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Bot Flies (Gastrophilus)



Scientific name: Gastrophilus intestinalis, G. Nasalis Common name: Bot flies Physical description of parasite: Adult flies are brown, hairy and bee-like, with one pair of wings, and measure about 3/4". The larvae (bot) is also 3/4" long with a narrow, hooked end and a broad, rounded body. Stages/lifecycles: After a 3-week developmental period in the mouth, bot fly larvae of both species, G. intestinalis and G. nasalis, migrate and attach themselves to the mucus lining of the horse's stomach and remain there during the winter. After about 10 months, they detach from the lining and are passed out of the body through the feces. The larvae burrow into the ground and mature. Depending on the conditions, adults emerge in 3 to 10 weeks. Adult females deposit eggs on the horse's legs, shoulders, chin, throat and the lips. Depending on geographic location, the life cycle of bot flies is not fixed to only certain times of the year and bot larvae can be active in horses anywhere from August to May. How the parasite enters the horse's system: Egg laying begins in early summer. Eggs of the different species differ in color and placement. G. intestinalis lay up to 1,000 pale yellow eggs on the horse's forelegs and shoulders. Moisture and friction from the horse licking itself cause the eggs to hatch in about seven days. G. nasalis lays about 500 yellow eggs around the chin and throat of the horse. These eggs are not dependent on the horse licking them to hatch. After hatching, G. intestinalis larvae are licked into the mouth. G. nasalis burrow under the skin to the mouth, there wandering through the mouth for about a month before migrating to the stomach for overwintering. Effects of parasite if left untreated: Horses that show no outward signs of illness can be severely infested, giving no clue of the potential damage occurring inside. However, some horses do show signs of infestation, including an inflamed mouth area and stomach irritation. Infestation with bot larvae may cause holes in the stomach lining. If the infestation is severe, the opening from the stomach to the intestines may be blocked, which can cause irritation, ulcers and even colic. The burrowing larvae can cause small tears in the skin, which can become infected. "Dive bombing" adult flies cause nervousness in horses.