Here, we look at what causes sweet itch, how to spot the signs that your horse might be gnat food and offer tips on how you can protect him from these miniscule, mighty-mouthed foes. Plus, we look at a study that confirmed horses can inherit a susceptibility to sweet itch.

Sweet Itch: At A Glance

Definition: Technically known as equine insect hypersensitivity (EIH), sweet itch is an allergic reaction to the saliva of biting insects, most often the Culicoides species of midges and gnats.

Signs: Intense itchiness, wheals and swelling, primarily on the neck (under the mane), face, shoulders, withers, belly and rump (over the tail). The skin may be rubbed until it is crusty, weepy, raw and hairless. Location of the lesions may depend on the specific species of gnat; most prefer to bite on the neck and along the topline, but some target the chest, belly, tail and groin.

Diagnosis: Skin scrapings and cultures may be needed to distinguish EIH from other causes of itchy dermatitis, such as mites, lice and fungal infections.

Treatment: Antihistamines or corticosteroids may help ease the itch and inflammation, providing relief to the itchy horse; antibiotics or antimicrobial shampoos can help prevent secondary infections. Pentoxifylline can be used to decrease skin reactivity, and in some cases allergy serum is worth trying.

Of Note: The condition is usually at its worst during the spring and summer when insects are most active, and it may subside during the winter. Less than 10 percent of horses in the United States develop sweet itch. Most horses won’t begin to show signs until they are 2 to 4 years old, and the condition is more common among Shires, Welsh Ponies and horses imported from Iceland, but it can occur in any breed.

Protect Your Horse

Research is underway to develop a reliable immunotherapy treatment for sweet itch. In the meantime, veterinarians may prescribe antihistamines and corticosteroids to help relieve the signs of the condition. But steroids carry a low but real risk of serious side effects, including laminitis. So, if your horse is susceptible to sweet itch, you’ll want to reduce his exposure to the insects that cause it. Here are steps you can take:

1. Stable your horse during peak biting hours. Culicoides spp. are most active at dusk and dawn, so bring your horse into a stall during those hours. For additional protection, install ultrafine screens in stall windows and set up fans to provide a continuous breeze over stabled horses. Gnats are weak flyers and even a slight breeze will keep them away. You might also consider applying a spray repellent designed for use on stall walls and other surfaces inside the barn.

2. Keep your horse covered. One way to reduce bites is to keep a fly sheet on your horse during insect season. Made of fine mesh and equipped with closures to keep out bugs, clothing designed specifically for horses with sweet itch has extensions to cover the neck, belly and tail, the areas where Culicoides spp. are most likely to bite. A fine-mesh mask may also be needed to protect the horse’s face and ears.

3. Use fly-control products. The most potent products combine repellents with pesticides. Look for ones labeled for use against gnats and midges. You’ll need to be diligent about applying the sprays and wipes as often as the manufacturers’ instructions allow.

4. Restrict your herd’s access to marshy areas. Gnats breed in marshy, shady ground with rotting vegetation. If possible, move your horse to a pasture in a drier, more exposed location away from boggy terrain. Around your barn, clear up standing water in ditches and gutters and keep them free of leaves and other debris. Place manure and compost piles as far away from the horses as is feasible.

5. Try over-the-counter products. You’ll find a number of products for horses with sweet itch, from feed supplements formulated to reduce inflammation, to topical preparations meant to soothe itchy, inflamed skin. Thick, oily or sticky ointments are messier but may keep gnats from landing on the horse’s skin. They may also help keep bugs from getting to the skin to bite. Anecdotal evidence suggests that these products offer some relief, at least in some horses. When trying a new topical product, start by applying it to a small area to make sure it will not further irritate sensitive skin. Be careful with tea tree oil, lavender and other herbals—they are soothing to some horses but can irritate the skin in others.

More Tips & Tricks

Here are a few home remedies recommended by riders to help prevent or reduce your horse’s exposure to the gnats that cause sweet itch:

• Apply Avon’s Skin So Soft bath oil to the most vulnerable areas.
• Add apple cider vinegar and/or garlic powder to your horse’s feed to make his sweat’s smell less attractive to gnats. (There are also supplements available that contain both apple cider vinegar and garlic.)
• Apply menthol products like Vick’s VapoRub to susceptible parts of your horse’s body.
• Rub a scented dryer sheet over his body.
• Apply baby oil to the roots of your horse’s mane and tail head; gnats can’t seem to get a grip on the skin to feed.

Genetic Connection for Sweet Itch?

A long-term study from the Czech Republic confirms that horses can inherit a susceptibility to the allergic skin condition known as “sweet itch,” which is technically known as insect bite hypersensitivity (IBH). The study focused on Old Grey Kladruber horses, a native warmblood breed, at the National Stud in Kladruby and 10 private breeding facilities.

