Corro Stories

Ask A Vet: What Are Summer Sores & How To Heal Them

By Amy Smith


Horses, while strong, can be quite delicate. As owners and riders, we know that seemingly tiny things can cause big problems—parasites and common flies are no exception. With the advent of dewormers, summer sores aren’t as prevalent as they used to be. However, changing temperatures that extend warmer months seem to be on the rise. Here is the 411 on all things related to summer sores. 

What are summer sores?

Image courtesy of Dr. Laura Stokes-Green

Sometimes referred to as “fly sores,” summer sores, or Cutaneous Habronemiasis as your vet would call them, are “non-healing cutaneous lesions caused by infective nematode larvae that are deposited in the skin by flies,” explains Dr. Heather Crather, DVM with Kopec Veterinary Associates. Basically, it’s an infection of the skin caused by the larvae of a stomach parasite.

These particular parasites are of the large-mouth variety (Habronema and Draschia) and are generally harmless to horses if they live as adults in the horse’s stomach. The problem and the sores arise when these little guys have their life cycle interrupted and don’t make it back into the stomach. “You’ll mostly see them in the spring and summer,” says Dr. Crather, “with the lesions regressing in the winter.”

What causes summer sores in horses?

The infection-causing larvae, produced by the adult worms living in a horse’s stomach, are shed through a horse’s manure. Typically, the larvae would make their way back into the horse’s stomach by catching a ride from a common fly. That fly will land around a horse’s nose and mouth and, sensing the moisture, the larvae bail and find a way to be ingested - life cycle complete.

But, sometimes the larvae get sidetracked. After hitching a ride on a fly, that fly may land on a horse’s scratch, open wound, or moist area of the skin rather than their nose or mouth. The overzealous larvae will sense the moisture and jump ship there, not realizing it isn’t their stop - silly larvae.

These lost little guys will burrow and dig, trying to find their way into the stomach. In doing so, they cause irritation and inflammation in the skin. Further compounding the problem, your horse may rub and bite at the affected area. What was once a tiny scratch is suddenly a painful, oozing sore.

Image courtesy of Dr. Laura Stokes-Green

How do you treat summer sores?

You’ll usually find summer sores on the legs where scratches are common, in an existing wound, around the eyes, and sometimes in the area of the sheath or vulva. So, what do you do if you suspect your horse is dealing with a summer sore? 

1) Call your vet: You’ll want to make sure that it is, in fact, a summer sore. Sometimes they can be mistaken for more serious conditions like proud flesh, skin cancer, or a fungal or bacterial infection. “Any wound that doesn’t seem to heal in a normal time frame, has developed more granulation tissue (proud flesh) than you would expect, or becomes painful, hot, or swollen, it’s time to contact your vet,” says Dr. Crather.  

With limitations surrounding the pandemic, pictures and a verbal description of the situation might be all you need for a diagnosis. However, your vet may ask for a visit in order to scrape the area to confirm the presence of these particular larvae.

2) Address the wound: Utilizing anti-inflammatories, whether topical, systemic, or both, will help reduce discomfort for your horse and slow the progression of tough scar tissue. Slowing the growth of this tissue is important in staving off the need to have the tissue surgically cut away or a potential secondary infection.

3) Ditch the dependents: Similar to the anti-inflammatories, your vet may prescribe a dose of dewormer like Ivermectin or Moxidectin to combat the adult worms in the stomach but this can also be used topically to kill the parasites directly on the sore.

4) Keep bacteria at bay: Open sores pose a risk for infection so it’s a good idea to work with your veterinarian to understand if antimicrobials or antibiotics should be included in your treatment plan. A combination of topical medications including steroids, dewormers, and antimicrobials can be used but “occasionally horses may need oral steroids for refractory cases,” explains Dr. Crather.

Additional treatment tips & tricks

Horseware Ireland Rambo Plus Fly Mask with Ears is great for horses who have summer sores around their eyes

Treating summer sores can be tricky depending on where the summer sore is located on the horse. Dr. Laura Stokes-Green of Steel and Associates recommends using fly masks and fly boots when possible to keep the sore covered, but able to breathe.

Horseware Ireland Rambo Plus Fly Mask with Ears is one of her top recommended fly masks for horses who have summer sores around their eyes or somewhere else on the face that is covered by the fly masks because of the wire-like material and brow band that keeps the mask from touching the horse’s eyes and rubbing the sore. “The mesh on other fly masks can get in their eyelashes, and often with these Habronemiasis on their eyes, you need to use an eye cream as well. So, the band on the Rambo Plus Fly Mask prevents the mesh from getting in their eyes,” shares Stokes-Green.

Rambo Plus Fly Mask with Ears

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When it comes to summer sores behind the knees, Stokes-Green has seen using human leggings to be very effective (yes, you read that right!). “Behind the knees is hard to wrap because the horse is constantly flexing,” said Stokes-Green. She recommends taking a pair of extra-large leggings that are made of cotton or Lycra an cutting off the top of the pants. Think of cutting it like you would if you were turning a pair of pants into shorts, but instead of keeping the top part, you’d keep the tubes of the legs. like you would if you were to make shorts out of a pair of pants).  “Take the largest part of the leg to go towards the top of their leg, and use Elastikon bandage tape – the 4-inch tape is the best – to secure the pant leg around the part that doesn’t flex, so it covers the area where the sore is without pulling on it.”

Fly boots are great, but if you notice that the boot is rubbing up against the sore, you may want to get a little crafty. One of Stokes-Green's clients took fleece liners and added it to the inside of their fly boot to create a gentle barrier between the sore and the boot (see the picture below). 

One of Stokes-Green's clients took fleece liners and added it to the inside of their fly boot to create a gentle barrier between the sore and the boot.

Are summer sores contagious?

While they’re not contagious, “some studies have suggested that Arabians, grays, and dilute colored horses like palominos, duns, and buckskins are more predisposed,” says Dr. Crather. Similarly, “southern states and more humid climates tend to see more cases but as long as flies are present, the potential for summer sores is there.”

How can I prevent summer sores?

The easiest step to take in preventing summer sores is making sure you work with your vet on outlining and maintaining a proper deworming schedule as it’s “useful in stopping part of the life cycle of the nematode, says Dr. Crather, “but it does not address the larval stage living within flies.” The key, Dr. Crather explains, “is environmental management that decreases the fly’ population.”

Whether you’re addressing your own property or cognizant of your barn’s fly management protocols, things like proper manure control and disposal can greatly reduce the chances of summer sores popping up. If manure is whisked away, so is the shed larvae. Additionally, if there’s no manure, the flies that carry the larvae back to the horse won’t have an excuse to hang around.

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