While habronemiasis is often thought of as a southern horse problem, it can occur in horses in other areas as well. Other names for habronemiasis include summer sores, granular dermatitis and jack sores. The problem is most common in the Southeast United States This is a parasite problem that can range from minor to severe.

Credit: Habronemiasis (swamp cancer). By permission. Knottenbelt DC, Pascoe RR, Diseases and Disorders of the Horse, Saunders, 2003

The parasites behind habronemiasis are stomach worms. The three nematodes most commonly named are Habronema muscae, Habronema majus and Draschia megastoma. These are all worms that can live in your horse’s stomach. They pass their embryonated eggs into manure where houseflies and stable flies may ingest them. The larvae then move to the mouthparts of the flies and get deposited on moist areas of your horse while the flies are feeding.

While any horse is susceptible to these parasites, one study showed Arabians at high risk compared to Thoroughbreds. That difference in breed prevalence may be related to how much time the horses spent outdoors in warm weather. Temperatures above 70 degrees F are ideal for the flies that spread the larvae.

Moist areas include around the eyes, mucous membranes near the mouth or around the penis of a stallion or gelding. Larvae deposited near your horse’s mouth will get swallowed and go on to complete the life cycle. Any skin abrasions or open wounds are also potential sites for the larvae to set up housekeeping. The larvae do some damage directly simply by migrating into tissues and causing irritation. However, much of the reaction is from the horse’s own immune system responding to the invaders.

The hyperreactivity reaction leads to granulation tissue (think proud flesh) and delayed healing of wounds. The proliferative granulation tissue may contain small yellow granules that are actually the remains of the larvae. Any of the areas of habronemiasis may be ulcerated, bleed easily and stimulate your horse to itch and rub. Of course, the rubbing and itching leads to more tissue damage and more sites for potential larvae deposits.

The three big areas that habronema parasites and their larvae damage are around the eyes, on the skin and in the stomach. The three areas require different strategies for treatment and prevention.

Ocular habronemiasis generally has lesions on the conjunctiva (moist tissues around the eyeball), on the third eyelid itself or on the eyelids. Owners may first notice what appear to be ulcerated growths, especially in the corner of the eye. Again, horses often rub these areas and make things worse. Applying corticosteroid ointment may help reduce the reactivity for small and fairly fresh lesions. Chronic sores may require surgical removal. Prevention is by far the best way to go.

Preventing habronemiasis around the eyes means strict fly control. A properly fitted fly mask can be ideal. If your horse won’t tolerate a fly mask, consider the “roll on” fly repellants around the eyes. These will need to be applied at least once a day.

On the skin, habronemiasis may need to be differentiated from cancerous growths, proud flesh and fungal infections. The only way to do this for sure is by a surgical biopsy. Occasionally a skin scrape of the area may reveal larvae when examined microscopically.

Obviously fly control is the best way to prevent habronemiasis on the skin as well as around the eyes. Most flysheets don’t cover the whole body and certainly the prepuce and penis are usually left exposed to potential fly visits. The granulomatous sores on the skin may regress on their own somewhat during the cold months but without treatment, may recur with warmer weather. Since the horse’s own body contributes to the reaction, some individual horses may have more of a reaction that others.

Use of fly sprays, fly repellants and environmental modifications can all help. Be sure to put fly spray around your male horses’ prepuces. Encourage bug eating birds like purple martins. Consider use of fly parasites – sort of like fighting fire with fire – that lay their eggs in fly pupae and thereby destroy the fly larvae before they hatch. Practice good manure management – cleaning stalls and pastures frequently – dragging pastures, etc. Put fly strips around your barn and consider the use of fans to keep flies away from the horses.

It is important to make sure any open wounds are covered to prevent fly damage and larval deposition. A heavy ointment may suffice or you may need to bandage or cover the area physically.

The sores found on the skin may respond to symptomatic treatment such as corticosteroid ointments, antibiotic ointments and cleaning. Systemic steroids and anti-inflammatory medications may help. Silver colloid ointments have also been used successfully. Many end up requiring surgical removal or cauterization to kill the larvae and halt the granulation buildup. 

The third area where these nematodes can cause problems is in your horse’s stomach as they drain nutrients. This is generally a minor problem but attacking the worms here can help to prevent the other manifestations. Ivermectin has been the dewormer of choice but moxidectin also appears to be effective.

If you find habronemiasis infections on horses in your area, you need to be prepared for yearly battles. Fly control and proper management of manure are important with or without habronema parasites. Having a preventive plan in place along with scheduled dewormings, as needed, can minimize your horse’s risk of developing this problem. Try to do a daily visual and hands on check of your horses. As with so many health problems, if this condition is detected early, treatment is generally more successful and less expensive – a win/win situation all round!

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Article by Contributing Veterinary Editor Debra M. Eldredge DVM.

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