In the warm summer months, flies of the species Gasterophilus lay eggs on the haircoats of horses. These eggs, and the flies they produce, are called bots. There are three primary species of Gasterophilus. Flies of the G. intestinalis species lay their eggs anywhere on the horse's body, but most commonly on the shoulders and front legs. G. hemorrhoidalis usually glue their eggs to the mouths of horses. G. nasalis deposit eggs on the bottom part of the lower jaw and throat. G. intestinalis are the most common form of bot flies.
After the eggs have been laid, they will hatch within a few days. The larvae migrate into a horse's mouth, and bury themselves between the molars and in the tongue. The bots will stay there for approximately one month, causing the horse pain and irritation. Eventually, the bots are swallowed and move to the stomach, where they continue to develop for nine to 12 months. From the stomach, the larvae move throughout the rest of the intestines and are passed into the feces. Over the course of the next month, the larvae mature into adult flies. The adult fly's remaining life span is typically about two weeks.
Larvae affect a horse most dramatically when they are living in its stomach, where they hook onto its lining and cause the formation of ulcers. While many horses with bots have no clinical signs, some severely infected horses will manifest symptoms that include gastritis and stomatitis.
