Summer sores in horses—also known as cutaneous habronemiasis—are frustrating and unsightly skin lesions caused by the larvae of stomach worms (Habronema spp.) deposited by flies into open wounds or moist areas. These equine skin sores can become inflamed, non-healing, and extremely uncomfortable for your horse.

While the term “summer sores” might make any horse owner uneasy, knowledge is your best defense. By understanding how these lesions develop, the role of parasites and flies, and how your horse’s immune system reacts, you can take proactive steps to protect your horse.

In this guide, we’ll cover the root causes of summer sores, effective treatment options, and essential prevention strategies—including proper wound care, deworming protocols, and fly control products. With the right approach, you can keep your horse healthy, comfortable, and free from summer sores all season long.

What to Know about Summer Sores in Horses

• Summer sores are caused by Habronema and Draschia spp parasite larvae deposited on horse’s skin by flies, not just simple wounds or infections.
• Key signs include non-healing lesions with white/yellow granules, intense itching, and common locations like mouth, eyes, and existing wounds.
• Treatment involves deworming medications, topical anti-inflammatories, possible surgery for severe cases, and implementing strict fly control measures.

Understanding Summer Sores in Horses

Summer sores in horses might sound like a minor inconvenience, but these pesky lesions are far more complex. Also known as habronemiasis, jack sores, swamp cancer, or bursati, these troublesome wounds are enough to make horse owners wince. But what exactly are we dealing with here? Summer sores occur when certain microscopic worm larvae, specifically from the Habronema and Draschia species, decide to take an unscheduled detour from their usual life cycle. Normally, these tiny parasites are content to live in a horse’s stomach, rarely causing much fuss. However, when their larvae end up in the wrong place at the wrong time, that’s when the trouble begins. These misplaced larvae cause an intense local reaction, resulting in those characteristic sores that just won’t seem to heal. The parasitic nature of summer sores sets them apart from other equine skin conditions. Unlike simple bacterial or fungal infections, these sores are the product of a complex biological process involving multiple organisms. It’s a bit like an unwanted parasite party on your horse’s skin, with flies serving as the uninvited guests who bring the troublemakers along.

The Causes and Risk Factors of Summer Sores

Habronema and Draschia Parasites

At the heart of the summer sore saga are the microscopic villains known as Habronema and Draschia. These parasitic worms, specifically Habronema muscae, H. microstoma, and Draschia megastoma, are the primary instigators of our equine friends’ discomfort. In their adult form, these worms typically reside in a horse’s stomach, where they generally cause little trouble. However, it’s their larvae that stir up all the fuss when they end up in the wrong place. The life cycle of these parasites is a fascinating, if somewhat disturbing, process. Adult worms in the horse’s stomach produce eggs, which are then passed through the horse’s manure. Once in the environment, these eggs hatch into larvae, eagerly awaiting their next move. This is where our story takes an unexpected turn – instead of finding their way back to a horse’s stomach, some unlucky larvae end up on the horse’s skin, setting the stage for summer sores.

Flies 

Flies play a crucial role in this parasitic drama, serving as unwitting transporters of our troublesome larvae. House flies, stable flies, and face flies are the main culprits. These insects feed on horse manure, picking up the Habronema and Draschia larvae in the process. When the flies later land on a horse, particularly around moist areas like eyes, genitals, lips or existing wounds, they deposit the larvae, initiating the summer sore process.

Weather

Mother Nature herself can contribute to the summer sore problem. Warm weather and extended fly seasons create ideal conditions for both the parasites and their fly taxi service. As our climate changes and warm seasons lengthen in many areas, we’re seeing longer periods of fly activity, which translates to more opportunities for summer sores to develop.

Stabling

The way we care for and house our equine companions can significantly impact their risk of developing summer sores. Dense horse populations, such as those found in many urban and suburban stables, can lead to higher fly populations and increased parasite transmission. Poor manure management practices exacerbate the problem by providing ample breeding grounds for flies and allowing parasite eggs to accumulate in the environment. The usual life cycle of horse Habronema and Draschia is horses ingesting infective larvae which end up in the stomach and in 8 weeks adults start to produce eggs that pass in manure. These eggs mature into infective larvae that stable and house fly maggots ingest and eventually the adult flies with these Habronema/Draschia larvae inside them,  land on the horse and deposit these larvae onto the skin.

