Summer Sores in Horses


Habronema muscae are stomach worms found in horses. The worms generate eggs and larvae from within the intestinal tract of horses. Habronema larvae are then passed in manure.

In the normal stomach worm life cycle, flies pick up the stomach worm larvae in horse manure and deposit them near the horse’s mouth. The horse ingests the larvae that travel to the stomach and, in approximately two months, mature into adult worms that usually cause very little damage to the horse. The adults lay eggs that are passed in the horse’s manure. Flies pick up the hatched larvae and cycle starts all over again.

The problem occurs when the stomach worm larvae are deposited by flies on mucosal surfaces, or any surface where there is a moist interface. The larvae cannot mature into adult worms, so they migrate around in the area, causing local inflammation and severe itching.

As the parasites dies, the dead larvae serve as a nidus of irritation, creating chronic. granulomatous inflammation. The parasite itself does not migrate far, but the natural inflammatory response of the body causes large lesions.


A biopsy of the affected tissue is required to make a definitive diagnosis.
However, an experienced veterinarian will be able to make a diagnosis based on the clinical appearance, location and behavior of the lesion.

The lesions are usually red, raised masses with a reddened, ulcerated surface. They can range in size from small lesions 5mm in diameter to ones that are almost the size of a baseball! They typically develop near wounds, or the eyes, the edges of the mouth, near the external genitalia, and sometimes around the fetlock or coronary band.


The best treatment is EARLY RESPONSE. First and foremost, contact us as soon as you notice a sore. A small lesion can become very large in just a few days.

Treatment of summer sores can be difficult and may require a number of approaches.

  1. Deworming with either ivermectin or moxidectin. Dewormers not containing either of these ingredients will not be effective. Repeated doses may be necessary
  2. Debulking and /or cryotherapy may be necessary in some lesions.

We recommend a two-prong approach to preventing Summer Sores.
1. Control the causative parasite, Habronema muscae by deworming with either Moxidectin (eQuest) or Ivermecin (eg. Equimax)

2. Control the intermediate host – flies!

Fly control is essential to prevent additional stomach worm larvae from entering the lesions. Remove manure, and other materials at least twice weekly to prevent fly breeding sites and the hatching of fly larvae. Insecticides, fly traps and baits, fly prevention face masks and repellents are all beneficial.

If  you have any questions of any of the information above, or if you suspect your horse may have a Summer Sore,
please contact us on 0427 072 095.