Courtesy of SmartPak
By Dr. Lydia Gray
An allergy is an exaggerated response from the immune system to a substance in the environment, called an allergen. Allergic reactions are extremely common in horses, whose immune systems can overreact or become hypersensitive to things they touch, inhale, eat, or that are injected into them.
Think insects, hay dust and mold, tree and grass pollen, products such as fly spray or shampoo, and drugs such as IM penicillin. The two most common sites for allergies to appear are the skin (hives, itching) and the respiratory system (coughing, nasal discharge).
Allergies in horses are cumulative, meaning a horse can be allergic to several things but it is the combination of separate allergies finally reaching the individual’s limit or threshold that causes outward signs to appear. A simplified way of looking at this is to imagine each allergen being worth one point and a particular horse’s allergic threshold being five points. Once a sixth allergen arrives, signs may appear even though that sixth allergen is not the sole reason for the horse to become symptomatic. By the same token, removing or limiting a horse’s exposure to only one or two of the substances he is allergic to may lead to improvement.
Common Signs & Symptoms
Hives (urticaria) and intense itching (pruritus) of the skin, as well as coughing and nasal discharge, are the most frequent displays of allergies in horses. Hives are fluid-filled, raised swellings or wheals on the skin, which can appear minutes to hours to days after exposure, may or may not be itchy, and can show up nearly anywhere on a horse’s body. When itching, scratching, and rubbing are the main signs of a skin allergy, the pattern of distribution on the body, time of year, and geographic location are helpful clues to the underlying cause.
Respiratory allergies in horses can present in two ways. One is the simple exaggerated response to inhaled tree and grass pollens similar to “hay fever” in people. Horses with allergic rhinitis or rhinoconjunctivitis can have a runny nose as well as red, swollen, and watery eyes. The other is the more complex Recurrent Airway Obstruction (RAO) and summer pasture-associated RAO in which horses cough, wheeze, demonstrate poor performance and labored breathing (even at rest), and have mucus discharge from the nose. Formerly known as “heaves” and COPD, it has recently been classified under the general heading “Equine Asthma Syndrome” for its similarity to asthma in people.
Because not every case of hives and itching – or coughing and nasal discharge – is an allergy, and because a case of skin allergy can actually be caused by something the horse inhaled, not touched, it can be quite challenging to identify the trigger factors or offending agents. What veterinarians do know is that insect hypersensitivity is the most common cause of allergies in horses as well as the most common reason for horses to itch. While horses can be allergic to many different insects, such as black flies, horn flies, and stable flies, it is the biting midge (Culicoides) or “no-see-um” that leads to the seasonal skin allergy called “sweet itch.” Also known as Seasonal Recurrent Dermatitis, “Summer Itch,” and “Queensland Itch,” certain breeds appear predisposed to sweet itch, developing the classic “rat tail” and “buzzed mane” appearance due to constant scratching and rubbing.
Environmental allergies are the second most common allergy seen in horses, and while they can also manifest as hivesand itching, constant exposure to barn dust and airborne allergens can lead to Recurrent Airway Obstruction in susceptible horses. Breathing in the small particles that make up agricultural dust means the horse’s tissues and immune system are exposed to bacteria and bacterial endotoxins, insect and mite debris, fungi, molds, vegetative material such as pollens, and noxious gases like ammonia. Dry hay and dusty bedding are responsible for the vast majority of particulate matter getting into the horse’s airway, as both are directly in the horse’s “breathing zone” and both contain high levels of reactive dust and allergens.
Like most conditions, allergies are diagnosed by starting with the owner’s description of the problem, including when it first started and what changes may have occurred recently in the horse’s diet and environment. Then, the veterinarian performs a physical exam on the horse, noting any abnormal findings and observing any clinical signs. Based on the these results, specific tests can then be performed such as skin scrapings, biopsies, and cultures for skin conditions — to rule out parasites, bacteria, or a fungus – or use of a rebreathing bag, imaging, or fluid samples to assess airway health and function.
