Allergic Dermatitis in Horses: A Review

Allergic dermatitis--simply, inflammation of the skin caused by an irritating stimulus--is a common yet often treatable and manageable problem in horses. Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, associate professor of Large Animal Clinical Sciences at Michigan State University's College of Veterinary Medicine, gave an overview of the disorder at a recent veterinary convention.

At the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., Rashmir-Raven explained that a wide variety of irritants can cause allergic dermatitis. She reviewed a few of the common causes and management options during her presentation.

Urticaria--Even if you've never heard the word before, chances are you've seen the ailment it describes: hives. Rashmir-Raven said that horses develop urticaria more than any other species, and veterinarians believe the condition is most commonly caused by insect bite hypersensitivity (other less common causes include contact dermatitis and drug reactions--more on those in a moment).

"In addition to hives, insect bite hypersensitivity can also manifest as intensely pruritic (itchy) dermatitis and is usually seasonally recurrent, although severely affected horses in Southern climates do not enjoy seasonal relief," she explained, adding that the condition often worsens with age.

A variety of insects can cause insect bite hypersensitivity, including Culicoides spp. (midges or "no-see-ums"), Simulium spp. (black flies), Haematobia irritans (horn flies), Stomoxys calcitrans (barn flies), Tabanidae spp. (horse flies), Culicidae spp. (mosquitoes), and Onchocerca cervicalis (neck threadworms).

Managing insect bite hypersensitivity-induced urticaria is multifaceted; not only must owners treat the hives and lesions, but they also need to manage the root cause of the hives--exposure and reaction to insects.

Rashmir-Raven suggested treating existing lesions with systemic corticosteroids, antihistamines, medicated shampoos, or topical ointments, or a combination of treatment options. Treatments should be tailored to each individual horse, so she stressed that horse owners should work closely with a veterinarian to obtain appropriate management and relief.

It's also crucial to reduce the affected horse's exposure to insects. Rashmir-Raven gave numerous suggestions on how to best accomplish this:

  • Apply long-lasting insect repellants, especially when insects are most active at dawn and dusk;
  • Outfit the horse with a fly sheet, fly mask with ear protection, and leg wraps during turnout (these can also be doused with fly spray if necessary);
  • Adhere permethrin (fly repellant) tags to the horse's mane, tail, and forelock;
  • Keep the horse stalled with a fan during peak insect hours ("Many biting insects are poor flyers," she noted.);
  • Place an automated fly sprayer in the horse's stall, using caution to ensure the chemicals don't contaminate the food and water supply or spray into the horse's eyes;
  • Try feeding an omega fatty acid supplement (such as ground flaxseed), as omega fatty acids have been shown to reduce allergic reactions;
  • Avoid turning horses out near standing water sources, manure piles, or cattle, as all these attract insects; and
  • Use fly predator wasps to reduce the number of flies on the premises.

For severely affected horses that cannot be managed with the aforementioned tips, Rashmir-Raven said veterinarians can prescribe low-dose corticosteroids and other anti-inflammatory drugs to help reduce clinical signs. Rashmir-Raven cautioned owners and veterinarians to rule out insects such as lice and mites and other infectious agents prior to implementing treatment for insect-induced urticaria.

Atopy--Simply put, atopy is a genetic predisposition to environmental antigen hypersensitivity that causes allergic dermatitis. Rashmir-Raven noted that a variety of irritants contribute to this condition including pollens, grasses, weeds, trees, molds, dust, feather, cotton, wool, and other fibers. "Arabians and Thoroughbreds in early adulthood appear to be predisposed to the disease," she added.

Dermatologic clinical signs of atopy include:

  • Alopecia (hair loss);
  • Excoriations (abrasions from rubbing);
  • Crusts;
  • Scales;
  • Erythema (skin redness caused by congestion of the capillaries);
  • Urticaria;
  • Lichenification (thickened and leathery skin); and
  • Hyperpigmentation (skin darkening).

"Knowing and avoiding the particular allergen that bothers the horse is the best treatment for the horse," Rashmir-Raven said. If a particular allergen isn't known, she suggests intradermal allergy testing. Combination therapy with omega fatty acids, hyposensitization (allergy "shots" based on intradermal allergy testing), antihistamines, and/or corticosteroids, will "likely provide the best long-term control."

Contact Hypersensitivity--Similar to atopy, contact hypersensitivity is an allergic reaction brought on by contact with or exposure to an offending substance or material. "The offending substance does not necessarily have to be one that has been recently introduced to the animal," Rashmir-Raven explained. "The horse can suddenly develop a hypersensitivity to a substance to which it has been exposed for years without any previous problem."

Horses with contact hypersensitivity will develop lesions (including vesicles, papules, thickened skin, and urticaria) at the area at which contact occurs, she explained. Lesions on the muzzle, pasterns, and fetlocks, for example, can point to a pasture plant allergy. Reactions to a saddle or girth will prompt lesions on the back or around the barrel. Lesions all over the body might suggest a shampoo or fly spray allergy. Rashmir-Raven cautioned that insect bite hypersensitivities can be confused with contact allergies.

Treatment of contact hypersensitivity is similar to that of atopy. Rashmir-Raven suggested washing the horse in cold water with a gentle soap (such as Ivory soap) to remove the irritants and ensuring the horse isn't allowed further contact with the substance. Again, if a particular allergen can't be avoided or can't be determined, she suggested therapy with omega fatty acids, hyposensitization, antihistamines, and/or corticosteroids.

To treat existing lesions, she recommended using topical antipruritics to prevent itching, antibacterial ointment on open lesions, and frequent baths to remove potentially irritating substances from the body.

Take-Home Message

Regardless of the cause, allergic dermatitis often results in a very itchy horse. Working closely with a veterinarian to manage the pruritis and its underlying cause will give the horse the best chance of returning to a normal state. Rashmir-Raven suggested aiming for "tolerable itchiness" if complete resolution of this often treatable condition isn't possible.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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