How to Treat and Prevent Summer Sores in Horses

Summer sores in horses are on the rise. Learn more about these parasite-related skin infections.
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Summer sores in horses are on the rise. Learn more about these parasite-related skin infections.
© Amy K. Dragoo

© Amy K. Dragoo

It started with a superficial cut—just a scratch, really—on your horse’s pastern. You cleaned it up and didn’t think much more about it. But the cut didn’t heal and now, weeks later, it’s an oozing, festering mess. Your horse keeps biting and rubbing it, so you know it’s driving him nuts. What is going on?

If you’ve been around horses for anything less than 30 years, you can be forgiven for not recognizing the condition long known as a summer sore. Since the mid-1980s these sores have become extremely rare, “so rare that veterinarians who graduated after that time might never have seen one and might not recognize it,” says D. G. Pugh, DVM, a professor in the department of pathobiology at the Auburn University College of Veterinary Medicine and director of the Alabama Department of Agriculture Veterinary Diagnostic Laboratory System.
Summer sores are still rare, he adds. But reports of cases have increased in the last three to four years. If your horse has a sore that won’t heal, the condition should be on your radar. The good news is that a summer sore will heal with the right care. Even better, these sores can be prevented. To know how, it helps to understand how they form.

Parasites Off Course 
A summer sore results from a wrong turn in the life cycle of certain stomach worms. These worms (Habronema and Draschia species) are not the most dangerous internal parasites of horses—as adults they live in the horse’s stomach and rarely cause serious harm. “Their larvae, however, can be associated with problems,” says Dr. Pugh.

Although still rare, summer sores (above) are becoming more common, in part due to warm weather that comes earlier and stays longer than in years past. | © Paula da Silva/arnd.nl

Although still rare, summer sores (above) are becoming more common, in part due to warm weather that comes earlier and stays longer than in years past. | © Paula da Silva/arnd.nl

The adult worms produce eggs that are shed in the horse’s manure and quickly hatch. The tiny larvae that emerge have to get back into a horse to complete their life cycle, and they need help for that. Their accomplices are maggots—the larvae of house, face and stable flies—that live in manure. Maggots ingest the worm larvae as they feed, and the worm larvae develop inside the maggots as the maggots develop into adult flies. 

As adults, the flies are drawn to the secretions around the horse’s mouth, eyes, nostrils, wounds and other openings. When they land to feed at these places, the larvae sense the moisture and bail out. Lucky larvae find themselves near the mouth, are swallowed by the horse and mature into adult worms in the stomach. But trouble starts when the worm larvae are deposited in other areas—in a wound, say, or on moist membranes around the eyes, the sheath or the vulva. 

The larvae are at a dead end in these places because they can’t get to the horse’s stomach; but they keep trying, migrating through the tissue at the spot. As long as they have moisture they can survive, causing local inflammation and intense itching. The horse may bite or rub the area in an effort to relieve the itch, but that just makes the problem worse. The result is a raw, swollen lesion, oozing blood-tinged fluid and filled with reddish, lumpy granulation tissue, like the proud flesh that can develop when skin doesn’t close over a wound. White or yellowish granules of calcified material may be sprinkled through the tissue. 

These sores, technically known as habronemiasis, were a familiar problem before the deworming agent ivermectin was introduced in the early 1980s in North America. Ivermectin, moxidectin and other drugs in their class turned out to be highly effective against the stomach worms whose larvae cause the sores, and widespread, routine use of the drugs dramatically reduced their numbers. But they were not wiped out. 

When deposited in a horse’s wound or on his moist membranes, worm larvae cause local inflammation and intense itching. | © Paula da Silva/arnd.nl

When deposited in a horse’s wound or on his moist membranes, worm larvae cause local inflammation and intense itching. | © Paula da Silva/arnd.nl

“Deworming kills these parasites, but not 100 percent of them. If there are adult worms in a horse’s stomach, they can produce eggs. If larvae are in the manure, some fly larvae can serve as intermediate hosts to these stomach worms,” Dr. Pugh says. It’s not clear why more sores have started to appear now. For some theories, see the box on page 56. Some horses seem more prone to summer sores than others, he adds. These horses may be hypersensitive to the parasite larvae—more likely if they have adult worms living in the stomach—or they may have a genetic susceptibility. 

