Summer Sores in Horses: Causes, Symptoms, and Effective Management

Summer sores, also known as habronemiasis or cutaneous habronemiasis, are a frustrating and sometimes painful skin condition that occurs in horses during the warmer months. They are caused by parasitic larvae that normally live inside the horse’s stomach but end up deposited onto wounds or moist areas of skin by flies. The resulting reaction can lead to persistent sores that fail to heal and cause significant irritation.

Every horse owner should understand what summer sores are, how to recognise them early, and the best ways to prevent and treat them effectively.

What Are Summer Sores?

Summer sores are skin lesions caused by the larvae of stomach worms, specifically Habronema and Draschia species. These parasites are transmitted by flies, particularly houseflies (Musca domestica) and stable flies (Stomoxys calcitrans), which act as intermediate hosts.

When these flies deposit infective larvae onto broken skin or around moist body openings such as the eyes, mouth, or genitals, the larvae burrow into the tissue instead of being swallowed and developing in the stomach. The horse’s immune system reacts strongly to their presence, causing intense inflammation and the formation of a granulomatous (lumpy) sore.

How the Life Cycle Works

Understanding the parasite’s life cycle is key to controlling summer sores.

1. Adult Habronema or Draschia worms live in the horse’s stomach, where they lay eggs.
2. The eggs pass through the digestive tract and are excreted in the manure.
3. Flies feeding on the manure ingest the eggs or larvae.
4. The larvae develop inside the fly for about one to two weeks.
5. When the fly lands on a horse’s eyes, lips, sheath, or wounds, it deposits the larvae.
6. If swallowed, the larvae mature normally in the stomach. If deposited on the skin, they cause an allergic reaction, leading to summer sores.

Because this cycle depends heavily on flies, the condition is most common in late spring and summer when fly populations peak.

Common Locations

Summer sores can appear anywhere the skin is broken or moist, but they are most often seen on:

• The corners of the mouth and lips
• Around the eyes or eyelids
• The sheath, penis, or udder
• The legs or chest where wounds are present
• Any area with open cuts or persistent skin irritation

Signs and Symptoms

Early recognition of summer sores is important because they can worsen rapidly. Common signs include:

• Raised, moist, red or yellow lesions that do not heal
• Itchy, crusted, or bleeding sores that attract flies
• Thickened, lumpy skin around the lesion
• Presence of yellowish granules (larval debris) within the wound
• Formation of proud flesh (excessive granulation tissue)
• Persistent irritation or discomfort, especially around the face or genital area

If untreated, summer sores can become large and ulcerated, sometimes resembling a tumour or sarcoid.

Diagnosis

Veterinary diagnosis is usually based on the appearance and location of the lesion, along with the time of year. To confirm the diagnosis, a veterinarian may:

• Perform a microscopic examination of discharge or tissue scrapings to identify larvae
• Take a biopsy to rule out other conditions such as sarcoids, squamous cell carcinoma, or fungal infections
• Assess for signs of gastric Habronema infection through faecal tests (though larvae are rarely found in faeces)

Because summer sores can look like other skin diseases, professional examination is essential for accurate diagnosis.

Treatment Options

Effective treatment aims to eliminate the larvae, control inflammation, and encourage healing. The sooner treatment begins, the better the prognosis.

1. Antiparasitic Medication

Systemic dewormers are vital to kill any migrating or gastric larvae. Ivermectin or moxidectin are commonly used and are highly effective against Habronema species.

2. Anti-Inflammatory Therapy

Corticosteroids, either topical or injected, are often prescribed to reduce inflammation and suppress the hypersensitivity reaction caused by the larvae.

3. Topical Treatments

Veterinarians may recommend creams or ointments containing antiparasitic agents, antibiotics, or corticosteroids to reduce infection and promote healing. Some formulations also deter flies from returning to the wound.

4. Fly Control

Fly control is critical during treatment. Fly masks, fly sheets, repellents, and proper manure management can drastically reduce reinfection risk.

5. Surgical or Mechanical Removal

In advanced cases, surgical debridement may be necessary to remove dead tissue, larvae, or proud flesh. Laser therapy may also be used to encourage clean healing.

6. Wound Care

Clean the affected area daily with a mild antiseptic solution and keep it covered when possible to prevent further contamination by flies.

Prevention

Prevention focuses on breaking the life cycle of the parasite and reducing fly exposure. Key measures include:

• Implementing an effective fly control programme, including traps, repellents, and regular cleaning of stables and paddocks
• Removing manure daily and storing it away from horses
• Regular deworming with ivermectin or moxidectin, especially during spring and summer
• Prompt wound management – treat even minor cuts immediately to prevent fly contamination
• Protective gear such as fly masks, fly sheets, and genital covers during peak fly season
• Environmental management, like reducing standing water and decaying organic matter where flies breed

Prognosis

With timely and appropriate treatment, most summer sores heal completely. However, chronic or untreated lesions can leave scar tissue, cause cosmetic defects, or recur during future fly seasons. Horses that have experienced summer sores once are more likely to develop them again due to hypersensitivity to the larvae.

When to Call the Vet

You should contact your veterinarian if:

• A wound fails to heal or worsens during summer
• Lesions appear around the eyes, genitals, or mouth
• You notice moist, red, or ulcerated skin attracting flies
• Your horse seems uncomfortable, rubbing or biting at affected areas

Prompt veterinary care is the best way to stop the cycle and prevent long-term complications.

Conclusion

Summer sores are a seasonal but serious skin condition in horses caused by fly-transmitted stomach worm larvae. They thrive in warm weather and on unhealed wounds, leading to painful and persistent lesions. Fortunately, with good fly management, regular deworming, and quick treatment of wounds, the condition can be controlled and even prevented.

Early recognition and veterinary attention are key to successful treatment. A comprehensive approach combining parasite control, wound care, and environmental hygiene can keep your horse comfortable and healthy throughout the fly season.