Breed-Specific Risks and Susceptibility to Summer Sores in Horses

All breeds of horses, all ages, all sexes can get summer sores. Factors such as a horse’s individual immune response, overall health status, and even genetics may play a role in determining susceptibility. Horses with existing stomach worm infections may be at higher risk, as they’re already shedding parasite eggs in their manure. Additionally, horses with open wounds or moist skin areas may be more likely to develop summer sores when exposed to infected flies.

Symptoms of Summer Sores

Identifying summer sores early can make a world of difference in your horse’s comfort and treatment success. These pesky lesions have a few distinctive calling cards that set them apart from other skin issues:

Lesions

Summer sores often start as small, red, seemingly innocuous ulcerated areas. However, they can quickly evolve into more dramatic lesions. As they progress, these sores can develop into raised, granulomatous masses that may resemble proud flesh – that excess tissue that sometimes forms over healing wounds. In severe cases, these lesions can grow to alarming sizes, with some reported to reach several inches in diameter.

Sulfur-Like Granules

One of the most distinctive features of summer sores is the presence of small, white or yellowish granules within the lesion. These granules, often described as having a rice-grain appearance, are actually calcified tissue resulting from the body’s inflammatory response to the parasites. This feature is considered a telltale sign of summer sores and can help differentiate them from other types of wounds or skin conditions.

Itching and Discomfort

Summer sores are downright uncomfortable for a horse. Affected horses often display signs of intense itching, frequently trying to bite, rub, or scratch the affected area. This discomfort can lead to further irritation and damage to the sore, creating a vicious cycle of itching and wound aggravation.

Where Summer Sores Appear on Horses 

While summer sores can theoretically appear anywhere on a horse’s body, they do have their favorite locations. These parasitic lesions tend to gravitate towards moist areas and mucous membranes, where the larvae can easily penetrate the skin. Common hotspots include:

– Around the eyes and mouth

– The sheath or vulva

– The inner thighs

– The lower legs, particularly in areas prone to small cuts or scrapes.

Additionally, any existing wound on the horse’s body can become a target for summer sores, as these areas attract flies and provide an easy entry point for the larvae.

Progression of Untreated Summer Sores

Left to their own devices, summer sores tend to worsen over time. The progression of untreated summer sores can be quite dramatic: Initially, the sore may appear as a small, ulcerated area that doesn’t seem to heal. As time goes on, the lesion typically grows larger and more inflamed. The horse’s constant attempts to relieve the itching through rubbing or biting can cause further tissue damage and inflammation. In advanced cases, summer sores can develop into large, cauliflower-like masses of exuberant granulation tissue. These can become several inches in diameter and may even require surgical intervention to resolve. Interestingly, summer sores often appear to improve during the colder months when fly activity decreases. However, this improvement is usually temporary, with the sores flaring up again when warm weather returns. This seasonal pattern is another characteristic feature of summer sores and can help in their identification.

How to Diagnose Summer Sores 

Any non-healing wound, especially one that appears during fly season or shows signs of excessive granulation tissue, warrants professional attention. Early intervention can prevent the sore from progressing to a more severe state, potentially saving your horse from discomfort and you from more intensive (and expensive) treatments down the line.

Diagnosing summer sores involves a bit of veterinary sleuthing. There is no blood test for summer sores.Your vet will start with a thorough physical examination, paying close attention to the characteristics of the lesion. The presence of those telltale sulfur-like granules can be a strong indicator, but it’s not always definitive. In some cases, especially with older or more severe lesions, these granules may not be visible. To confirm the diagnosis, your vet may perform a skin scraping or biopsy of the affected area. These samples can be examined under a microscope to look for the presence of Habronema or Draschia larvae. However, it’s worth noting that finding the actual larvae can be challenging, as they often die off quickly in the tissue. Instead, veterinarians often look for characteristic inflammatory patterns and eosinophils (a type of white blood cell associated with parasitic infections) in the tissue samples.