What To Do
A summer sore will rarely heal on its own. These sores usually appear in spring and summer, when flies are most active, and just keep getting worse as summer progresses. The inflammation may fade in winter and you may think recovery is underway, but in spring the sore usually erupts again.

The first step in dealing with the problem is to have your horse’s veterinarian examine the skin lesion and attempt to make a diagnosis, Dr. Pugh advises. Other conditions can have similar signs. Summer sores in the skin can look like proud flesh, various growths (sarcoids, squamous cell carcinoma, mast cell tumors) or pythiosis (“swamp cancer” caused by a fungus-like organism). In the eye, a summer sore may mimic a growth, onchocerciasis (caused by the filarial worm Onchocerca), inflammation from a foreign object or certain bacterial or fungal infections. Some of these problems are potentially more dangerous to the horse than a summer sore, so the sooner you consult the veterinarian the better. The diagnosis is based on clinical signs and laboratory analysis of a scraping or biopsy. 

To eliminate the sore, follow a three-part plan of attack:

Flies, which carry the worm larvae, are drawn to the secretions around a horse’s wounds, mouth, eyes, nostrils and other openings. | © Paula da Silva/arnd.nl

Flies, which carry the worm larvae, are drawn to the secretions around a horse’s wounds, mouth, eyes, nostrils and other openings. | © Paula da Silva/arnd.nl

• Treat it. Your veterinarian may prescribe topical or systemic glucocorticoids, which are powerful anti-inflammatory drugs, or a topical mixture of glucocorticoids and dimethyl sulfoxide (DMSO). Reducing inflammation should slow the proliferation of granulation tissue in the summer sore, but that alone may not be enough for healing to begin. Sometimes the excess tissue has to be surgically “debulked”—shaved or frozen off—for healing to take place. If a secondary infection has taken hold, the horse may also require antibiotics.

• Kill the parasites. Treating the horse systemically with ivermectin or moxidectin should remove the adults from the stomach. Sometimes these drugs are applied directly to the sore as well, along with the anti-inflammatory treatments, to hit the larvae. 

• Control flies. Any open sore is a fly magnet, and flies will irritate the lesion and perhaps deposit more worm larvae. Fly-repellent ointment may discourage them, but farm-wide fly control is the best way to deal with these pests. “All fly-control programs should be built around reducing places where flies breed—manure, wet feed, wet organic material—and be part of a broad prevention strategy,” Dr. Pugh says.

Prevention
Flies and parasites are herd problems—they pose risks for every horse on the property, not just the lone horse who develops a summer sore. To prevent these sores, you need to control both

problems. 

Go after flies where they live, breed and feed. Effective control can include these steps:

• Clean up. Pick stalls once or twice a day and clean paddocks at least twice a week to get rid of manure, spilled feed, trampled hay and other materials that attract and provide feeding and breeding sites for flies. 

Fans and fly-proof screens will  help protect stabled horses during the times of day when flies are  most active. | © Stacey Nedrow-Wigmore/arnd.nl

Fans and fly-proof screens will help protect stabled horses during the times of day when flies are most active. | © Stacey Nedrow-Wigmore/arnd.nl

• Manage manure. How you do this will depend on your setup. You can compost it. Done right, composting generates enough heat to kill fly larvae as well as parasite eggs and larvae. You can spread some on fields as fertilizer (but not on horse pastures—that would encourage parasite transmission). You can stockpile it in an area far from the barns and paddocks where horses are or you can have it hauled away. Except when it’s spread on fields, keep it covered.

• Try fly predators. Added to manure piles, these tiny parasitic wasps lay their eggs in fly pupae. The wasp larvae feed on the pupae and destroy them. Suppliers usually ship the predators several times a season.

• Use feed-through fly-control agents. These products contain insect growth regulators or larvicides that pass through the horse undigested and end up in manure, where they keep fly larvae from developing. They’ll also affect fly predators, so these approaches shouldn’t be combined.

