The list of substances that can trigger horse allergies is almost endless. It includes pollens, fungal spores, molds, insect saliva, the droppings of insects, foods, chemicals, fabrics, etc. Obviously every horse is bombarded with these substances on a daily basis, and most handle it just fine. Then there are the others, the ones who get horse allergies.
Genetics plays a role in determining how susceptible the individual horse is to developing allergies. A large study performed at two German warmblood/Lipizzaner breeding farms found that if both the sire and dam battled allergies, the foal had about a 50% chance of developing them. If neither are allergic, the foal’s risk was under 20%. Their results were similar to data for humans, showing that a genetic tendency may increase the chances of — but doesn’t guarantee — allergies will develop. Exactly why isn’t clear, although management may come into play. Note, though, that while the predisposition to allergies may be genetic, the exact trigger/allergen is not hereditary.
The Immune System
The basic genetic link to a tendency to develop allergies likely lies in the immune system itself. For example, people with immune-system deficiencies that make them unable to produce other types of antibodies typically compensate with an overproduction of the antibody IgE (immunoglobulin E). It doesn’t have to be extreme, either. All it would take is a decreased ability to produce one or more of these other antibodies for the immune system to start producing more IgE than it should. Once that happens, allergies are more likely to develop.
Infections may make a horse more susceptible to developing allergies even without a genetic risk factor. Remember that an invading infection calls the body’s T-cells into action. T-cells are immune-response fighters in the white blood cells. These T-cells have ”helper” cells, commonly known as the Th1 (responsible for infection-fighting immune responses to attacks within a cell) and Th2 (responsible for immune responses to attacks outside a cell). Many invading organisms interfere with the host’s sophisticated first-level immune response from the Th1 cells. When this occurs, the body calls the Th2 cells into action, which drastically raises the body’s inflammatory response, putting it in a favorable position for allergies to develop.
Equine respiratory infections also predispose the body to allergies simply by damaging the natural protective lining of the respiratory tract so that inhaled allergens get direct access to the tissue. Inhaled irritants set the stage for allergic reactions in the respiratory tract. Ammonia fumes don’t reach the lower lung, but they do cause a lot of inflammation in the nasal passages. Aerosolized dusts and mold spores that reach the lower airway do a lot of damage in their own right and might also predispose to true allergy (by now, you’re probably thinking ”heaves” or COPD, and you’re correct).
Corticosteroids are a well-known treatment for allergy, but recent research is finding that the treatment causes an elevation of cortisol to levels found with stress that can favor allergic responses. Basically, it’s appearing that steroids don’t suppress the immune system across the board like we used to think they did. They suppress the Th1, infection-fighting response, but not Th2. This means that horses under stress, such as long-distance shipping or heavy exercise, could be at a higher risk of developing allergies. It also explains why horses with Cushing’s disease and increased cortisol production have a reduced ability to fight infections and also often develop allergies for the first time in their lives.
Types Of Allergies
Before we can discuss allergy tests and treatments, we need to understand the varying reactions that occur in the horse’s body to different allergies. For some of us, all the IgG / IgE talk coming up can make your head spin, but hang in there. These reactions are important to understand because they explain what allergy tests can, and cannot, tell you. Note: ”Ig” stands for immunoglobulin, which is an antibody. An antibody is a protein used by the body to fight antigens. Antigens are invading foreign objects, like bacteria or viruses. The letter after the ”Ig” is a specific type of immunoglobulin.
Eye Allergies. The conjunctiva is the inner, pink lining of the eyelids. It’s a mucus membrane, similar to the linings of the mouth, respiratory tract and gut. Like all mucus membranes, it contains immune-system cells to defend it against infections and foreign materials. These defense mechanisms may take the form of an allergic reaction. Symptoms include swelling, itching, redness and tearing. These are usually IgE-related allergies.
Upper Respiratory Allergies: Just like people, horses can develop allergies involving the nose, sinuses and throat. Symptoms include increased clear or and frothy nasal discharge, sneezing, snorting, head-shaking, swollen nasal membranes and coughing. These are usually immediate-type, IgE-related allergies.
Skin Allergies: Hives are the classic manifestation of a skin allergy. They are related to IgE and histamine release. Hives may be the result of something that has come in contact with the skin but may also be triggered by something that the horse inhaled or ate.
For example, many horses with respiratory allergies develop hives at the same time as their breathing difficulties. Insect-bite hypersensitivity/allergy is a bit more complicated. Several studies looking at Culicoides hypersensitivity (AKA ”Sweet Itch”) have found that IgG antibodies can also be involved.
Food Allergies: The horse’s intestinal tract has a greater concentration of immune-system tissue than anywhere else in the body, and for good reason. The gut is constantly exposed to a wide range of both the infectious and beneficial organisms that inhabit it, parasites, a host of proteins, complex carbohydrates and a wide variety of toxins. Both IgA and small amounts of IgE are produced in response.