Summer sores can sometimes be mistaken for other equine skin conditions, which is why professional diagnosis is so important. Conditions that may resemble summer sores include proud flesh (excessive granulation tissue), squamous cell carcinoma, sarcoids, and even certain fungal infections like pythiosis (sometimes called “swamp cancer”).  In recent years, more advanced diagnostic techniques have been developed. A polymerase chain reaction (PCR) test can detect genetic material from the parasites, offering a more definitive diagnosis. However, this test isn’t yet widely available in all veterinary settings.

How to Treat Summer Sores in Horses

The cornerstone of summer sore treatment is the use of specific equine dewormers, particularly ivermectin products to get Habronema species such as Ivermectin, Zimectrin® Gold or Eqvalan Paste. Or for Draschia and Habronema, products like Quest or Quest plus which have moxidectin are effective. Ivermectin products are not effective against Draschia.

These medications work to kill both the adult worms in the horse’s stomach and the larvae causing the skin lesions. Your vet may recommend multiple treatments to ensure all parasites are eliminated. In some cases, these medications may even be applied topically to the sore itself for a more direct approach. To address the inflammation and promote healing of the lesion, your vet may prescribe topical treatments. Glucocorticoids, a type of steroid, are often used to reduce inflammation and itching. These may be combined with dimethyl sulfoxide (DMSO), which helps the medication penetrate deeper into the tissue.

Other topical options might include antimicrobial treatments to prevent secondary bacterial infections. In severe cases where the summer sore has developed into a large mass of exuberant granulation tissue, surgical debridement may be necessary. This procedure involves removing the excess tissue to allow for proper healing. It’s typically reserved for cases that don’t respond well to medical management alone. Recurrence is a potential issue with summer sores. Horses that have had summer sores once are often more susceptible to developing them again in the future. This is partly because the adult worms may still be present in the stomach, continuing to produce eggs. Additionally, the horse may have developed a hypersensitivity to the parasite larvae, making future reactions more likely. To minimize the risk of recurrence, your vet may recommend ongoing preventative measures. This could include regular deworming treatments, especially during fly season, and implementing comprehensive fly control strategies. Long-term management is key to keeping your horse comfortable and summer sore-free.

Methods for Preventing Summer Sores in Horses

Parasite Control 

Effective parasite control is crucial in preventing summer sores. Even with targeted deworming programs based on fecal egg counts (FECs), adult horses still require routine parasite protection. To implement a deworming program, regular fecal egg count monitoring using an Equine Fecal Egg Count Test is essential. This involves analyzing a sample of your horse’s manure to determine the number of parasite eggs present. Horses with high egg counts (typically over 200-500 eggs per gram, depending on the parasitologist’s recommendation) are dewormed.

Both Habronema and Draschia eggs are not seen on routine fecal tests. Routine fecal tests will only monitor strongyle and  roundworm eggs so a fecal test showing no Habronema or Draschia eggs does not mean there are no adults of these in the stomach producing eggs. You need to deworm to address these.

Horses with low FECs should be dewormed at least once, and in some cases twice a year. Because horses are natural grazers, they are in constant contact with parasite-contaminated environments—even if they are not actively shedding eggs. Horses with medium to high fecal egg counts will need more frequent deworming, particularly during grazing season when parasite transmission is highest. It’s also important to consider modern management practices: most horses are turned out in herds or rotate through shared pastures. If even one horse in a shared turnout has a high parasite burden, others in the group are at risk and should be treated accordingly. Foals are especially vulnerable and should follow a dedicated foal deworming schedule starting at a young age, as they are more susceptible to roundworms and other internal parasites. A consistent, strategic deworming plan protects not only individual horses but the entire herd.

