Information on Fly Strike in Horses

Fly strike can be a deadly problem, it is more common in sheep and rabbits than in horses. Fly strike is a disease that if neglected can lead to a slow and painful death.

What is Fly Strike

Fly strike is more correctly known as myiasis. It is a problem that develops when certain species of flies lay their eggs on a living animal. The larvae hatch within 24 hours and begin to eat their way into the host animal, in this case a horse. Approximately 24 hours later bacteria may start to grow in the open wound created by the fly maggots. It is the bacterial infection that results in septicemia or toxemia, contributing to the animal becoming lethargic, anorexic, and eventually killing the animal.

Concerns in Horses

In horses it takes several flies to create a life threatening situation, but problems should not be ignored even still. Typically the flies will lay their eggs around the horses anus, or vulva. The sheath is another area of concern, as are any areas with open wounds.

Types of Fly Strike

There are three types of fly strike. Obligatory myiasis being caused by the maggots of Cochliomyia, or Chrysomyia, who willingly burrow into living flesh. These flies are of a concern to horses in South and Central America, Africa, and Asia. Facultative myasis is caused by blowflies feeding on dead tissue in wounds including skin made sore by being coated with feces. They do not eat into living tissue but secret enzymes that may create more open flesh. Accidental myasis is when a fly species who normally does not feed on living animals lays its eggs on a wound while eating and its larvae have no other option for their survival.


Botflies will sometimes lay their eggs inside the horses nasal cavity and this can result in myiasis, or fly strike. Botflies typically lay their eggs on a horses leg, hoping that the horse will ingest the eggs when itching with their mouth. The larvae incubate in the horse’s tongue for three weeks before moting to the stomach. In this case the maggots grow up in the stomach and cause problems with digestion and absorption of nutrition, which can be also termed as myiasis.

Signs of Fly Strike

Maggots will be seen in open wounds and in some cases wounds will appear to get worse, rather than better. In the case of an infestation in the sheath, vulva, or nasal cavity, the horse will appear to have discomfort from that area, and the owner should check for maggots.


A veterinarian should be contacted to remove the maggots properly, and to clean out the wound. If the veterinarian determines that a bacterial infection is also a problem, they will want to make sure that is treated.

Prevention of Fly Strike in Horses

Wounds should be kept clean, dry, and bandages should be changed regularly. Application of fly insecticides can be made with caution to use only those safe around wounds, or open flesh. If horses have feces caked on their rumps, as may occur with foal scours, it should be cleaned off. The sheath area should be kept clean, and try at all times.