Summer Sores - Strange round marking on horse lips - What are they?

“Summer sores” is a horseman’s term for a skin condition caused by larvae of Habronema or Draschia species worms.

The adult worms of these species live on the inside wall of the equine stomach and do not migrate internally. In fact, most horses with gastric habronemiasis or Draschia species infestation do not show any clinical signs, and actual damage or impact to an adult horse is thought to be minimal. (One of the reasons most people don't believe the sores come from worms)
Heavy infestation in some individuals, however, has been known to cause stomach irritation and can even lead to perforation and possible stomach rupture. 

The normal cycle for Habronema or Draschia species worms begins when the stomach worms’ eggs are excreted in the manure and into the environment. These embryonated or developing eggs are ingested by the larvae of various types of flies. House, stable and face flies are all commonly associated with Habronema and Draschia species egg development since these flies start as larvae in manure piles.
That's why it is always best to remove all manure piles from our fields !

Fly larvae grow and develop, and the adult flies serve as both incubators and carriers, allowing the stomach worm eggs to develop into more advanced larval stages inside the flies that then transport them to re-enter the horse. When these infected flies land near a horse’s lips, the larvae are released and swallowed by the horse, completing their life cycle.

So what are these round wounds you see? 
The specific skin condition known as “summer sores” occurs when stomach worm larvae are deposited on injured or irritated skin tissue or mucous membranes. Moist areas of the body—such as the eyes, commissure of the lips, ears, ventral abdomen, prepuce, penis and urethral process—are at risk.
Yes, not only lips are prone to these ! 

Areas on the limbs, especially from the fetlock to the coronary band, are frequently prone to mild cuts, scrapes and trauma and thus can also be susceptible to summer sores.

Parasites (e.g., ticks, flies) also can irritate the horse, and the animal’s subsequent rubbing and scratching can damage skin, allowing entry to Habronema or Draschia species larvae.

This is an abnormal step in the usual life cycle for these worms and where problems begin.
These “out of place” larvae cannot grow into adult worms in these locations but can induce a severe local inflammatory reaction characterized by intense swelling, ulceration, redness and itching. These lesions tend to grow rapidly and usually cause horse owners to seek veterinary advice within a few days.

Summer sores, or, more correctly, cutaneous habronemiasis or Draschia species infestation, can occur in all equine species (horses, donkeys, mules and zebras) and are most prominent in parts of the world with a tropical or temperate climate.

Lesions associated with proud flesh, or proliferative granulation tissue, can look similar to summer sores, and both problems tend to occur after skin trauma or injury. Various skin tumors, such as sarcoids or squamous cell carcinomas, can also look like summer sores.

So not every round marking you see are Summer Sores ! 

Also included on the differential diagnosis list would be various fungal skin infections, foreign body granulomatous reactions and pythiosis. This is why you should contact your veterinarian for advice prior to treating any lesion in question.

The best diagnostic methodology requires deep tissue biopsy of the lesion for histopathologic confirmation of the larvae. This is sometimes difficult because of the location of lesions and because larvae are not always easily recovered from even active lesions. Therefore response to treatment is often relied upon.

Treatment usually includes larvicidal, anti-inflammatory and antimicrobial components.
Ivermectin has commonly been used to kill both the adult worms in the stomach and the larval forms in the skin tissue. A single dose of ivermectin is generally sufficient to kill Habronema species larvae. But some horses don’t respond or they become reinfected, necessitating multiple doses or the use of moxidectin. Ivermectin resistance has also become an issue.

Corticosteroids reduce the inflammatory hypersensitivity reaction, and antimicrobials treat secondary infections, usually a result of self-inflicted biting, rubbing and itching. Refractory lesions may require extensive surgical debridement to adequately remove degenerative larvae within the tissue. In terms of prevention, fly control, manure removal and the quick and appropriate treatment of skin wounds and infections can greatly reduce the incidence and severity of summer sores.

Please always consult with your veterinarian should you have questions and/or concerns.