As the hot and humid season arrives in Florida, horse owners are being warned to be on the lookout for signs of “summer sores”, caused by stable flies and an equine parasite.
Summer sores are a massive inflammatory reaction to the larvae of the equine stomach worm is Habronema muscae, which are deposited onto wounds and mucocutaneous junctions including the sheath, eyes and corners of the mouth. This reaction then forms granulomas: the “summer sore”.
Adult stomach worms shed larvae into the environment through the manure of infected horses. These larvae are then ingested by maggots and develop within these maggots as they become adult flies. They reside in the mouthparts of the flies where they are deposited onto the wounds or mucocutaneous junctions of horses causing an infection.
Horses can ingest these larvae by consuming dead flies in feed or water. The larvae continue development in the stomach of the horse and begin laying eggs within eight weeks but cause very little clinical signs in the horse.
Usually, the horses shed the eggs in their manure to contaminate the environment. Some horses are shedders, but even a wormed horse can get summer sores. This is because the larvae that cause the massive inflammatory reaction are actually already dead. It is the body’s immune response to the dead larvae that causes the intense itchiness and summer sore formation. Resolution of these wounds is very intensive.
Veterinarians at Palm Beach Equine Clinic say that some immune responses are greater than others, but the best way to prevent such reactions is an early response.
Debulking is often necessary to remove the dead larvae from the sore. Usually this is aggressive and has to be repeated. The area needs to be covered at all times if possible and a medicated ointment applied daily.
Cryotherapy has also proven successful in some cases, the clinic says. “This involves the freezing of summer sores with liquid nitrogen. Some of our veterinarians have found success using immune mediating products.”
An oral deworming product is often administered to the entire stable of horses when a summer sore is seen in case a horse is carrying the adult Habronema worms in the stomach. Fecal worm egg counts do not detect Habronema larvae so worming should always be a primary step in the control of summer sores.
Fly control is also an important step, with one solution the use of the likes of Solitude IGR, an oral feed additive that acts as an insect growth regulator containing cyromazine.
This prevents the larvae from developing exoskeletons so they cannot become adults. It does not get absorbed by the horse but passes through their digestive tract to be passed in their manure. The maggots then ingest the cyromazine while feeding on the manure and the immature fly will die before spreading Habronema larvae. This significantly reduces the fly population in just 4 to 6 weeks.
Other ways to control flies are less effective but should be attempted. Removing manure as often as possible from the stall and keeping it away from the stable are good ways to limit fly transmission of Habronemiasis. Open wounds should also be covered as soon as they appear.
In the event of stubborn resistant summer sores, the answer is often surgery. Some horses need surgical intervention to remove all of the granulomatous reaction and dead larvae within the summer sore.