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Insect Bite Hypersensitivity In Horses

By Susan L. White DVM, MS, diplomate ACVIM

Article Archives LinkInsect Bite Hypersensitivity In Horses

Insect Bite Hypersensitivity In Horses

Virtually all horses are subject to bites of flying insects. Some horses develop an allergic reaction to salivary antigens of the biting insects (here after referred to as Insect Bite Hypersensitivity or IBH). IBH is characterized by skin reactions that are seasonal (warmer months when insects are active) or considerably worse when insect pressure is high in those geographic areas with warm temperatures throughout the year.

Acute reactions to insect bites consist of reddened or inflamed skin (visible in non-pigmented areas), papules, often with a central crust, and wheals. Secondary lesions caused by self-inflicted trauma due to pruritus often result in more extensive lesions of alopecia, excoriation, extensive crusts and secondary superficial infections. Horses with chronic IBH may have lichenification and depigmentation of affected skin. Most horses develop IBH when middle aged or older but some horses that show IBH when very young (1-6 years of age) may have atopy. Atopy is a genetic programmed abnormality of the immune system in which the horse becomes sensitized to environmental antigens that in non-atopic animals does not create disease. Both innate and acquired immune function is affected as well as the barrier function and structure of the skin. All horses affected with IBH tend to have more severe reactions as they age.

Insects Breeding Area and Feeding Times Table

Any biting insect may be implicated in IBH however the most common insects that initiate IBH are stable flies, horn flies, black flies or buffalo gnats and Culicoides spp gnats (no see-ums). House flies, that look similar to stable flies, do not bite but feed on body secretions or fluids and may aggravate excoriated skin lesions. Diagnosis of insect bite hypersensitivity is made from the signalment (generally an adult individually affected animal, although related horses may be similarly affected), history, and response to avoidance (improve and exacerbate seasonally). All other dermatologic diseases should be ruled out. The distribution of lesions on an affected horse is dependent on the biting characteristics of the insect responsible. Table 1 gives the general times of the day that different insects are active. Since there is still much to be learned about the identification and feeding habits of many of the insects implicated in allergic dermatoses, it may not be possible to identify the exact etiological agent.

Regardless of the offending insects the most important factor in controlling IBH is avoidance of insect bites. Complete avoidance can be difficult to impossible since several different insects with different feeding patterns may be involved (Table 1). However, stabling of horses during peak feeding times of biting insects identified in clinical disease can substantially reduce the antigenic load. The use of fans to prevent insects from landing on the horse are advisable in stalls and loafing sheds. Fly sheets and face masks (that preferably include ear protection) may be used both during stabling and turn out.

Sanitation on premises, both barns and paddocks, should be practiced. Stable flies reproduce in wet, rotting vegetation as well as manure. Thus removal of accumulations of wet vegetation and draining of low areas is advisable in addition to manure removal and proper composting. The use of fly predators offer the best overall and consistent control of flies for long term management. Feed through fly inhibitors are also effective, although these products are only active in manure and will not offer complete control in environments were wet vegetation is found.

Because pruritus, regardless of the cause, is exacerbated by heat, humidity and solar radiation, run in sheds and stabling or other provisions for shade and fans are desirable. Cool water hosing reduces the sensation of pruritus and calms inflamed skin. Topical skin care is an important component in controlling IBH. Kinetic Vet* offers a dermatologic product line, available only to veterinarians, that substantially improves the clinical condition of IBH. Gently cleansing of the skin with a non-detergent based shampoo with 1% hydrocortisone and 1% praxomine (Equishield IR® Shampoo) removes dirt, sweat and environmental antigens while providing pruritus relief and decreasing the inflammatory reaction. Topically applied Equishield IR® Spray (1% hydrocortisone and 1% praxomine) can be applied to affected areas of the horse as needed. Equishield IBH® Salve contains an antiseptic (chlorhexidine 2%) and insect repellant (citronella oil 1%) in addition to praxomine and hydrocortisone.

In many horses environmental management, avoidance practices, and skin care with topical steroids as needed will substantially improve the horse. Horses with generalized intense pruritus may need initial therapy with systemic steroids to break the itch-scratch cycle. Prednisolone, 0.5-1.5 mg/kg orally/day, may be given in the morning until the pruritus and secondary skin trauma are under control. Prednisone may be used but because of its limited and variable oral bioavailability not all horses will respond to its use. In severe nonresponsive cases dexamethasone, 0.04-0.75 mg/kg parenterally or orally, may interrupt the inflammatory response. The steroid dose should be gradually decreased before it is discontinued. Long term use of steroids should be avoided.

Other medications that may contribute to IBH control are antihistamines and polyunsaturated fatty acids. Hydroxyzine hydrochloride (0.5-1.5 mg/kg every 8-12 hours orally) is more effective in controlling urticaria than pruritus, but is a useful part of the management strategy. Cetirizine (0.2-0.4 mg/kg BID orally) has been effective in many horses. Omega 3 fatty acid supplementation contributes to overall skin health and is often helpful in IBH cases. EquiShield SA® Powder, which contains omega-3 and quercertin, is a palatable supplement that can help diminish allergic reactions. When purchasing products containing omega 3 fatty acid it is important to have a manufactured date and preferably an expiration date on the label.

The severity of IBH clinical signs will vary dependent on insect pressure (often weather dependent) and the degree of allergies to other substances. Thus management practices to decrease dust along with insect avoidance and skin care is advisable. Clients should understand that IBH can be controlled but not cured.

*Visit www.kineticvet.com to see all of the products for dermatologic conditions in horses

Susan L. White DVM, MS, DACVIM is the Josiah Meigs Distinguished Professor, Emeritus in large animal medicine at the University of Georgia’s College of Veterinary Medicine. Dr. White is board certified in Large Animal Internal Medicine and has a masters degree in veterinary pathology. Dr. White is a recipient of the ACVIM Kirk Award for professional excellence and is currently president of the American College of Veterinary Internal Medicine. She has published over 75 articles and book chapters and is a frequent speaker at CE meetings. Dr. White has had a long standing interest in equine dermatology. She has lectured both nationally and internationally in large animal dermatology for over 30 years. She maintains a consulting service in equine dermatology nationally.