Flies Are More Than a Nuisance: Tips For Preventing Summer Sores 

Kenton Morgan, DVM, Equine Technical Services Veterinarian, Zoetis

Flies are more than a nuisance—these insects can transmit diseases, cause bacterial infections and trigger equine allergic dermatitis. They also leave behind parasitic larvae that can fester into painful, difficult-to-treat summer sores (cutaneous habronemiasis). 

Developing an effective fly control plan to prevent these nonhealing wounds begins with understanding which fly species to target, how they reproduce and how they can irritate your horse. Plus, there’s an added benefit—proactively reducing fly populations that cause summer sores can help prevent other diseases they carry too. 

Know your fly populations 

House flies and stable flies share similarities: They look alike and can be linked to the spread of strangles, equine influenza, Salmonella, summer sores, vesicular stomatitis, E. coli, pigeon fever, and equine dermatitis (Onchocerca). The females of both species lay eggs in decaying organic material like rotting hay, bedding and leaves. These insects are distinguished by their mouthparts: 

  • House flies have sponge-like mouthparts and feed on the mucous and moisture around the eyes and mouth, wounds and genitalia
  • Stable flies have biting mouthparts that pierce the skin like a needle so they can take a blood meal

What causes summer sores? 

Despite their differences, both species can deposit infective larvae of the equine stomach worm (Habronema muscae) or (Habronema microstoma) in open wounds or around moist areas while feeding on your horse. These larvae can then trigger severe itchiness, inflammation and ulceration around the affected site. Proud flesh or granulomatous tissue commonly develops, causing a chronic wound condition that’s difficult to heal.1

  • This condition can be complicated by secondary bacterial infections. These lesions may also contain small, rice-like, calcified structures that add to this chronic irritation.
  • Treatment is difficult and typically includes a combination of topical and systemic medications. In extreme cases, surgery is necessary to remove proud flesh that has formed. Some lesions will appear to resolve during the winter months, only to flare up again as the days begin to warm in the spring and summer. Summer sores rarely heal on their own and are challenging to treat. Call your veterinarian as soon as you notice minor lacerations that don’t heal quickly or if you see any abnormal lesions on your horse. 

The fly lifecycle

Horses pick up equine stomach worm larvae from their feeding activities and then, once ingested, they mature into adults in the stomach of the horse. Once mature, the adult worms lay eggs. Those eggs are shed into the environment in your horse’s manure, where flies feed on the hatched larvae.1

Take preventive steps today

Getting a head start on fly season can help provide the most effective control to prevent immature flies from developing into adults. Flies begin to emerge once temperatures rise above 50 F, so depending on where you live, that could be between February and April. In mild climates, house and stable flies breed year-round, with peak populations occurring during humid months.

  • About one month prior to the start of fly season in your area, it may be helpful to feed Solitude® IGR, an insect growth regulator supplement that prevents the production of chitin. Chitin is a natural compound that house and stable flies need to develop their exoskeletons. So larvae exposed to Solitude IGR in the horse’s manure cannot develop into adult flies and they die off. 
  • The pelleted supplement can be administered after the start of fly season, but it takes a few weeks to “catch up” to the existing fly population. For maximum effectiveness, begin treatment about 30 days prior to the onset of fly season and continue treating all horses on your property daily throughout fly season.

Select a dewormer with your veterinarian

American Association of Equine Practitioners recommends making deworming decisions based on your horse’s age, egg shedding levels and the length of the grazing season in your area. Deworming your horse at the beginning of the grazing season is also an important component in reducing stomach worm populations. Moxidectin, the active ingredient in Quest® Gel (moxidectin) and Quest® Plus Gel (moxidectin/praziquantel) dewormers, works to kill stomach worms that produce eggs capable of developing into larvae and eventually contributing to the development of summer sores.2

A nod to good horsekeeping 

Breaking the fly lifecycle begins with good horsekeeping practices. In addition to using a feedthrough product such as Solitude IGR and deworming your horse, be sure to:

  • Remove wet organic material such as old hay, bedding and leaves
  • Change round bale locations frequently and discard damp leftovers
  • Dump manure as far from the barn as possible and compost it if it cannot be removed from the property
  • Eliminate standing or pooling water
  • Use fly traps, sprays and fly predators
  • Use fly masks, sheets and boots to protect your horse when practical
  • Protect open wounds, even minor ones, to keep flies off the area
  • Understand that having cattle on the property increases total fly populations

Schedule a call with your veterinarian 

Involving your veterinarian sooner rather than later is essential to treating summer sores using the least invasive protocols. What looks like a minor wound can trigger an inflammatory reaction that spirals out of control, becoming a nonhealing wound that lingers or worsens over time. 


Do not use Quest Gel or Quest Plus Gel in foals less than 6 months of age or in sick, debilitated and underweight horses. Do not use these products in other animal species, as severe adverse reactions, including fatalities in dogs, may result. Consult your veterinarian for assistance in the diagnosis, treatment, and control of parasitism.


  1. Lenz T. Summer sores. American Association of Equine Practitioners. https://aaep.org/horsehealth/summersores#:~:text=Summer%20sores%20result%20from%20a,areas%20on%20the%20horse’s %20body. Accessed March 1, 2024. 
  2. American Association of Equine Practitioners. Internal parasite control guidelines.https://aaep.org/sites/default/files/2021- 03/Internal_Parasite_Guidelines.pdf. Accessed March 1, 2024. 

All trademarks are the property of Zoetis Services LLC or a related company or a licensor unless otherwise noted. Bimectin is a trademark of Bimeda Inc. EquiMAX is a trademark of Virbac. IverCare is a trademark of Farnam. Panacur is a trademark of Merck Animal Health. Safe-Guard is a trademark of Intervet Inc. Zimecterin Gold and Eqvalan are trademarks of Boehringer Ingelheim. © 2024 Zoetis Services LLC. All rights reserved. SOL-00037

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