Once the evidence points to allergies as the culprit, specialized allergy testing can be performed to confirm the diagnosis and assist with a treatment plan. At this point, some veterinarians may partner with a board-certified dermatologist or internal medicine specialist. Most experts agree that intradermal (skin) allergy testing is better than serologic (blood) allergy testing at identifying allergens for an individual horse. However, when food allergies are suspected, there is no reliable test other than a food trial or elimination diet where the horse is put on the simplest diet possible until all signs improve, then “challenged” with one new food item each week to try and identify the offending feedstuff.
Prescription Medications Available
The first line of defense against allergies in horses is usually steroids such as dexamethasone, prednisolone, and others. They can be administered directly on the skin (topically), by mouth (orally), by injection, or intranasally, which may be the preferred route when dealing with a respiratory allergy such as RAO. By administering a corticosteroid (and bronchodilator) directly to affected airway tissues, more medication is deposited right where it is needed and less travels throughout the body, potentially causing less unwanted side effects like laminitis. Antihistamines are also commonly given to help relieve the clinical signs of allergies, and even antibiotics are sometimes used when secondary bacterial infections are present.
Like people and other animals, allergic horses can be desensitized with a series of “allergy shots” to triggering substances in the environment identified by proper allergy testing. This allergen specific immunotherapy or ASIT is fairly effective, not labor-intensive, has minimal side effects, is cost-effective, and best of all, may result in a permanent cure, not just temporary symptomatic relief. The newer sublingual immunotherapy (ie under the tongue) has been successful in humans and dogs and shows promise in horses.
Supplements that May Lend Support
A number of studies have confirmed the value of omega 3 fatty acids in horses with seasonal allergies. According to the NRC Nutrient Requirements of Horses:
Supplementation of horses with recurrent seasonal pruritus (“sweet itch”) with large amounts of flax seed (1 lb/1000 lb horse) was associated with a significant decrease in the allergic skin response to Culicoides extract, suggesting a possible benefit of flax seed in the management of horse with this condition.
Omega 3 fatty acids are also recommended by the American College of Veterinary Internal Medicine who recently published a revised consensus statement about equine inflammatory airway disease:
Another way to modulate the inflammatory response is by supplementing the diet with polyunsaturated omega-3 fatty acids. . . .supplementing the diet with omega-3 fatty acids, in particular DHA or docosahexaenoic acid (1.5 g/day for 2 months), in addition to switching horses to a low-dust diet, was shown to provide more rapid improvement (within 1–2 weeks) in clinical signs of IAD and RAO when compared to only low-dust diet (at least 4–5 weeks).
Because MSM has a long track record of safe use in the horse and has been shown to be a potent antioxidant in horses, some veterinarians routinely recommend its use in seasonal allergic conditions of the skin and respiratory tract. Other ingredients reported to be of benefit include adaptogens such as Ashwagandha, American Ginseng, Astragalus, Siberian Ginseng, and others; spirulina; and turmeric (curcumin).
The main way to avoid allergic reactions in horses is to avoid exposing them to their known allergens. Because horses can be allergic to more than one thing, it may not be necessary to remove every single substance that causes a reaction, just enough of them so the horse doesn’t have uncomfortable or unsightly clinical signs.
- Protect the horse from insectsby using fly spray, a fly-deterrent supplement, fly masks, fly sheets, and fly boots; putting fans in stalls; using mesh curtains on barn openings; and keeping the horse inside during the times of day when bugs are worst.
- Reduce insect numbers by using feed-through insect control supplements, Fly Predators,and good management practices such as removing manure and eliminating standing water.
- Note items that contact skin such as bedding, horse care products (e.g. shampoos), and tack and tack care products (e.g. leather cleaner)
- Minimize dust in the barn by changing the type of bedding used, improving ventilation, wetting aisles before sweeping, and other practices.
- Provide a dust-free diet by wetting or steaming hay; swapping long-stem hay for chopped, cubed, or pelleted hay; or switching to a complete feed.
In the worst-case scenario, a horse must be moved to another barn or even another state to avoid allergens specific to that geographic location. Hopefully an accurate diagnosis, correct identification of allergens, and the combination of prescription medication, immunotherapy, supplements, and management practices can keep both owner and horse comfortable.