01/06/2023
Summer sores
Habronema Muscae, Habronema Microstoma and Drashia Megastoma are small usually harmless worms that live within the stomach of the horse. They rarely make headlines as infection seldom causes any clinical signs – that is until larval activity goes awry.
Summer sores can affect all equines and most commonly occur in places with a tropical or temperate climate. Here in the UK they’re quite unusual but certainly not unheard of, especially in the Summer, hence the name.
Habronema or Drashia larvae cause a localised inflammatory reaction in the skin, creating granulation tissue in a circular red, weeping sore. They can occur anywhere on the body but are most common on the lips and face, legs.
Like all intestinal parasites, the adult worms shed eggs onto the pasture via the horse’s dung. The eggs hatch and if luck is in their favour, they get ingested by the maggots of the house and stable flies that are laid alongside them in the faeces.
The worm larvae develop inside the maggots as they mature into adult flies. These gravitate back to the horse to feed off secretions of the nose, mouth, eyes, under the belly and any open wounds. When the timing is right the larvae aim to jump ship and get deposited around the mouth of the horse so that they can find their way back into the horse’s stomach to continue the lifecycle.
Summer sores occur when the larvae are deposited where there is moisture and fruitlessly try to burrow into the skin tissue, resulting in a raw, swollen lesion.
Summer sores are very unlikely to heal of their own accord without intervention. A single dose of Ivermectin is effective for both adult Habronema worms in the stomach and the larval forms in the skin tissue. In some cases, steroids and antimicrobials may be prescribed alongside this by your vet to reduce inflammation and treat secondary infections - usually a result of self-inflicted biting, rubbing, and itching. Good fly control goes hand in hand with the treatment and prevention of Summer sores.
