How to Treat and Prevent Summer Sores in Horses

Last Updated on February 19, 2022 by Allison Price

Horses are experiencing more summer sores. Find out more about parasite-related skin conditions.

(c) Amy K. Dragoo

It began with a superficial cut on your horse’s pastern. It was quickly cleaned up and you didn’t really think about it. The cut was not healed and it is now a swollen, bleeding mess. It’s obvious that your horse is causing you pain by constantly biting and rubbing at it. What’s the problem?

You can’t be blamed if you haven’t been around horses for more than 30 years for not being able to recognize the sore, which has been known for many years as a “summer sore”. These sores have been extremely rare since the mid-1980s, and “so rare that veterinarians who graduated later might not have ever seen one.” D. G. Pugh DVM is a professor at Auburn University College of Veterinary Medicine and director of Alabama Department of Agriculture Veterinary Diagnostic Laboratory.

He adds that summer sores are still uncommon. However, cases have been reported to be on the rise in the past three to four year. You should immediately take action if your horse develops a painful sore. A summer sore can be healed with proper care. These sores can also be prevented. It is important to understand how these sores form.

Summer Sores in Horses

Parasites Off Course

Summer sores are caused by a mistake in the life cycle for certain stomach worms. These worms, Draschia and Habronema species, aren’t the most harmful internal parasites for horses. As adults, they live in horses stomachs and rarely cause any serious injury. Dr. Pugh says that their larvae can cause problems.

Summer sores are still very rare but they are becoming more common due to warmer weather. | (c) Paula da Silva/

The eggs are quickly hatched by the adult worms, which shed their eggs in horse manure. To complete their life cycle, the tiny larvae must be reintroduced to a horse. Maggots, which are the larvae of house and stable flies, are their accomplices. The maggots consume the worm larvae while they feed and the larvae grow inside the maggots.

Adult flies will be drawn to horse’s eyes, nose, nostrils, mouth and other openings by the secretions. The larvae will leave these areas to find moisture, and they will return to their nests. Lucky larvae are found near the mouth and become adult worms in their stomachs. Trouble begins when the worm larvae end up in other places, such as around the eyes, the sheath, or the vulva.

These larvae are stuck in these areas because they cannot reach the horse’s stomach. But they persevere and migrate through the tissue. They can survive as long as there is moisture. This causes local inflammation and itching. Although the horse might try to soothe the itching by biting or rubbing the affected area, this only makes matters worse. This results in a raw, swollen area, filled with blood-tinged fluid, and reddish, lumpy, granulation tissue. It’s similar to the proud flesh that can form when skin isn’t closed over a wound. The tissue may also contain calcified material in the form of white or yellowish granules.

These sores were known as habronemiasis and were common before the introduction of ivermectin, a deworming agent, in the 1980s in North America. Ivermectin, Moxidectin, and other drugs of their class proved to be extremely effective against the stomachworm larvae that cause sores. The widespread use of these drugs significantly reduced their numbers. They were not eradicated.

Worm larvae can cause severe itching and inflammation if they are left on horses’ wounds or on their moist membranes. | (c) Paula da Silva/

These parasites can be killed by deworming, but not all of them. Horses can have eggs if they have adult worms in their stomachs. Dr. Pugh states that fly larvae in manure can be used as intermediary hosts for these stomach worms if they are present.” It is not known why sores are appearing more frequently now. See the box at page 56 for some theories. He adds that some horses are more susceptible to summer sores than other horses. These horses could be hypersensitive to parasite larvae. This is more likely if there are adult worms in their stomach.

What to do

A summer sore won’t heal by itself. These sores are more common in summer and spring, when the flies are at their most active. They only get worse with each passing summer. Although the inflammation may subside in winter, you might think that recovery is under way. However, in spring, the sores usually flare up again.

Dr. Pugh recommends that you have your horse’s veterinarian examine and diagnose the skin problem. Similar signs can also be present in other conditions. The skin may develop summer sores that look like proud flesh, squamous cells carcinoma, mast cell tumours, or pythiosis (“swamp-cancer” caused by a fungus. A summer sore in the eye may look like a growth, Onchocerca (caused from the filarialworm Onchocerca), inflammation due to a foreign object, or other fungal or bacterial infections. These problems can be more serious than a simple summer sore. It is important to consult your veterinarian as soon as possible. A scraping or biopsy can be used to diagnose the horse.

Follow these steps to eliminate the sore:

Fly larvae carry the worm larvae and are attracted to horse’s wounds, eyes, nostrils, mouths, noses, and other openings. | (c) Paula da Silva/

It can be treated. Your doctor may recommend topical or systemic glucocorticoids. These powerful anti-inflammatory drugs are also available in a combination of glucocorticoids (DMSO) and glucocorticoids. Although reducing inflammation will slow down the growth of granulation tissue in the sore’s summer, it may not be enough to allow healing to occur. Sometimes, the excess tissue must be removed surgically (shaven or frozen) in order to allow healing to occur. Horses may need antibiotics if they have a secondary infection.

Get rid of parasites. The adults should be removed from the stomach by treating the horse systemically using ivermectin and moxectin. These drugs can be applied directly to the sore along with anti-inflammatory treatments to kill the larvae.