When everything works smoothly, the intestinal tract develops a ”tolerance” for commonly encountered things, but allergies can develop. Immediate-hypersensitivity food allergies have not been reported for horses. An example of an immediate, IgE controlled food allergy would be peanut or strawberry allergy in people, where a life-threatening reaction occurs sometimes before they even swallow.
If the reaction is to a partially digested portion of the food, it may be delayed for an hour or more, until the food breaks down in the stomach and moves into the small intestine. Symptoms of this are nausea and vomiting, but since the horse can’t talk and can’t vomit, it would be difficult for us to recognize that type of problem. However, the horse may well come to associate discomfort after eating certain things and refuse to eat them. Food allergy in other species may involve predominantly IgG reactions. Wheat sensitivity is a common example in people, and inflammatory bowel diseases (which horses also can get) may involve allergy.
Lower Respiratory/Lung Allergies. The classic example of this is ”Heaves,” also known as chronic obstructive pulmonary disease (COPD) an d, the newer term recurrent airway obstruction (RAO). Some of these horses have obviously increased numbers of eosinophils or mast cells (anti-inflammatory cells). These cells are identified through a test called a tracheal wash, which obtains a sample of mucus/fluid from the lung. However, in other cases, including in young horses with similar symptoms, the type of cells seen is predominantly neutrophils, which indicate inflammation but do not confirm allergy. IgE measured in the fluid may also be low.
The bronchi react to irritation, whether it’s allergy-mediated or not, by constricting and causing the typical symptoms of wheezing, labored/heavy breathing and nostril flaring as they work harder to try to move enough air through the constricted pathways in their lung. We connect this automatically with allergy, but it isn’t necessarily the case.
An Australian study presented at the Third World Equine Airways Symposium found that endotoxins (potentially toxic natural substances) from bacterial contamination of bedding could induce airway inflammation in almost half of the young Thoroughbreds recently put into training and exposed to this bedding. Sawdust and shavings had considerably lower levels of contaminating endotoxins than straw or other organic beddings. The same researchers also tested blood from 12 horses with RAO and confirmed circulating IgE to common allergens in the environment was present in four of them, demonstrating that true allergy can be involved as well, but it wasn’t always the case.
To further complicate the picture of lung allergies, there’s the potential for IgG to be involved. In humans there is a condition called ”Farmer’s Lung,” which is similar to heaves in horses and triggered by many of the same molds and inhaled fine dust particles that cause heaves. In this condition, the hypersensitivity is predominantly associated with IgG antibodies. A high number of people with this condition showed delayed and late reactions on intradermal skin tests.
The most important, but most frequently overlooked, part of reaching a diagnosis of an allergy is the horse’s history. Things like a family history of allergy, a seasonal pattern, age, appearance of symptoms after a change in the environment/management or after multiple exposures to a substance, and recurrent problems with symptoms can all make allergy a more likely diagnosis.
Blood Tests. Pulling a blood sample to test for allergies is quick and simple. Blood tests used to be performed by a technique called RAST (radioimmunoassay), but today’s tests use ELISA technology that links the test material to a dye that is released when IgE antibody from the horse’s blood reacts with the test substance. Sensitivity has improved over the initial tests.
Blood testing for allergy in dogs with skin problems has been documented to correlate well with intradermal skin testing. The response to desensitizing immonotherapy injections based on blood testing results also has a high success rate in small animals. However, the same documentation is lacking for equine allergies. A recent study from Switzerland looked at the cellular lung response and serum IgE levels of horses with RAO/heaves compared to normal horses and found too much overlap between the serum IgE levels to make the test useful. In humans, IgG testing for specific allergens known to potentially cause chronic lung disease is available but hasn’t made its way into equine medicine yet.
Blood testing isn’t considered reliable for detecting food allergies. High circulating antibody levels can be found in a high percentage of people with no food intolerances at all. Another drawback is that blood tests only detect IgE antibodies, and some types of both skin and lung reactions may involve IgG. Much more work needs to be done to determine the types of equine allergies that can be accurately diagnosed by blood testing
Despite being easy to do, blood tests for allergies are useful only in some circumstances. They’re also expensive to do and require that results are interpreted with caution.
Intradermal Skin Testing. This test involves the injection of small amounts of diluted antigens into the upper layers of the skin. Plain saline is also used as a negative control, and histamine as a positive control injection. The reaction around each material injected is measured and compared to the positive and negative controls. Classic allergy reactions involving IgE will appear within 15 minutes or so. Delayed reactions are also sometimes detected as swelling that show up hours to days after the test material is placed. Most veterinary dermatologists and allergists still consider this test method the most reliable in the horse.
Challenge and Elimination Tests. A challenge test would be when you deliberately expose a horse to a suspected source of allergic reactions and see how he responds. An elimination test is the opposite: You protect the horse from the suspected allergen and see if the symptoms disappear. This is the only reliable way of testing for suspected food allergies. It’s also the most reliable way to determine lung allergies, but it’s rarely done because of the risk of triggering severe reactions. Instead, keeping a detailed diary of when symptoms are worse — such as at a particular time of year or time of day, when the horse is inside vs. outside, or a type of diet — will help narrow down the list of possible triggers.