The same person visually inspected the horses over a period of 13 years, from 1996 to 2009, scoring the severity of any signs of IBH at each observation. A total of 1,209 observations were entered in a database, along with information about each horse, including location, age and pedigree.

In statistics, the degree of a trait’s heritability is expressed on a 0 to 1 scale, with 0 meaning that none of the variability of the trait among individuals is the result of genetics and 1 meaning that genes are responsible for all the variability. The Czech researchers found the heritability of sweet itch susceptibility among the study horses ranged from 0.305 to 0.626, depending on the factors considered in the calculations. These numbers, the researchers say, indicate the heritability of sweet itch is “medium to high, so there is a possibility to reduce the prevalence of IBH by selection.”

Reference: “Genetic parameters of insect bite hypersensitivity in the Old Grey Kladruber horse,” Journal of Animal Science, January 2017

For More:

• For more tips on how to protect your horse from pesky and disease-carrying insects this summer and for our shopping guide on protective fly gear—including fly sheets designed specifically for sweet-itch protection—click here.
• Learn how to avoid the top-three summer equine health issues here.

• Aural plaques: flaky white patches on the inner surface of the ear; occur when chronic fly irritation causes inflammation. Topical application of an antiviral or steroid cream can clear the plaques and soothe skin, but any pigment loss under the skin is permanent. Ear hair provides a natural defense against flies, so minimize trimming as a preventive. Protect clipped areas with repellent or a fly mask with ear covers.

• Dandruff: skin oil that sheds from the coat as either small, dry flakes or large, waxy crusts. Dry dandruff typically forms at the base of the mane and tail and on the girth area; oily dandruff usually appears on the elbows, hocks and lower legs. Either type results from a variety of causes, including allergies, insects, poor nutrition and undiagnosed illness, but the most common causes for crusty, scaly skin are bacterial infections (Staphylococcus). Some breeds, such as Thoroughbreds and Arabians, are more affected than others. Anti-dandruff shampoo formulated for horses can dissolve flakes and scales; products containing an antibacterial such as chlorhexidine will help to resolve large, waxy crusts; and a feed supplement may improve skin and coat condition.

• Hives: soft, raised, itchy bumps that develop and disappear often suddenly as part of a reaction to an insect bite or sting, medication or exposure to an allergen. May be seen on any part of the body, but mainly occur on the back, flanks, legs, neck and eyelids. Hives usually resolve without treatment; identifying and preventing exposure to the causative agent may be necessary in chronic cases.

• Insect bite hypersensitivity: allergic skin disease caused primarily by the saliva of insects of the genus Culicoides (midges, gnats, no-see-ums). Signs include itching, skin thickening, lesions, scaling, crusting, hair loss and hives. Appears primarily on the face, mane, ears, trunk and tail. Corticosteroids can relieve the cycle of itching, but prevention is key: Apply fly spray; stable horses at dusk and dawn (the insects’ preferred feeding times); use fans to create air currents too challenging for the flies to navigate; and maintain a clean, dry environment to disrupt breeding and reduce numbers.

• Lice: tiny wingless parasitic insects that live in the haircoat and are visible upon inspection. There are two types: chewing, which feed on hair and dead skin, and blood-sucking. Both cause itchiness, and horses may bite at or rub infested areas, causing hair loss and skin damage. Sucking lice create small wounds that may become infected. To treat: Use a spray containing pyrethrin, synthetic pyrethrin or the organophosphate coumaphos.

• Mange: infestation by a parasitic mite that feeds by piercing the skin or burrowing into it. Signs include red, moist, crusty skin patches and intense itching over the body (sarcoptic mange), ears (psoroptic mange) and lower legs (chorioptic mange). The condition is passed from horse to horse through direct contact or via shared equipment. A topical or oral acaracide, such as ivermectin or moxidectin, will eliminate mites. A topical shampoo may help to remove crusting and scabs.

• Melanoma: tumor of the skin cells that produce pigment (melanocytes); common in aging gray horses due to the gray gene; appears as a raised, round, firm black nodule; typically found under the tail, around the anus, on the eyelids and in the corners of the mouth. Most melanomas are benign, and horses may require no medical intervention. Treatment options include removal via surgery, cryotherapy or laser; local chemotherapy; oral medication and immunotherapy.

• Photosensitivity: painful inflammation of lightly or unpigmented patches of skin; much more extensive than sunburn. Results from the ingestion of certain plant toxins that eventually circulate in the capillaries near the skin surface and are activated by sunlight. The toxins are present in a number of common pasture plants, including St. John’s wort, buckwheat, bishop’s weed, spring parsley and some varieties of clover, particularly alsike. Photosensitivity may also be related to a few types of antibiotics (tetracyclines) and impaired liver function. To treat: Relieve pain, prevent infection, eliminate exposure to the cause, apply sunblock and outfit horses in UV protective sheets and masks.

