Bit & Bridle

Equine Summer Sores

Other Names

  • Cutaneous habronemiasis
  • Habronematidosis
  • Granular dermatitis
  • Jack sores

What are Summer Sores in Horses?

Summer sores, known medically as cutaneous habronemiasis, are an intensely itchy and frustrating skin condition caused by the aberrant migration of larvae of the stomach worms Habronema muscae, Habronema majus, and Draschia megastoma into the horse’s skin. Rather than completing their normal life cycle through the gut, the larvae become deposited in skin wounds, moist skin areas, or the conjunctiva of the eye by house flies and stable flies that are acting as intermediate hosts.

The result is an intense, granulomatous tissue reaction as the horse’s immune system responds to the misplaced larvae. The lesions are characteristically itchy, ulcerated, and fail to heal — distinguishing them from ordinary wounds or proud flesh. Summer sores are most prevalent during the warm months of the year when fly activity is high, hence the seasonal common name. Although primarily a warm-climate problem, they do occur in the UK during summer, particularly in horses with poor fly control.

Causes

The life cycle of the stomach worms responsible involves both horses and flies:

  • Adult Habronema and Draschia worms reside in the stomach of the horse, producing larvae that are passed in the faeces
  • House flies (Musca domestica) and stable flies (Stomoxys calcitrans) ingest the larvae from faeces, where they develop to the infective L3 stage within the fly
  • When flies feed around the horse’s mouth, eyes, or wounds, they deposit infective larvae
  • If larvae are deposited in the mouth and swallowed, they complete the normal life cycle in the stomach. If deposited in wounds, moist skin folds, or the conjunctiva, they cannot complete their development and trigger an inflammatory reaction — causing summer sores.

Horses with open wounds, sweet itch, or other skin conditions that attract flies are at higher risk. Horses in areas with heavy fly populations are more frequently affected.

Symptoms and Signs

Summer sores have a distinctive clinical appearance:

  • Ulcerated, granulating lesions that do not heal despite appropriate wound care
  • Intense itchiness — the horse rubs, bites, and traumatises the area constantly
  • Yellow-brown, gritty or caseous material within the lesion (degenerated larvae and inflammatory debris)
  • Characteristic small, hard, yellowish-white granules embedded in the tissue (“sulphur granules” appearance)
  • Common locations: lower limbs and fetlock region, conjunctiva (inner corner of the eye), prepuce and sheath, lips and face, urethral process of male horses
  • Seasonal recurrence — lesions typically worsen in summer and partially regress in winter
  • Fly attraction to affected areas, perpetuating the cycle

Diagnosis

Diagnosis is primarily clinical but can be confirmed histologically:

  • Clinical examination: Characteristic appearance, location, seasonality, and failure to heal despite wound care are strongly suggestive
  • Biopsy and histopathology: Tissue sections show eosinophilic granulomas and, in some cases, identifiable larval fragments. This is the definitive diagnostic method.
  • Response to treatment: A good response to ivermectin-based treatment supports the diagnosis
  • Differentials to consider: Pythiosis (“swamp cancer”), exuberant granulation tissue (proud flesh), sarcoids, squamous cell carcinoma, and fungal granulomas

Treatment

Treatment requires both addressing the parasitic component and managing the granulomatous tissue reaction:

  • Ivermectin or moxidectin: Systemic anthelmintic treatment is essential. Ivermectin (at standard worming doses) kills the larvae within the lesion and reduces the parasite burden in the gut, interrupting the lifecycle. Some cases require repeated treatment at two-to-four week intervals.
  • Corticosteroids: Topical or systemic corticosteroids reduce the intense inflammatory reaction and help shrink granulomatous tissue. Commonly used as combined topical preparations with ivermectin and dimethylsulfoxide (DMSO).
  • Topical combination therapy: A compounded paste or cream containing ivermectin, corticosteroid (such as dexamethasone), and DMSO can be applied directly to lesions
  • Debridement: Surgical trimming of excessive granulation tissue may be necessary for large lesions
  • Wound management: Keep lesions clean, dry, and covered where possible to reduce fly access
  • Fly control: Rigorous fly control is essential throughout treatment to prevent re-infection and new deposits of larvae

Prevention

  • Fly control: The most important preventive measure. Use fly repellents, fly rugs, masks, and stabling during peak fly activity times
  • Wound management: Cover all wounds promptly to prevent fly access. Use wound dressings or bandages during the summer months.
  • Anthelmintic programme: Regular worming with ivermectin or moxidectin reduces the stomach worm burden and therefore the larval supply available to flies
  • Manure management: Prompt removal of droppings reduces fly breeding sites and larval availability
  • Identify high-risk horses: Horses that have had summer sores previously are at higher risk and should be subject to more intensive fly control measures during summer

Prognosis

With appropriate treatment, the prognosis for summer sores is generally good, particularly for lesions caught early. Lesions in the conjunctiva require specialist attention to prevent corneal damage. Chronic, large, or repeatedly traumatised lesions may require more aggressive intervention and take longer to resolve. Recurrence in subsequent summers is common if fly control and worming programmes are not maintained. Lesions involving the urethral process of male horses can occasionally cause urethral obstruction, which requires prompt veterinary attention.

When to Call the Vet

  • A wound on your horse is not healing after two to three weeks despite normal wound care
  • You notice itchy, ulcerated lesions that the horse persistently rubs or mutilates
  • There are lesions on or around the eye — ocular habronemiasis requires prompt treatment to protect vision
  • Lesions on the sheath or prepuce are causing the horse discomfort or interfering with urination
  • The condition recurs each summer — discuss a proactive prevention and treatment protocol with your vet

Frequently Asked Questions

How do I know if a wound is a summer sore or just proud flesh?

Both conditions involve excessive granulation tissue, which makes differentiation difficult on appearance alone. Summer sores are intensely itchy — the horse will repeatedly traumatise the area by rubbing and biting — and typically contain characteristic yellowish granules within the tissue. They may also be seasonally recurrent. Proud flesh does not typically cause significant itchiness. If in doubt, a biopsy provides a definitive answer.

Are summer sores contagious between horses?

Not directly. The infection requires the fly intermediate host. However, horses sharing a yard are exposed to the same fly population, and a horse with high stomach worm burden contributes larvae to that shared fly population. Good herd-level fly control and anthelmintic management reduces the risk for all horses.

Will ivermectin worming paste from a tube work?

Standard oral ivermectin at normal dosing (0.2 mg/kg) does kill Habronema and Draschia larvae and is part of the treatment protocol. For stubborn lesions, your vet may compound a topical preparation containing ivermectin directly applied to the lesion alongside systemic treatment.

Can summer sores be prevented by worming alone?

Worming reduces the stomach worm burden and therefore the larval supply available to flies, but it does not eliminate the risk entirely. Fly control is equally — if not more — important. The combination of regular ivermectin-based worming and rigorous fly control gives the best prevention.