The Bot Fly

Bot Fly Gastrophilus intestinalis; G. nasalis; G.haemorrhoidalis

Most horses in the U.S. are infested with one to three different species of Bot Flies sometime during the year. Bots may damage horses directly and indirectly, the adults resemble a honey bee and may cause some horses to hurt themselves or their handler while trying to flee from the egg laying adult. The larval stage also may cause a variety of digestive tract problems.

Bots mate in the vicinity of horses and lay eggs while in flight by darting in and laying a pale yellow or grayish egg, usually attached to the forelegs (primarily G. intestinalis), but can be found in other areas. Adult females may lay 500-1000 eggs. Incubation is about 5 days, after which they are stimulated to hatch within a few seconds from the warmth and moisture of the horse’s tongue and lips.

An adult Bot Fly laying an egg on a horses’ leg.

All species penetrate the soft tissue of the horse’s mouth where they spend approximately 3 weeks, which causes pain and irritation to the horse’s mouth, causing rubbing. They then migrate to the stomach or small intestine where they attach and feed for several months.

In the digestive tract severe damage can be caused in the form of edema, blockage, ulcers, rupture, etc. When conditions become right, they detach and pass out with the feces. Pupation takes place usually in the top layer of soil under the dung pile. Adults emerge two weeks to two months later and repeat the process.