Summer sores develop from the deposition of Habronema worm larvae in fresh wounds and moist areas of the horse's eyes and sheath by infected house and stable flies. It is most common from spring to autumn when flies ‘worry’ wounds, the face and sheath.

Adult Habronema worms live in nodules in the stomach wall. Eggs passed in the manure are ingested by developing fly pupae. Re-infection and spread is mainly by ingestion of live or dead flies in feed, or when horses lick wounds in which the larvae are present.

Progression of symptoms

Typically, open wounds that start healing well with normal pink granulation tissue become infested with Habronema larvae introduced by house or stable flies. The larvae migrate into the wounds to form hard nodules of raised red-brown tissue.

Similar nodules may form on the conjunctiva of the eyes, lining of the sheath and urethral opening of the penis. The lesions enlarge and spread slowly, and to relieve the irritation the horse will bite or rub the wounds.

Larger nodules may bleed, ulcerate and weep yellowish or clear tissue fluid. The exudate attracts more flies, and if they are carrying Habronema larvae, the size of the lesions increase and the degree of irritation worsens.

Treatment and control of summer sores

Larvae in the summer sore lesions, as well as adult worms in the stomach, can be controlled with a worming paste or liquid such as Equimax, Equest, Ammo or MecWorma, administered at regular worming intervals of 6 to 8 weeks during the high-risk period. Consult your vet for advice on a worming program.

Ongoing control of flies is essential to prevent wound infestation with larvae:

  • Keep wounds clean and dry to promote healing and make them less attractive to flies.
  • Septicide Antiseptic Cream or Cetrigen Spray will repel flies from healing wounds – also apply each morning to the skin surrounding fresh open wounds to reduce fly worry and the risk of summer sores.
  • Stable and yard hygiene is essential to reduce fly breeding. Environmental insect repellants are available that can be sprayed in stables or bedding.
  • Fly veils fitted during daylight for paddocked horses will reduce the incidence of lesions in the eyes.

Daily applications of fly repellant cream or spray to the front border of the sheath, particularly when flies are in plague proportions, are also recommended.

Once infection appears, veterinary treatment to remove large nodules, particularly around the eyes or sheath, may be necessary. Consult your vet if any nodules are of concern.