This method allows for more targeted use of dewormers, reducing unnecessary medication and helping to preserve the effectiveness of these crucial drugs for the future. Fecal egg count monitoring should be performed at least twice a year, typically in spring and fall. However, horses with historically high egg counts or those prone to summer sores may benefit from more frequent testing. Your veterinarian can help you develop a customized parasite control program based on your horse’s individual needs and your farm’s specific situation.

Fly Control

Since flies play a crucial role in transmitting the larvae that cause summer sores, comprehensive fly control is vital for prevention. A multi-faceted approach to fly management can significantly reduce the risk of summer sores. Environmental management is the foundation of effective fly control. Proper manure handling is crucial, as fly larvae thrive in moist organic matter. Removing manure from stalls, paddocks, and high-traffic areas at least twice a week can dramatically reduce fly populations.

For larger operations, consider implementing a manure composting system. Biological control methods offer a natural way to combat fly populations. Fly predators, tiny parasitic wasps that feed on fly pupae, can be an effective addition to your fly control arsenal. These predators are harmless to horses and humans but can significantly reduce fly numbers when released regularly throughout the fly season. Chemical control measures using premise sprays such as Elector PSP Premis Spray or Cylence Ultra Premise Spray can provide additional protection against flies. Feed-through fly control agents contain insect growth regulators or larvicides that pass through the horse’s digestive system and end up in the manure, where they prevent fly larvae from developing. Fly sprays such as Edure Sweat Resistant Fly Spray for Horses and repellents such as Repel-Xpe Emulsifiable Fly Spray Concentrate applied directly to the horse can provide short-term protection, especially during peak fly activity periods. Some horses may be sensitive to certain ingredients, so monitor for any adverse reactions when using a new product.

Natural Fly Repellent Options for Horses

If you’re searching for a natural fly repellent for horses, there are effective solutions that don’t rely on chemicals. EquiShield® FG (Fly Guard) and Sore No-More® Classic Ricochet Horse Spray provide powerful equine fly protection using essential oils like citronella, cedarwood, lemongrass, and peppermint. These plant-based ingredients naturally deter flies while being gentle on your horse’s skin.

Wound Care

Proper wound care is essential in preventing summer sores, as any open wound can become an entry point for Habronema larvae. Promptly clean and treat even minor cuts or scrapes, using an antimicrobial solution such as EquiShield CK Spray or Banixx Wound Care Cream to keep the area clean and promote healing. For larger wounds, consult with your veterinarian about appropriate dressings or bandages to protect the area from flies. Regular grooming can help you spot potential problem areas early. Pay special attention to areas prone to small injuries, such as the lower legs, and address any issues promptly. Keep an eye out for signs of skin irritation or excessive rubbing, which could indicate the beginning of a problem.

Protective Gear for Your Horse

Physical barriers can provide an extra layer of protection against flies and the summer sores they can cause. Fly masks are particularly useful for protecting sensitive areas around the eyes and face. Look for masks that extend to cover the ears and nose for maximum protection. Some horses may benefit from full-face masks that cover the entire head, including the muzzle. Fly sheets can offer body-wide protection, especially for horses that are particularly sensitive or prone to summer sores. These lightweight, breathable coverings prevent flies from landing on your horse’s body.

For additional protection, consider using leg wraps or boots designed to repel flies. When using any protective gear, it’s important to check your horse regularly to ensure the equipment fits properly and isn’t causing any rubbing or irritation. Remove fly masks and sheets daily to groom your horse and check for any developing issues.

Final Tips for Preventing Summer Sores in Horses

Preventing summer sores in horses requires a proactive, multi-layered approach. No single method guarantees full protection, but combining targeted fly control, consistent deworming, and diligent wound management offers your best defense. Regularly inspect your horse for early signs of skin irritation or lesions, and maintain clean, dry living conditions to reduce exposure to flies and parasites.

As we’ve explored, summer sores—also known as equine cutaneous habronemiasis—can become a chronic issue without timely intervention. Partnering with your veterinarian to create a personalized prevention and treatment plan ensures your horse receives the care best suited to their needs. With vigilance, proper hygiene, and consistent parasite control, you can help keep your horse healthy, comfortable, and summer sore-free all season long.