allergy-testingReprinted from our July/August 2014 issue
Written by Dr Emily McNally, VDM
Rappahannock Equine Clinic, Locust Grove, VA

It’s that time of year when you may find yourself reaching for the tissues to help combat your runny nose and watery eyes. Allergies. Not all of us have them, but we all know how irritating they can be. Our horses are no different. Skin allergies make our horses miserable, and by mid-summer, some of them have rubbed themselves raw. In a healthy horse, the body produces proteins called antibodies to be used as weapons against foreign substances. An allergic reaction, also known as hypersensitivity, is an exaggerated immune response by the body to a substance from the environment called an allergen. When present, allergies in horses are seen in either the respiratory system or, as this article discusses, the skin.

Allergens encompass a wide range of culprits including insect bites, feed, airborne particles, and objects a horse touches. No matter what exactly the horse is allergic to, skin allergies often manifest in the same way. Horses that react to skin allergens often develop one or more of the following: hives, itching, hair loss, scaling and crusting, whitening of hairs and thickening of skin in the affected areas, and a loss of hair at the tail base from rubbing. An allergic reaction can be a simple one-time occurrence, but in some horses, it can be a chronic and frustrating aspect of life.

Allergies are often diagnosed from clinical signs and an owner’s descriptions of the problem. Your vet will consider where the horse lives, its age, seasonality of the clinical signs, and the location of the lesions on the horse’s body. Skin scrapings, cultures and biopsies can be performed to rule out other possible causes such as bacterial, fungal, and parasitic infections and tumors. Once allergies are suspected as the cause of the horse’s skin problems, treatment is often geared towards improving the symptoms, not necessarily treating the cause.

Medications that are used in treatment of skin allergies include corticosteroids and -antihistamines. Steroids work by broad suppression of the immune system and the products involved in allergic reactions. Side effects may be a consequence of chronic or aggressive use of steroids and include behavioral changes, increased susceptibility to infection, poor wound healing, and laminitis. Prednisolone and dexamethasone are the steroids of choice in most horses. Antihistamines are safer alternatives to steroids and can be used for longer periods of time. Individual horses have different reactions to antihistamines and they do not work in all cases. The only side effects reported include light sedation and personality changes. Your veterinarian can discuss with you what the best option would be for your horse.

To get to the root of the issue, further testing is required to discover which specific allergens the horse is allergic to. There are two allergy tests available: intradermal (skin) testing and serologic (blood) testing.  The current gold standard in allergy testing is intradermal testing, which is similar to the test performed in humans. The horse is sedated, and a large area of skin, often one side of the neck, is clipped and prepped. Multiple allergens are placed into the horse’s skin and response to those allergens is recorded and interpreted. If the patient is allergic to the allergen, a local reaction will occur at the injection site, causing redness and swelling. Steroid and antihistamines must be withdrawn for 2–6 weeks before testing. Despite these downsides, this test remains the most specific and reliable allergy test.

Serum testing requires submission of a blood sample to a veterinary laboratory, and the procedure is no more complicated than a single blood draw. Advantages to this method when compared to intradermal skin testing include the following: medications do not have to be withdrawn for a certain period of time before testing, no specialized equipment or personnel is required, no sedation is needed, hair does not have to be clipped, and there is a lower cost with more widespread availability. Despite being more convenient, serum testing is controversial due to its decreased reliability and specificity. However, it still may be a good place to start before jumping in to the high cost of referral for the intradermal skin testing.

If allergy testing has been done, the horse can go on a series of allergy shots to help desensitize its immune system to the specific skin allergens that have been identified as the causes of its allergies. Injections are given under the skin. Although improvement may be seen rapidly in some cases, a minimum of 12 months is often necessary to determine the efficacy of treatment in any given horse.

As an owner of a horse with allergies, I can share other things you can do to help your horse get some relief. Acupuncture and Chinese herbs can be useful in treatment of skin allergies. Supplements that have a high concentration of omega-3 fatty acids have been shown to decrease inflammation in horses. Feeding ground flax seed or flax seed oil can help decrease the increased immune response seen in horses with skin allergies. Other supplements that might help include MSM, APF, and probiotics. Smart Pak offers a supplement called “SmartBug-Off” for those horses with insect bite hypersensitivity that combines flax seed, MSM and other antioxidants to help combat biting insects and the effects that they have on the immune system. Platinum Performance also has a “Skin and Allergy” supplement for horses that need additional allergy support.

Other management practices aim at reducing exposure to possible allergens. Protect your horse from insects by using fly masks, fly spray, fly sheets, and fly boots. You can also use high-powered fans in the stall, aisle ways and run-in shed to lessen the likelihood of flies landing on your horse.  Reduce the number of insects in the area by using good management practices such as clearing manure frequently and eliminating standing water. A good product we recommend is “Fly Predator” to also help decrease the insect population. Minimize dust in your barn by changing bedding if necessary and improving ventilation.

Medicated shampoos are available to offer your horse some relief from itching. Cool water should always be used for the itchy patient. For itchy horses, shampoos containing oatmeal with or without the topical anesthetic pramoxine can be used, and shampoos with steroids can also be valuable. Antiseborrheic shampoos that contain zinc pyrithinone or selenium sulfide are recommended for horses with excessive scale, and antimicrobial shampoos can be used in those with secondary infections.

Skin allergies can be a difficult problem to treat and diagnose. If you know your horse has allergies, starting treatment early in the season before flies come out will give better results. Using a multi-modal approach, you can hopefully help your horse be itch-free throughout the spring and summer! If you have any questions about this article or would like to schedule an appointment, please call our office at 540-854-7171.