Female botflies buzz about their victims, idling in midair long enough to lay eggs. Just where those eggs are laid depends chiefly on the type of botfly. Three species parasitize horse: the common bot (Gasterophilus intestinalis) lays eggs on the horse’s legs, abdomen, flank, and shoulders; the nose bot (Gasterophilus nasalis) drops eggs around the muzzle; and the throat bot (Gasterophilus haemorrhoidalis) deposits its eggs on the underside of the neck and lower jaw. Common bots are most bothersome to horses.
Once botflies lay eggs, mature eggs or fresh-hatched larvae enter the mouth when the horse licks or bites the affected area. They remain in the mouth, usually burrowing in the tongue, gums, or other tissue, for the next month or so, and then molt and migrate to the stomach, where they attach themselves to the stomach lining and remain for 9 to 12 months. Once larvae have fully matured, they detach from the stomach, pass in the manure, and finally develop into flies in soil or dried manure, usually emerging in the summer or fall.
Without proper deworming, horses may develop health problems. Low or moderate infestations are detrimental because bot larvae consume nutrients intended for their host. Severe infestations of bot larvae damage the lining of the stomach, sometimes to the point of ulceration and rupture.
A thoughtful deworming program can virtually eliminate the internal threat caused by botflies. Most deworming preparations contain drugs, known as botcides, effective against bots. Though parasitologists have identified resistance of certain parasites to particular drugs, bots seem to be fully susceptible to currently available anthelmintics.