Sweet itch can make a horse miserable. The intense itchiness can cause him to rub away patches of hair, leaving the skin underneath raw and weepy. If a case is severe enough, the horse may become restless and thin, as he spends more time scratching than eating.

Technically known as equine insect hypersensitivity or insect bite hypersensitivity, sweet itch is a severe allergic reaction to the saliva of biting insects, primarily Culicoides spp. midges and gnats but also possibly blackflies or deer flies. Sweet itch reactions can occur anywhere on a horse’s body. However, they are usually seen on the areas where the insects tend to bite: on the underside of the belly, under the mane or the dock of the tail.

The condition is usually at its worst at the times of year 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 won’t begin to show signs until they are at least 2 to 4 years old. The condition is more common among Shires, Welsh Ponies and horses imported from Iceland, but it can occur in any breed.

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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.

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In focus: Sweet itch

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.

This article first appeared in EQUUS issue #452, May 2015.