Treating a horse systematically with ivermectin or moxidectin should remove adult worms from his stomach. Sometimes these drugs are applied directly to a summer sore as well to fight the stomach-worm larvae. | © Paula da Silva/arnd.nl

Treating a horse systematically with ivermectin or moxidectin should remove adult worms from his stomach. Sometimes these drugs are applied directly to a summer sore as well to fight the stomach-worm larvae. | © Paula da Silva/arnd.nl

• Protect horses. Face and ear masks and topical repellents—sprays or, around wounds, ointments—can help. So can stabling horses during the times of day when flies are most active, especially if the stable has fans or fly-proof screens.

• Kill flies with traps, baits and residual fly sprays in areas where they congregate. Sprinkling sodium bisulfate on stall floors can also reduce fly numbers as well as ammonia, in the barn. 

Go after the parasites with a selective deworming program. When a horse gets a summer sore, it makes sense to treat his stablemates with ivermectin or moxidectin as a preventive measure. They’ve been visited by the same flies as the affected horse and may be harboring adult stomach worms. But when it comes to routine parasite control, the approach long followed by many horse owners—deworming every horse every eight weeks—should be off the table, Dr. Pugh says. Such indiscriminate dosing encourages resistance, which develops when a few worms survive treatment and pass the traits that helped them survive to their offspring. “This is critical in the case of other internal parasites, such as small strongyles [cyathostomes],” he says.

Already some dangerous equine parasites have found ways to resist common deworming medications, and the problem is spreading. Widespread resistance has developed against two of the three broad classes of these drugs, benzimidazoles (such as fenbendazole) and pyrantel salts (pyrantel pamoate or pyrantel tartrate). Ivermectin and moxidectin belong to the third class, the macrocyclic lactones. So far they’re still effective against small strongyles, the most widespread and dangerous equine internal parasites. But researchers believe it’s just a matter of time until worms resistant to all three classes of dewormers develop. 

As part of an effective fly-control program, manure should be cleaned out of paddocks at least twice a week. | © Dusty Perin

As part of an effective fly-control program, manure should be cleaned out of paddocks at least twice a week. | © Dusty Perin

To delay that day and protect your horse, work with your veterinarian to set up a selective parasite-control program that’s tailored to your situation. The recommended program will vary depending on where you live, how many horses are on the property, how old they are, how much pasture they have, how often they travel to shows, how often new horses come onto the property and other factors. Fecal egg counts will help identify horses who are high shedders of strongyle eggs. These horses may need deworming more often than others, while less susceptible horses may need to be checked and dewormed only a couple of times a year.

As part of an effective fly-control program, manure should be cleaned out of paddocks at least twice a week. | © Dusty Perin

As part of an effective fly-control program, manure should be cleaned out of paddocks at least twice a week. | © Dusty Perin

An Old Problem Returns 
Experts aren’t sure why summer sores are becoming less rare in some areas. Here are the leading theories: 

Resistance. Constant reuse of the same dewormer allows parasites to develop resistance. Are Habronema and Draschia becoming resistant to ivermectin and other drugs of its class? “We don’t know if this is happening,” Dr. Pugh says. “No one has documented it.”

New deworming programs. Resistance to commonly used dewormers has developed in other dangerous parasites. To counter that trend, in 2013 the American Association of Equine Practitioners issued guidelines recommending a selective, individualized approach to deworming. The new approach targets the parasites that are the biggest threat to horse health (like small strongyles) and generally involves longer intervals between dewormings. Are Habronema and Draschia taking advantage of the longer intervals and staging a comeback? Although that’s possible, Dr. Pugh says, “We began to see cases before the AAEP advocated the new protocols.”

Today, more horses are densely concentrated rather than spread out over many acres. | © Frank Sorge/arnd.nl

Today, more horses are densely concentrated rather than spread out over many acres. | © Frank Sorge/arnd.nl

Weather. In recent years, warm weather has arrived earlier and hung around longer in many parts of the country. Warmer weather means a longer fly season, giving flies more opportunities to breed and produce offspring.

Management. Weather may be a factor, Dr. Pugh says, but the past 40 years have brought changes in how horses are kept. More horses are densely concentrated in urban and suburban stables, rather than spread out. In these situations, “Poor manure handling and lax fly control give fly populations a chance to increase. If you have more flies, you have more summer sores,” he says.

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