"Bots" is the term used for a common group of insects that parasitize horses to complete their life cycle. Parasitisation of equines by insects is called "mydriasis". The scienific names for the common bots are Gasterophilus intestinalis, G. nasalis, and G. haemorrhoidalis. Bot flies lay their eggs by attaching them to the legs, chest, and face of horses. These hatch out as infective larvae that by way of the mouth make their way to the stomach and intestines of horses where they can do damage and have been associated with stomach rupture. This article discusses the life cycle, diseases, treatment, and prevention of this parasite including photos of all the life stages.

Adult flies are brown, hairy and bee-like, with one pair of wings, and measure about 3/4 inch. They may look like small honey bees flying around the head or legs of horses, occasionally going in and touching the horse to deposit their effes. The intestinal stage, or bot, is also 3/4" long with a narrow hooked anterior end and a broad rounded body. G. nasalis larvae can be distinguished by their single row of spines per segment, compared to a double row of spines on G. intestinalisand G. haemorrhoidalis. In large infestations they may occasionally be seen in the stools.

The Life Cycle of Bots

In the late winter or early spring, larvae are released from the stomach and pass onto pasture in the feces. They burrow into the ground and pupate. Adults emerge in three to 10 weeks, depending on temperature.

Egg laying begins in early summer. Eggs of the three different species differ in color and placement. G. intestinalis lays pale yellow eggs on the forelegs and shoulders. Moisture and friction from the horse licking itself cause the eggs to hatch in about seven days.

G. nasalis lays its yellow eggs around the chin and throat. These eggs hatch in about a week without stimulation. G. haemnhoidaiis lays only about 150 black eggs around the lips of the horse. These eggs hatch in 2 to 3 days.

After hatching, G. intestinalis larvae are licked into the mouth. G. nasalisani G. haemorrhoidalis burrow under the skin to the mouth. All three species of larvae wander through the mucosa of the mouth for about a month before migrating to the stomach for overwintering.

Larvae overwinter attached to mucosa of the stomach. They can be so numerous that stomach tissue cannot be seen.

With warmer weather, larvae are released in the feces to continue the life cycle.