Habronematidosis or habronemosis, more commonly known as summer sores, is a parasitic skin infection that can occur in horses and donkeys. In some areas, these sores are called granular dermatitis or jack sores. [1]

As the name implies, summer sores usually occur during the summer months. This is because they are associated with biting flies which proliferate during this time of year.

A summer sore is a non-healing wound that may spontaneously disappear in the winter and reappear during the warmer months. Horses around the world can be affected by summer sores, but they most commonly affect those living in tropical, subtropical, and temperate regions.

Summer sores are rare in cooler climates, but they can still occur under certain circumstances. Infection rates in horses are increasing, possibly due to increased range and activity of biting flies related to climate change.

What Causes Summer Sores in Horses?

Summer sores can occur in all equids including horses, donkeys, mules, and zebras. In equids, they are caused by parasitic stomach worms, specifically the following species: [1][2]

  • Habronema microstoma
  • Habronema muscae
  • Draschia megastoma

Stomach worms have an indirect life cycle with equids as definitive hosts and flies serving as intermediate hosts.

As their name implies, stomach worms normally take up residence in the horse’s stomach. Eggs are shed by adult worms in the horse’s gastrointestinal tract and passed in feces where they are ingested by fly larvae. [3]

Infected adult flies feed on discharge around the horse’s mouth, where they release nematode larvae. The larvae are swallowed by the horse and complete their life cycle, maturing into adult worms in the stomach. [3]

If larvae are deposited near the horse’s nasal cavity, they may migrate to the lungs, causing a condition known as pulmonary habronemiasis. [4]

When infected flies release parasite larvae near the horse’s eye, genitals, or around wounds anywhere on the body, they can penetrate the skin. As a result, the larvae’s development stops, but a hypersensitive inflammatory process occurs, leading to a summer sore. [3][5]

Symptoms of Summer Sores

Summer sores cause a local inflammatory reaction that is painful and itchy. Ulcerated and non-healing lesions develop, which can appear as cancer-like masses that attract more flies. This can lead to super-infections that are difficult to resolve. [5]

Summer sores often occur in injured parts of the horse’s body such as legs, the sheath, coronary band, inner eye, male genitalia, and under the belly. The most common place to find summer sores is on the: [1][5]

  • Chest
  • Fetlocks
  • Inner sides of the legs

Summer sores often have a greasy appearance with blood-tinged fluid draining from them. They may also contain yellow or white calcified material. [6] In some cases, sections of worms can be seen within the lesion. [1]

Left untreated, they usually disappear during the winter months, but often reappear in the spring. [6] Researchers are not sure if the parasite larvae remain in the lesion in a dormant state during winter and reactivate during warm weather or if the wounds are reinfected during the subsequent fly season. [1]

When worm larvae are released in or around the eyes, horses may also develop conjunctivitis, inflammation of the eyelids, thick discharge, dermatitis with sensitivity to light, and tearing of the eyes. [1] This phenomenon is less common than other forms of Habronema infection. [1]

When equines have lesions on the genital areas, they may have frequent urination due to fibrosis of the area. [1]

Forms of Summer Sores

Clinical signs of summer sores can range from mild to severe. They may appear as a single lesion or multiple lesions. Horses can also develop several forms of summer sore lesions including: [1]

  • Dry: generally circular and hairless, covered by grayish scales
  • Wet: Associated with discharge and clumping hair
  • Edematous: Hairy and without a regular shape, characterized by swelling and small nodules

Diagnosing Summer Sores

Diagnosing summer sores in horses can be challenging for veterinarians. This is because the lesions can appear similar to those caused by other skin diseases such as botryomycosis, pythiosis, phycomycosis, onchocercosis, sarcoids, and squamous cell carcinoma. Diagnosis may be more difficult if both sarcoids and summer sores occur simultaneously. [1][3]

Horses are usually diagnosed with summer sores based on clinical history, physical examination, impression smears, and biopsy. [4] Most veterinarians take tissue samples from the sores, analyzing them to detect worm larvae.

