Published: August 2021 | Updated: June 2023
Your horse has developed a sore on his lower abdomen that just won’t go away. He’s constantly scratching at the area and it begins to look worse by the day. What is it? Don’t rule out a summer sore, technically known as habronemiasis.
Though these lesions have been rare in recent decades due to advances in parasite control, they’re beginning to make a comeback, unfortunately. And the problem with summer sores is that, unlike some types of lesions that a horse can develop, this one won’t typically go away on its own; intervention is definitely needed.
What Causes Summer Sores on Horses?
At first glance, summer sores (also known as granular dermatitis or jack sores) may look like any other type of sore that a horse can develop or it might even look like a sarcoid, but upon closer inspection, you’ll find several differences.
Equine summer sores typically appear as visible granulation containing small yellow, rice-like protrusions within the skin with reddish or pus-like discharge around the area. They are not your typical lesion because summer sores result from a complex association between the horse, stomach worms (Habronema), and the stomach worm’s intermediate hosts: house, face, and stable flies.
Normally, flies pick up stomach worm larvae in horse manure, old bedding, or other rotting organic material around the barn or pasture and deposit them near the horse’s mouth. The larvae are then ingested by the horse where they travel to the stomach, mature into adult worms (causing little damage to the horse, interestingly enough), and lay their eggs, which are then shed in manure.
However, stomach worm larvae can actually do more damage when flies deposit them around wounds or moist areas on the horse, such as the prepuce, lower abdomen, or corners of the eyes. When deposited in these locations, the larvae have essentially reached a ‘dead end’, but they are able to survive if moisture is present.
In an attempt to continue their life cycle, the larvae migrate into the horse’s skin tissue where they cause localized inflammation and severe itching. The horse will naturally chew on or scratch the area, leading to a lesion that will worsen over time and may last for years.
Summer sores may also lead to the formation of proud flesh, especially around existing wounds. This is often the result of irritation and a horse’s hypersensitive reaction from the larvae.
If left untreated, summer sores may improve in winter, but then they will return in the warmer months. As noted before, summer sores appear to be making a comeback, in part due to warm weather that arrives earlier and stays longer than in years past.
Other reasons for the recent uptick in occurrences could be related to changes in farm management, including more dense concentrations of horses in some areas, coupled with lax manure and fly management programs.
How to Get Rid of Summer Sores on Horses
Treating summer sores often requires a multi-faceted approach. Because these lesions are caused by a reaction to stomach worm larvae, they won’t respond to standard topicals intended for wounds or abrasions, and they also won’t typically heal on their own.
First and foremost, the parasite larvae must be targeted.
For smaller sores, deworming the horse with ivermectin or moxidectin will kill worm larvae and usually allow the sore to heal. However, with more severe lesions where proud flesh has developed, the granulated tissue may need to be debrided or removed surgically before other treatments can be used. In some instances, cryotherapy may be needed to remove proud flesh.
Once the proud flesh has been treated, the horse should be dewormed with ivermectin or moxidectin and a topical treatment mixing glucocorticosteroid and DMSO should be applied directly to the lesion to reduce inflammation and itching. If the lesion is on the horse’s leg, wrapping it can help to protect the wound and prevent the horse from chewing on it. Using fly repellent ointment around a healing summer sore in horses can also help to keep flies away and prevent reinfection from occurring.
For more severe cases, veterinarians often prescribe antibiotics in order to treat any secondary infections that may develop, and corticosteroids may be given to control inflammation.
Preventing Summer Sores
Preventing summer sores primarily revolves around two tactics: fly control and keeping your horse on a regular deworming program. Since flies transmit stomach worm larvae to areas where they congregate on your horse, reducing the fly population on your farm can help reduce your horse’s chances of developing summer sores.
Some tips for fly control include:
- Implement regular clean up of manure and old bedding in stalls in paddocks; compost manure or move it off site;
- Use fly sprays, feed-through fly control products, fly traps, and/or premise pest sprays;
- Use fly predators around your farm; and
- Protect your horse with fly barriers such as face masks, fly sheets, and leg protection.
A regular deworming program is important for many reasons, but it can also greatly reduce your horse’s chances for developing summer sores. As mentioned above though, only ivermectin and moxidectin are effective in killing stomach worm larvae.
Other Things to Know About Summer Sores
Horses living in areas with high temperatures and increased humidity may be at higher risk for developing summer sores, and researchers also believe that some horses might be more prone to summer sores than others.
This could be due to a hypersensitivity to the stomach worm larvae or even a genetic component. However, dewormer resistance cannot be ruled out in relation to the recent rise in cases as well.
If you suspect that your horse has a summer sore, it’s important to have your horse evaluated by your veterinarian for a diagnosis before beginning treatment. Continue to work with your veterinarian on implementing an effective parasite control program in order to prevent future recurrences.
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