Bot eggs

The bot (stomach worm) is not a worm but the larvae of Gasterophilus flies.

The adult horse bot fly emerges a during the summer or fall season. After the fly emerges from the pupa, it quickly finds a mate, lays the eggs on the horse's coat and, on grooming, the eggs make their way to the mouth where they hatch in mouth as larvae.

Over winter the larvae burrow into the stomach lining (where they spend 8-10 months) and are then passed out via the faeces and will hatch in the summer.

Significant trauma and ulceration of the stomach can occur following infection.

Disease is also caused by the larvae in mouth. Larvae here create borrowing holes, which in turn become infected. The 3rd stage larvae are passed in the faeces and over winters to develop the following summer.

Infected horses can often show no signs, which is why treatment should be given over the winter months to kill them.