Definitive diagnosis is challenging as larvae may be difficult to detect if they are mineralized or necrotic (dead). [1][4] Larvae are only found in about half of all cases because the larvae can only live for a few weeks in skin tissue. However, dead larvae can cause more tissue damage than a living parasite. [1]

Horses usually require sedation during the skin sampling procedure. One possible complication is that tissue sampling may worsen the ulcerated area, further delaying the healing process. [3]

Molecular testing is the best way to diagnose summer sores if larvae are not visible or found in a biopsy. This involves a polymerase chain reaction (PCR) test, which can identify DNA from larvae within the skin sample. PCR testing can also diagnose stomach worms living in the horse’s gastrointestinal tract. [1][4][5]

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Treating Summer Sores

Treating summer sores can require several different approaches to eliminate the parasitic infestation and support skin healing. Always consult a veterinarian to determine the best treatment protocol and before administering medications to your horse. Closely follow directions on the medication’s label and adhere to the dosage prescribed by your veterinarian.

Treatment strategies for summer sores can include anthelmintics (dewormers), wound cleaning, corticosteroid drugs, topical products, bandaging, antibiotics, surgery, cryotherapy and fly control.

Anthelmintics

For horses with smaller skin lesions, treatment with ivermectin or moxidectin paste dewormers may kill any living worms and allow the lesion to heal. [6] These dewormers are usually given orally. However, in some cases they can be applied directly to the lesion as well.

Ivermectin and moxidectin belong to a class of anthelmintics called macrocyclic lactones, which are products or chemical derivatives of soil microorganisms belonging to the genus Streptonyces. These dewormers treat stomach worms found in the gastrointestinal system, lungs, and skin. Other dewormers that do not belong to the macrocyclic lactone class are not effective in treating summer sores. [7]

Historically, macrocyclic lactones have had a high degree of efficacy against Habronema muscae parasites. However, due to growing anthelmintic resistance, other treatments may be needed as well. [1]

For example, one study showed that a single dose of ivermectin did not appear to be effective in treating summer sores. Additionally, some researchers have found that treating with ivermectin may worsen lesions around the eye. [1]

Cleaning

Summer sores should be kept clean to encourage healing. Gentle cleansing of the skin with a mild antiseptic soap and then drying is recommended. Avoid over-scrubbing the area as this may cause more damage and delay healing of your horse’s skin.

Corticosteroids

Corticosteroids such as dexamethasone may help reduce inflammation and hypersensitivity reactions caused by summer sores, especially around the eyes. These medications may be administered topically, injected directly into the lesion, or, in some cases, injected into the eye, depending on your veterinarian’s recommendation. [1]

Topical Treatments

Topical treatments can be applied to your horse’s skin to promote healing and provide a barrier against flies to prevent further infection. Topical treatments used for summer sores include: [3][6]

  • Glucocorticosteroids
  • DMSO
  • Topical nematode larvicideal compounds
  • Antimicrobial products

Bandaging

If the summer sores are on the horse’s legs, bandaging may help to improve healing. Properly applied bandages protect the wound and keep the horse from chewing on the limb lesion. Bandages also prevent flies from further aggravating the area or depositing more parasites. [6]

Before applying bandages, check with your veterinarian and make sure to follow proper bandaging techniques. Ensure the area stays dry to avoid further skin irritation. As part of best wound management practices, check the lesion daily, apply treatment if needed, and re-bandage.

Antibiotics

In some cases, summer sores can become infected with pathogenic bacteria. If a bacterial infection occurs, your veterinarian may prescribe antibiotics to clear the infection. [6]

Surgical Debulking

Although smaller lesions may heal with deworming and minor treatment, others may progress into large growth with proud flesh (exuberant granulation tissue). If this occurs, surgical debulking may be necessary. In this procedure, excess tissue is removed so the lesion can heal. [3]

Cryoth