Control Flies. Open sores are magnets for flies. They can irritate lesion and deposit more worm larvae. Although a fly-repellent cream might discourage them, farm-wide control is the best way for these pests to be controlled. Dr. Pugh states that all fly-control programs should focus on reducing the breeding areas of flies–manure and wet feed–and be part a comprehensive prevention strategy.


Flies and parasites can be a problem for the entire herd. They pose a risk to every horse on the property. You must control both parasites and flies to prevent them from causing sores.

You should go after the flies wherever they live, breed, and feed. These steps can be used to control flies effectively:

* Clean up. Pick up stalls at least once a day. Clean paddocks at the most twice a week to remove manure, spilled feed and other materials that attract and breed flies.

Stable horses will be protected by fans and fly-proof screens during peak times when flies can be active. | (c) Stacey Nedrow-Wigmore/

* Manage manure. Your setup will determine how you manage it. It can be composted. If done correctly, composting can generate enough heat to kill both fly larvae and parasite eggs. Some can be spread on the fields to fertilize, but not on horses’ pastures as that would encourage parasite transmission. It can be stored in an area away from horses’ barns or paddocks, or it can be hauled away. Keep it covered if it is spread out on fields.

Fly predators are an option. These tiny parasitic wasps lay eggs in fly pupae. The larvae of wasps feed on the pupae, and then destroy them. The predators are often shipped multiple times per season by suppliers.

* Use feed-through fly control agents. These products contain insect growth regulators and larvicides. They pass through horses undigested and end in manure. This prevents fly larvae developing. These products can also be harmful to fly predators so they should not be used together.

The treatment of a horse with ivermectin and moxidectin should eliminate any adult worms. These drugs can also be applied to the summer sore to combat the larvae of stomach-worms. | (c) Paula da Silva/

* Protect horses. Protect horses with ear and face masks as well as topical repellents sprays around wounds or ointments. Stabilizing horses at times when flies are active can be done, particularly if there is fans and fly-proof screens.

* Use traps and baits to kill flies in areas they congregate. Sprinkle sodium bisulfate onto stall floors to reduce the number of fly species.

You can target the parasites using a selective deworming method. As a preventative measure, treat horses with ivermectin and moxectin. These horses may have adult stomach worms because they were bitten by the same flies that the horse with the summer sore. Dr. Pugh suggests that routine parasite control should be abandoned. He says many horse owners have been following the same approach for years: every eight weeks, deworming each horse. This indiscriminate dosing can encourage resistance. Resistance develops when some worms survive treatment, and they pass on the traits to their offspring. He says that this is especially important for internal parasites such as small strongyles (cyathostomes).

Some dangerous equine parasites are already able to resist common deworming medication, and this problem is only getting worse. Two of the three main classes of drugs have been subject to widespread resistance: benzimidazoles (such fenbendazole), and pyrantel salts, such as pyrantel pamoate (or pyrantel tartrate). Ivermectinmoxidectin are members of the third group, the macrocyclic latones. They are still effective against small strongyles which are the most dangerous and widespread equine parasites. Researchers believe that it is only a matter time before worms are able to resist all three types of dewormers.

Manure must be removed from paddocks at minimum twice per week as part of an effective fly control program. (c) Dusty Perin

Your veterinarian can help you set up a customized parasite-control program to protect your horse and delay the inevitable. The program recommended will depend on your location, the number of horses on the property, their age, the amount of pasture they have, the frequency with which they travel to shows and how often they get on the property. Horses that shed a lot of strongyle eggs will be identified by their fecal egg count. Horses with high levels of strongyle eggs may require deworming more frequently than others. Horses that are less susceptible may only need to be dewormed once a year.

Manure must be removed from paddocks at minimum twice per week as part of an effective fly control program. (c) Dusty Perin

An Old Problem Resurfaces
Experts don’t know why summer sores seem to be becoming more common in certain areas. These are the top theories:

Resistance. Parasites can develop resistance by constant use of the same dewormer. Is Draschia and Habronema developing resistance to ivermectin or other drugs in its class? Dr. Pugh said that “we don’t know if it is happening.” It has not been documented by anyone.

Deworming methods have changed. Other dangerous parasites have developed resistance to common dewormers. In order to counter this trend, the American Association of Equine Practitioners published guidelines in 2013 that recommended a targeted, individual approach to deworming. This new approach focuses on parasites that pose the greatest threat to horses’ health, such as small strongyles. It also requires longer intervals between dewormings. Is Draschia and Habronema making a comeback by taking advantage of longer intervals? It’s possible but Dr. Pugh states that “We started to see cases before AAEP advocated for the new protocols.”

More horses today are concentrated than scattered over large areas. (c) Frank Sorge/

Weather. Warmer weather has been arriving earlier and staying around for longer periods of time in many areas of the country over recent years. Warmer temperatures mean a longer fly season which gives flies more chances to breed and produce offspring.

Management. Dr. Pugh believes that weather may play a role, but changes have been made to how horses are managed over the past 40 years. Instead of being spread out, horses are more concentrated in urban and suburban stables. These situations are conducive to fly populations increasing. He says that more summer sores are caused by more flies.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!