Other Tests. If a horse has skin lesions and the diagnosis isn’t clear, a biopsy may be warranted. The type of cellular infiltrate seen with allergy can actually vary quite a bit, but biopsy and culture is still a useful way to rule out other problems.
Horses with respiratory symptoms suspected to be allergic can often be confirmed by a careful, detailed history alone. Depending on symptoms, the veterinarian may also want to rule out chronic infection by cultures and/or skull X-ray and endoscopic exam (”scoping”). With lung symptoms, it’s always advisable to have a BAL (bronchoalveolar lavage) done for cultures and to determine the types of cells present in the airways. Horses with many eosinophils or mast cells may respond well to antihistamines, but those with predominantly neutrophils (a type of blood cell that forms an early response to infection) probably won’t. It’s important to rule out infection as either the real cause or a complicating problem that needs to be addressed as well.
Over-the-counter nutritional supplements, like the ones we’re recommending in this section, are simple to get and use. However, when you’re dealing with allergies you should always involve your vet in the process and be sure he or she knows what you’re using.
Antioxidants. Allergic/hypersensitivity reactions are inflammatory. Controlling their severity falls on the shoulders of antioxidant defense mechanisms. Several studies have a documented decreased antioxidant status in the lungs of horses with allergic lung disease, and at least one (Equine Veterinary Journal, November 2002) found improved exercise tolerance and lower inflammatory scores after supplementation with vitamin E, vitamin C and selenium. Additional antioxidant support is advisable (our last article on this subject is in our August 2004 issue, but plans are underway to update that).
MSM. Many owners of heaves-affected horses are convince d that MSM helps. The one formal study that looked at this found no benefit, but the disease in those horses was so advanced that even clenbuterol (a drug that is both a decongestant and a bronchodilator) didn’t help them breathe. The jury is still out, but it may be worth a try.
Spirulina. Field reports continue to mount that Spirulina platensis helps most horses with pulmonary allergies and some with skin allergies as well. This blue-green algae has known anti-histamine effects and has been shown experimentally to redirect the immune system away from allergic responses. The dose for an average size horse is 20 grams twice a day. That’s 1 ounce by volume measure of the loose powder (measure two tablespoons). This can be purchased by the pound from www.herbalcom.com, 888-649-3931. Many horses have been able to greatly reduce their reliance on drugs when being supplemented with Spirulina.
The anti-inflammatory omega-3 fatty acids that are high in ground stabilized flaxseed meal help control reactions to Culicoides bites and reduced inflammatory cell counts in horses with RAO, although they don’t change pulmonary function. High-quality ground stabilized flax products are available from Omega Fields (www.omegafields.com, 877-663-4203) or Horse Tech (www.horsetech.com, 800-831-3309).
Details regarding drug types, indications and dosages for your horse need to be determined by you and your treating veterinarian. However, the general categories of drugs commonly used for allergies by veterinarians are:
Antihistamines. These are just what their name implies — drugs that block the release of histamine. Histamines are released by mast cells during an allergic reaction/immune response and cause many of the allergic symptoms you see. They’re available in oral forms or may be given intravenously by your vet. Antihistamines are only of use in classic allergic reactions involving IgE. Skin, lung, drug or food allergies with an IgG component will not be responsive to antihistamines.
Bronchodilators. These drugs are often used to help ease breathing in horses with lung allergies, but they may not be effective as a sole therapy. They are available for both oral administration and via specially adaptations for inhaler medications such as the AeroMask, or via a nebulizer.
Corticosteroids. Whether inhaled, given orally or by injection, corticosteroids are a cornerstone medication for allergic reactions. With acute/severe symptoms there’s simply no substitute. However, because of the potential for both acute and chronic side effects their use should be reserved for situations that truly require them and the lowest dose possible used.
Epinephrine. This hormone, also known as adrenalin, is a life-saving drug used in treating acute severe allergic reactions involving severe bronchospasm or swelling of the head and throat to the point that breathing is compromised. Fortunately, reactions this severe are rare, but if your horse has ever had an allergy reaction severe enough to cause obvious breathing difficulties you should discuss possibly keeping epinephrine on hand with your vet and be sure you understand when, and how, to use it.
Immunotherapy. Immunotherapy, sometimes called ”desensitization” or ”allergy shots,” is the injection of gradually increasing doses of the offending allergens. Its popularity has waxed and waned in horses, and it’s still being actively studied. Immunotherapy directs the immune system response away from generating IgE. Allergens to be used are chosen on the basis of strong immediate reactions on skin testing or high levels of IgE detected by blood tests.
Dealing with an allergic horse is not easy. It takes a committment to find the cause, avoid it and know how to manage it. But, it can be done.
Enlist the help of your veterinarian and consider keeping an allergy diary, if necessary. It’s important to note the triggers and do your best to minimize exposure to the allergens. You’re going to have combine a good dose of common sense with a detailed history of the horse and weigh in the results from traditional allergy testing.