then very gradually introduce one new food at a time in an effort to build a balanced diet that will not trigger an allergic response.”
Davison says there is “anecdotal evidence” that feeding omega 3 fatty acids from a fat supplement may help resolve symptoms of skin irritation and inflammation.
Misery Loves Company
Davison has seen all kinds of allergic reactions, some right in her own backyard. Her father bought a mare “years and years ago” with the goal of breeding some quality racehorses. She laughs as she recalls getting all the foals that “didn’t make it to the All American Futurity.”
“I got to ride, run barrels and do various other things with those colts,” she says. “Every spring and summer, the mare broke out and just rubbed and itched. We now know it was probably the culicoides.”
What Davison found interesting was that none of the mare’s foals had any allergy problem until they were moved. As long as they were on the ranch where they were born, they never showed any allergic symptoms. However, when one was sold and she took another to college with her, both—although they were in totally different environments—developed hypersensitivity.
“One just totally went haywire and rubbed everywhere,” Davison recalls. “It rubbed all the hair out within a few days of moving. It is an example that you never know what all is going on with allergies and what totally triggers them.”
To make matters more complicated, she noted that it is not always a single allergen causing the trauma. The frustration level often rises when triggers are not only difficult to identify, but there are also multiple triggers involved.
Some time ago, Davison dealt with a breeder who owned about 200 horses. Every spring, the stallion, and only the stallion, would break out with hay fever type symptoms—big, soft welts, weepy eyes and looking miserable without the itching and scratching.
“When they changed the horse’s diet to straight oats, the symptoms were relieved within 24 hours,” Davison says. “He did okay with the allergy issue, but they didn’t like him on oats, and he didn’t like oats. Within a couple of months, they tried him back on his original diet. He would do fine the rest of the year, but when spring came around, they’d have to start the whole process again.”
Another horse owner who contacted Davison had a mare that was allergic to insects. Her story was enough to make an owner pull her hair out. According to Davison, the lady got very diligent about fly spraying everything in the barn. The next thing she knew, her stallion had rubbed all his hair out. It turned out that he was allergic to the fly spray.
Karen and her husband, trainer James Davison, moved from Fredericksburg to Gonzalves, Texas, a few years ago. Soon, one of their own mares broke out and started itching and rubbing.
“You couldn’t even put a saddle on her,” Davison says. “She had taken all the hair off everywhere. I did the blood analysis that everybody does. It came back with certain food [sensitivities] on there, so we made some changes in her diet. There was no improvement.
“A veterinarian that had worked for Purina many years told me I needed to get her scabs cultured. It turned out she did have a bacterial infection that a round of antibiotics cleared up. She went back on her original diet and ate that for the next 10 years with no problems.”
That incident peaked Davison’s curiosity about the results of the food test she’d run on the mare. After all, the horse showed food sensitivities, but no outward symptoms. She pulled blood on four horses in their barn with no sign of allergies. All four blood panels came back with positive foods listed.
“But they had no symptoms,” Davison reiterates. “And, some of the labs that run the blood analysis actually state on the results that they are not particularly accurate for food allergies.”
Because of the volume of calls Purina receives regarding food allergies, one of their veterinarians did some comparison shopping last year. Blood was pulled from a single horse and sent to two different labs. Samples from several different horses that had no symptoms were also sent to several labs. All came back with differing results regarding food allergies.
“That is one of the things I try to tell horse owners,” Davison says. “It is a bit of a hunt when you’re trying to find out what is causing an allergy. You have to start trial and error to figure out what is going on. That simple blood test is not the ‘be all, end all,’ and it sends a lot of horse owners off on a wild goose chase.
“Some of these horses will come back with so many different food allergies as positive or borderline that you can’t come up with anything to feed them that wouldn’t be on the list. It can get to be really difficult. Rarely does it ever end up being the food allergy, although once in awhile it does.”
People have a lot more choices when it comes to diagnosing their allergy troubles. Beyond a simple blood test that tells you only that you have some antigens from exposure, they do the skin grid scratch test among other sophisticated diagnostics.
“The blood test is a tool and it sure is a place to start,” Davison says. “But usually, it isn’t the end of the line. Most the time you make the changes in the diet, and the horse still has the allergies.”
Get the Vet
A mix-match of causes can really complicate the diagnostic and prognostic phases of figuring out why your horse is suffering from hypersensitivities. The horse owner may hear a lot of convoluted explanations. What might have seemed a dietary problem could be an insect reaction or possibly a bacterial infection.
“An allergic reaction to protein normally causes what are historically referred to as ‘protein bumps’ on horses,” Davison says. “Instead of large soft welts, protein bumps are usually hard little bumps like a large BB under the skin. They may be a reaction to a certain protein, not necessarily of dietary origin, but may be from a protein injected in the skin when insects bite.
“Other skin reactions cause scabby eruptions on the skin that usually itch, causing the horse to rub enough to lose hair and even cause sores. These may be from an allergic reaction or a bacterial infection. Scabs can be cultured to determine if there is a bacterial infection and a regimen of antibiotics may resolve the issue.”
When you notice your horse exhibiting any signs of allergies, Davison suggests you involve your veterinarian. Veterinary care can relieve discomfort associated with the symptoms. Your equine practitioner can also help determine what changes in environment or management of the problem will best alleviate the disruption. Of course, Davison says with a smile, seasonal problems will spontaneously disappear only until the next season.
Atophy: a genetic tendency to develop classic allergic diseases
Culicoides: a genus of minute biting gnats or midges
Dermatitis: inflammation of the skin
Histamine: a histidine-derived amine compound that is released mainly by damaged mast cells in allergic reactions
Leukotrienes: a lipid produced by white blood cells in an immune response to antigens that contributes to allergic asthma and inflammatory reactions
Mast cell: a large granular cell, common in connective tissue, that produces heparin, histamine and serotonin
Organophosphates: compounds include some of the most toxic chemicals used in agriculture
Urticaria: hives—raised welts on the surface of the skin—usually an allergic reaction
Karen Davison, PhD
Dr. Karen Davison is the Manager of Equine Technical Services for the Horse Business Group at Land O’Lakes Purina Feed LLC, in St. Louis, Mo. She attended Texas A&M University where she earned her Master of Science degree and PhD in Equine Nutrition.
Davison’s research includes the effects of fat-added diets on reproduction and lactation in broodmares and studying the effects of fat-added diets on growth, bone development and thyroid hormones in weanling horses. She previously spent eight years with the Texas Agricultural Extension Service in the State Horse Specialist Office.
Karen and her husband, trainer James Davison, currently live in Gonzales, Texas.
Annie Lambert is a veteran journalist and horsewoman based in California. E-mail comments on this article to