• Rainrot: crusted, painful skin infection caused by Dermatophilus congolensis, a microorganism that resembles bacteria and fungi and normally lives on skin. Wet conditions stimulate the release of spores and spread the infection. Treatment involves antibiotics. Topical antibacterial shampoo may be used to soak scabs so they can be removed. Clipping affected areas may be necessary. Isolate infected horses to reduce the spread of disease. 

• Ringworm: fungal infection that appears as an itchy, circular pattern of hair loss, usually on the girth and saddle areas. The fungi are found in soil and can be picked up when a horse digs, rolls or lies down. Typically, the infection is transmitted through shared tack and grooming supplies. Ringworm can clear without treatment. Antifungal shampoo or topical therapy can speed recovery. To prevent spread and recurrence, disinfect equipment and other potentially contaminated surfaces.

• Sarcoid: tumor caused by papillomavirus infection; usually not painful or itchy. There are six distinct types:

• verrucose–gray, flat, scaly with an irregular surface, slow-growing; often seen on the face, trunk or sheath; the least aggressive sarcoid tumor
• occult–small, flat, gray, hairless, slow-growing; may not change for years; usually occurs around the mouth and eyes, on the neck and relatively hairless areas, such as the inside of the forelimbs, armpits and thighs
• nodular–distinct solid lump; may ulcerate and bleed, can appear singly or number in the hundreds usually on the groin, sheath or eyelids
• fibroblastic–aggressive and fast-growing, irregularly shaped, firm, fleshy raised lump; usually smooth and hairless; may ulcerate and bleed; often occurs in clusters on the eyelid, groin, lower limbs, coronary band and the site of a skin wound or trauma
• mixed–a combination of two or more types
• malevolent–rare, aggressive tumor that spreads extensively through skin; may ulcerate and bleed; often forms on the jaw, face, elbow or inner thigh.

Treatment options for all types of sarcoids include surgical removal, immunotherapy, chemotherapy, cryotherapy and laser surgery. The recurrence rate in horses is high.

• Scratches: painful scabby or oozing skin infection above the heels; caused by a mix of bacteria, fungi and parasites that enter breaks in skin; most often occurs when footing is excessively wet, muddy or dirty. Mild cases may resolve by cleaning and clipping the affected area and keeping it dry. Severe cases may require topical treatment with an antibacterial, antifungal or steroid alone or in a mix; oral antibiotics may be necessary for extensive conditions.

• Summer sore: itchy, inflamed lesion caused by the migrating larvae of gastric worms (Habronema and Draschia species); may become swollen and raw as horse rubs, ooze blood-tinged fluid and fill with lumpy, reddish granulation tissue. Treatment includes topical or systemic glucocorticoids or a topical mixture of glucocorticoids and dimethyl sulfoxide (DMSO); excess granulation tissue may require surgical removal. 

• Sunburn: painful reddening, inflammation and possible blistering and peeling of pale or nonpigmented areas of the skin caused by overexposure to the sun’s ultraviolet rays. Treat with a soothing, moisturizing topical ointment, such as zinc oxide or silver sulfadiazine. To prevent: Stable during the day, provide shade outdoors, apply UV sunscreen (a horse-specific product or one that is child-safe, at least SPF 30), use protective fly sheets and masks.

• Sweet itch: allergic skin disease caused by biting midges (Culicoides). Salivary proteins injected into the skin cause intense itching. The mane, tail and withers are most often affected. Rubbing to relieve the itch can produce open wounds and secondary infections. Treatment may rely on topical corticosteroids and/or antihistamines. Preventive measures include insect repellent, fly sheets, face and ear masks and stabling from dusk to dawn when Culicoides are most active.

• Vitiligo: autoimmune destruction of cells that produce melanin, the natural pigment that colors skin and hair. Appears as white or light patches that vary in size and location; often develops around the lips and on the muzzle and eyelids. Vitiligo is more common in gray horses; it is linked to an immune response against melanomas, which may be present in the perineal and tail areas. There is no treatment.

• Warts: small, rough, gray-white growths caused by papillomavirus; often seen on the muzzle, ears, genitals and pasterns of horses age 4 and younger whose immune system has yet to fully mature. Most warts resolve on their own. Treat inflamed or sore spots with an over-the-counter antiseptic or topical moisturizing lotion. Surgery, topical cream or cryosurgery may be an option for warts causing discomfort. Papillomavirus is contagious, so quarantine any infected individual, don’t share equipment and disinfect all surfaces to kill lingering organisms.  

Practical Horseman thanks Rosanna Marsella, DVM, DACVD, for her technical assistance in the preparation of this article. A veterinary dermatologist, Dr. Marsella is a professor at the University of Florida College of Veterinary Medicine in Gainesville, where she also serves as the director of the laboratory of comparative dermatology.