During the spring and fall, adult flies of species of the genus Gastrophilus lay eggs on the hairs of horses and ponies. Depending upon species, the eggs are deposited on the hairs of the chest and legs as well as around the head and throat latch. Eggs appear as small seed-like, cinnamon-brown objects attached to the hair. Generally, the horse, in licking the body, ingests the eggs, that are passed to the stomach.
In the stomach, the eggs develop into larvae that are fierce-looking and grub-like. They are usually about three-quarters of an inch long and about three-eighths of an inch in diameter. They possess strong mouth parts by which they attach themselves to the stomach wall of the host. There are rows of spines on the larvae that run around the body like stripes and are very irritating as the larvae roll with the stomach movements. The number of larvae found in the stomach may vary from a few to hundreds. Their presence leads to intermittent colic in the horse or sometimes chronic indigestion. This generally results in the horse not maintaining flesh in accordance with feed intake.
The larvae are passed out in the feces in the spring, where they complete their life cycle under leaves or other trash, going into pupa then to the adult fly stage. The flies attack the horse to lay eggs, thus disturbing the animal. The efforts of the animal to avoid the flies may make it difficult or unpleasant to ride or to work.
For control, check with veterinarian.
