By Amanda Hedges, DVM, cVA, CVSMT, published in the San Mateo County Horsemen’s Association (SMCHA) Newsletter, Q1 2021
An Immune System Gone Awry
Molecules that irritate our horse’s bodies are all around. One of the jobs of the immune system is to protect us from the harmful effects of these irritants, maintaining the balance between stimulus and an appropriate response that keeps a horse healthy. An allergic reaction can occur when an irritant or combination of irritants disrupts or overwhelms the immune system’s balance. This reaction may be severe and life-threatening (anaphylaxis), sudden (acute), or more slow/insidious in onset (chronic).
Signs of an Allergic Reaction
Signs of allergies often appear as an immune system overreaction, resulting in local or systemic inflammation. We see hives, itching, oozing, scabs, hair loss, tearing, coughing, breathing changes, nasal discharge, hair loss, poor performance, and even gastrointestinal upset. Anaphylaxis is a severe acute allergic reaction characterized by increased respiratory effort, rate, or noise, recumbency, and/or shock. Anaphylaxis is a medical emergency necessitating immediate veterinary intervention. Sometimes an allergic episode (e.g., hives) is a one-off and other times a horse has developed allergies to something in his/her environment.
To restore balance to the immune system in non-life threatening cases, we need a two-fold approach: 1) decrease the irritants 2) calm down the immune system. Let’s look at some common equine allergens and what we can do about them.
Changes in the environment and the horse’s immune system can make it difficult to identify the cause of a horse’s allergic response. Some common equine allergens include:
Insects: The most common culprit of chronic skin allergies is the saliva of the Culicoides fly (a.k.a. gnats, no-see-’ums). Other fly species, other insects, and even arachnids (spiders) can cause an allergic response.
Environmental: Components of dust, different molds and mildews, different plants and pollens, and even some topical products can all cause allergies.
Note that food allergies are not common or well-understood; it is more common for a horse to be allergic to the components of dust on hay than the hay itself.
Treatment strategies focus on minimizing the presence of the allergen and influencing the immune system to restore balance. Common treatments include:
Environmental management: if you know what your horse is allergic to from an allergy testing profile (see “Desensitization Injections” below), then you can minimize his/her exposure to specific irritants. General recommendations to decrease irritants revolve around fly control and dust control, both of which will add fuel to the fire of an overactive immune system.
Fly control: Remove manure from the living area at least once daily. Make sure that your horse is stabled far from the manure collection area. Consider feed-through fly products, fly traps, automatic fly spray systems that use permethrin, overhead fans, fly sheets including belly coverage, fly masks including ears, fly boots, natural predators (bats, birds, wasps), and other fly control strategies.
Dust: Consider wetting down your horse’s hay at each feeding. Stable your horse away from dusty arenas, and avoid riding during times of peak arena use. Good ventilation is also good.
Diet change: While food allergies are not common, it is even more challenging to diagnose allergies due to dusts, molds, and contaminants in hay. A trial diet can help assess the contribution of diet to an allergy response. A novel food source, for example timothy pellets, is fed for 3 months, and the horse’s allergy signs are monitored. If allergy signs improve, then a food allergy is suspected. Other feeds can then be added to further identify the allergen. Often wetting down the feed is helpful to minimize the amount of inhaled aerosolized allergens.
Steroids: Short- or long-acting steroids may be used to help suppress the immune system. Steroids can have some unwanted side effects, so use the lowest dose needed for the shortest amount of time. Combine with environmental management, antihistamine, and omega3 fatty acid supplementation for better effects.
Antihistamines: A key molecule in the allergic response is histamine. Oral antihistamines, such as hydroxyzine, cetirizine, or diphenhydramine, can suppress the histamine response. Unlike steroids, these drugs are safer for long-term use though they can make some horses a bit sleepy. Your vet can advise on which drug may be the best for your horse, and the ideal duration of treatment. In horses these medications are better at preventing an allergic reaction than at treating a current one.
Supplements: Some supplement ingredients can help support the body’s immune system. In addition to good quality hay or pellets and a vitamin/mineral supplement, horses with allergies may benefit from:
Omega3 fatty acids: Research supports that horses fed a high dose of omega-3 fatty acids may have a decrease in allergy signs, possibly by decreasing the inflammatory response. This should be used long term as effects are not immediate.
Equine-specific research is poor or lacking for other compounds reported to help with allergic reactions such as ashwagandha (found in some plants in the nightshade family), American ginseng (plant in the ivy family), astragalas (herb in the legume family), MSM (an organosulfur compound), quercetin (a plant flavanol), spirulina (a biomass of cyanobacteria), and turmeric (in the ginger family).
Combinations of these ingredients can be found in brand-name supplements and in traditional Chinese herbal medicine formulations.
Immunotherapy (aka allergy shots): For a personalized treatment, consider requesting an allergy test. Two test protocols are available to identify the specific allergens to which your horse is reacting. The most precise test for skin allergies is called intra-dermal skin testing. To perform this test, a veterinary dermatologist injects a small amount of different environmental irritants under the skin and then monitors the strength of the horse’s immune response at 30 min, 4 hours, and 24 hours. The second option is blood sample, while this is a more convenient way to test for allergens, it is thought to be less specific than intra-dermal testing. Following testing, a personalized allergy shot protocol is developed for your horse, with a dosing regimen to slowly introduce the allergens to your horse’s immune system without overwhelming it, resulting in a more appropriate response. Environmental management is again key to maximizing the success of this treatment plan. While the frequency of injection decreases over time, most horses require life-long treatment to keep allergy signs at bay. Allergy desensitization is a great way to manage skin allergies long-term; it greatly reduces the allergic response in most horses (though give it up to a year to work fully). There’s not much proof that allergy desensitization shots work very well for respiratory allergies.
Allergies and Aging
With time, both the environment and your horse’s immune system will change. New irritants can come in the form of new landscaping, new products, changes in air quality, etc. As horses age, there is some evidence that they can experience immunosenescence, or the gradual weakening of the immune system over time. Both of these factors may mean that your horse’s allergy status and immune system needs may change with time. If you notice any allergy signs, contact your veterinarian to discuss further options!
It may take weeks, months, or even years to find the perfect combination of immune-support and environmental control to rebalance your horse’s body. This plan may need to be regularly adjusted depending on the season. After establishing a treatment plan with your veterinarian, it may take weeks to see results. Complete resolution of all clinical signs may not be possible. With patience and persistence, almost every horse can find some degree of relief from allergies.