Scientific name: Gastrophilus intestinallis, G. Nasalis
Common name: Bot flies or bots

Description: Bot flies are brown, bee-like flies that measure about 3/4 inch (19 mm) in length. During the larval stage they are referred to as bots. Two types of bots are prevalent in horses, G. intestinallis (common bot) and G. nasalis (throat bot). Left untreated, bots can cause gastric ulceration, colic and stomach irritation.

Life cycle: Adult flies, which are usually active from mid-summer until late fall, deposit pale yellow eggs on the forelegs (common bot) chin and throat (throat bot). When a horse licks egg-covered areas, moisture stimulates hatching and larvae burrow into the tongue, gums and other mucous membranes in the mouth. After developing in the mouth for 3 to 4 weeks, the larvae migrate to the stomach and attach to the stomach lining. Bot larvae grow in the stomach for 9 to 10 months before being passed out in manure. Discharged larvae burrow into soil or manure and emerge as adult flies in three weeks. Adult flies copulate and females begin to lay eggs on horses. The life cycle takes one year to complete.

Symptoms: Adult bot flies can be recognized by their “dive bombing” flight patterns. Horses tend to become nervous and excitable when adult flies lay their eggs. Unlike some parasitic eggs, the yellow, bot-fly eggs are visible to the human eye and can be seen in patches on the horse's legs, chest, throat, mouth and neck. Larvae in the mouth can cause ulcers on the tongue and gums. Colic and other signs of gastrointestinal distress are symptomatic of bot larvae irritating the stomach wall.

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