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The Culprit Behind Summer Sores in Horses: Habronema

July 17, 2021 3 min read

The Culprit Behind Summer Sores in Horses: Habronema

Summer sores may start off as minor circular lesions near your horse's mouth, eyes, genitalia, legs, or even near minor wounds, but they can quickly turn into a full-blown problem. These sores will appear as an ulcer filled with light red granulation tissue, almost like proud flesh. Summer sores, also known as habronemiasis, may contain pieces of calcified tissue and leak red-tinged fluid.The cause of this aggravating and painful scenario to horses and owners alike may be invisible to the naked eye, but causes big issues nonetheless. The culprit? Habronema larvae. 

These small little worms will make you wish you had known how to manage and prevent summer sores in horses a whole lot sooner. Keep reading to learn everything you need to know about the lifecycle of this diminutive little bugger and how you can stop him in his tracks. 

Life as Larvae

The lifecycle of the Habronema worm starts in an infected horse’s digestive tract where eggs laid by adult Habronema worms in the horse’s stomach are passed into the horse’s manure. Here the eggs grow and become larvae. These larvae are invisible to the naked eye and smaller even than the legs of a house fly. This small size makes them the ideal snack for the house fly or face fly maggots developing in the manure. As the maggots unknowingly ingest the larvae, they become an intermediary host for the growing Habronema worm. The larvae continue to develop into the L3 stage while inside the maggot. By the time the maggots are developing their wings, the Habronema inside them are now able to infect their final host (your horse) where they develop into adults. 

Finding a New Home

Now in the L3 stage, the Habronema larvae are ready to find a new home inside a larger host. The larvae are particular about which hosts they can inhabit. Habronema worms cannot infect humans or other livestock and are limited to living inside equids, including horses, donkeys, and mules. The Habronema inside the fly is unable to control where it is deposited on the horse. If it’s lucky, the fly will deposit the larva near the mouth of a horse where it is swallowed. If it makes it into the digestive tract successfully, the larva will mature into adults in about eight weeks and continue the cycle of infection. 

For the most part, Habronema worms living in the stomach of your horse will have no clinical signs. Many horses are carrying Habronema worms with no detrimental effects. Without any signs of infestation, the horse owner is none the wiser until a summer sore develops. However, if the horse has a particularly heavy infestation, it may develop gastritis: inflammation of the mucosa of the stomach wall. 

A Lost Little Worm

So, where does the problem start? Summer sores occur when Habronema larvae become a little lost after being deposited by the fly. If a Habronema worm is deposited on the horse’s lips, eyes, genital region, or near a wound, the larva will burrow into the tissue in search of moisture. This causes significant irritation and inflammation in the form of an oozing ulcer. The severe itching from the movements of the larva often leads to self-mutilation, as the horse attempts to find relief by biting at the area. Unless managed successfully, the summer sore will only become worse as the worm continues to burrow, causing more irritation, and creating a cycle of itching and biting. Over time, proud flesh and, when lesions are on the limb, lameness can develop, as well as secondary infection. 


How to Manage Summer Sores in Horses

Successful management of summer sores in horses relies entirely on your approach, how quickly the lesion was noticed, and when care was started. Like most issues in horses, early detection is key to a fast recovery. Researchers have found that Habronema are susceptible to a deworming protocol that uses a macrocyclic lactone treatment. Deworming your horse will kill the adult Habronema living in your horse’s digestive tract and the larvae that’s causing the sore. However, for larger summer sores with proud flesh, further care is needed.

Managing summer sores in horses has historically required extensive debridement surgery by a veterinarian, followed up with the use of non-competition safe corticosteroids. Now, there’s an option that many horsemen have found to be effective. Zarasyl equine barrier cream has been used successfully by veterinarians in management of summer sores. The topical cream can be easily applied by a horse owner, barn manager, or stable hand. Non-irritating and non-toxic,  Zarasyl is odorless, non-volatile, water miscible, and chemically stable with a competition and race-day safe formula. This equine barrier cream provides sustained delivery of orthosilicic acid to the skin, which is the bioavailable form of silicon associated with healthy connective tissue growth.

If your horse is struggling with a summer sore, try Zarasyl equine barrier cream. Learn more about Zarasyl here.