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.

Generally, fly sprays contain natural and synthetic insecticides to knock down or deter flies on contact. Others like UltraShield Gold combine tried-and-true ingredients like pyrethrin and permethrin with fatty acids to repel flies while also disrupting their navigation systems to stop them before they bite.

“I love UltraShield Gold’s new formula because it totally disrupts the flies’ GPS so they don’t bite your horse,” Ashker said. “I use it on my horses in the show ring, on the trail, in their stalls, in the cross-ties and especially when they’re turned out.”

Ashker also finds the UltraShield Sport ideal for competing in the hot and humid Florida weather where she’s based due to its water-based formula that’s sweat resistant and repels dust.

“When I’m working my horses in either grand prix or four-star eventing they’re always going to be nice and sweaty, especially in Florida,” she noted. “I love this product—I call it the ‘Blue One’—because I know it’s going to last through all of my work. That’s important because I don’t want to have to keep spraying my horses while I’m in the warm-up.”

Watch Ashker’s video below for her tips on how to keep your your equine partner fly-free this summer.

About Laine Ashker

Five-star eventer and dressage professional Lainey Ashker has competed in numerous FEI competitions at the five-star level, including the Blenheim Palace International Horse Trials CC14* and the Defender Burghley Horse Trials CC15*. The horsewoman, who trains out of her Keystone Acres farm in Chesterfield, Virginia, also won the National Eventing Championship (Advanced Level) in 2013 aboard her long-time mount, Anthony Patch. More recently, she won her first CDI in May 2023 aboard her upper-level dressage horse, Zeppelin.  

This video is brought to you by Absorbine.

For More:

• For Laine Ashker’s tips on how to create a healthy, shiny coat for the show ring, click here.
• For her winning recipe for hoof health, click here.

For More:

• If your horse tends to get squirmy when you apply fly spray, teach him how to stand quietly with these strategies.
• Learn more about chemical-free pest control options here.

Tackle the Source

Timely elimination of breeding sites and moisture control are important in reducing infestation. Houseflies visit moist material from manure and other organic matter, where the females will lay eggs as they feed. Likewise, stable flies will develop in decaying matter, such as dirty straw, spilled feed or hay, and urine or water. Here are a few simple tips to keep the breeding population down in your barn:

• Clean up manure daily in stalls and at least weekly in paddocks. Spread or compost the waste. 

• Replace stall bedding weekly. Shavings and sawdust have been proven to produce fewer flies than straw. 

• Place waterers in well-drained areas and away from places where horses are fed. 

• Keep feed areas dry and avoid ground feeding because any moist, decaying leftover feed is an ideal breeding site for stable flies and houseflies. 

There are other stable-management practices that can help, too. 

• Fly traps and sticky paper may be most useful as a means of documenting fly numbers over time. A significant increase in catch from one week to the next can be a warning to check on sanitation and to increase fly-control measures. 

• Fans that direct a downward and outward airflow will help to keep flies from entering barns. 

• Screening is an excellent way to keep flies out of feed and tack rooms and stalls. 

• Several commercial firms offer a fly-parasite (predator) release program that can be used to supplement fly control. 

• You also can apply residual insecticides to walls, ceilings and rafters of barns and sheds where nuisance flies rest. These insecticides are made for this specific purpose as opposed to those developed for direct application to horses—so check the label. General observations and accumulations of fly specks (waste drops) will help identify target spots. Be sure to protect water and feed when making applications. Do not apply the same insecticide or different products within the same chemical class repeatedly throughout an entire season to minimize insects developing resistance.

Maximize Your Fly Spray’s Effect

While addressing the source of fly-control problems is key, insecticides can help to provide some temporary reduction of pest problems. Specifically, how you use fly spray can make a big difference in helping your horse be comfortable. Here are some tips:

• Read the label for age restrictions. Some products should not be applied to foals younger than 3 months. Check to be sure the pests you want to control are listed on the label, and apply spray no more frequently than the recommended retreatment intervals. [Read more: What’s in a fly spray label?]

• Observe horses frequently to determine which body parts are most irritated by insects. Mosquitoes and gnats tend to feed on horses’ underbellies, which are easily overlooked and may be more difficult to cover evenly with fly spray. Stable flies feed on the lower legs, where insecticides and repellants are prone to being covered with dust and washed off with water. 

• Before applying insecticide, thoroughly brush your horse’s coat to remove dirt that can reduce the spray’s effectiveness.

• Lightly brush against the lay of the hair while applying the spray to ensure adequate cover. This allows the insecticide to reach the skin.

• Apply spray more frequently rather than more heavily. Reapply fly spray once your horse’s coat has dried after washing or exposure to heavy rain. 

Thanks to Lee Townsend of the University of Kentucky Extension Service for contributing information to this article. 

This article originally appeared in the June 2015 issue of Practical Horseman.