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Bots - Flies, Eggs, Grubs in Stomach

Synonyms: Gasterophilus spp.

Bot flies are bee-like insects that commonly infest horses. The adult fly lays pale, sticky eggs on the hairs of the body, legs and chest of horses. Another species lays eggs around the lips and mouth. These eggs cause mild skin irritation. Horses bite at the skin and ingest the eggs, which develop into fat, orange colored larvae attached to the lining of the stomach.

Bots are mostly not considered very harmful. Claims of perforation and ulceration are often exaggerated. But large numbers of larvae in the stomach probably cause some irritation. One bot species lays eggs around the mouth. This same species larvae also colonize the rectum of horses, and this can cause rubbing and even rectal prolapse. The adult flies are highly annoying to horses when they are laying eggs. Herds of horses bothered by bot flies will sometimes stampede.

Larvae are commonly seen upon veterinary endoscopic examination of the stomach but it is hard to know whether they are causing problems there. Probably not.

Bot flies are active in the summer and early fall in temperate climates. In the tropics and subtropics, they are active throughout the year. The grubs over-winter in the horse’s stomach and complete their life cycle in the spring.

Prognosis & Relevant Factors

Bots are generally not a problem unless very large numbers are in the stomach, or in the case of the species that occupy and irritate the rectum. Claims of perforation and ulceration are probably exaggerated.

Repeated use of deworming compounds to kill bots is probably not necessary, as bots usually don't cause the horse much problem. Repeated use of woming compounds favors resistance among both bots and other parasites. Use fecal egg counts to determine the numbers of parasites your horse has. Let that guide your parasite control program.


  • Given that bots are probably not very harmful to the horse, should I really be using lots of worming compounds to kill them, and potentially worsening the resistance problem?

    Bot egg removal is difficult and probably not worth the work. In temperate climates, deworming with ivermectin or moxidectin after the first few killing frosts (or in the winter) kills larvae in the stomach and rectum and breaks the cycle. Other commercial dewormers do not kill bots.

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    Author: Doug Thal DVM Dipl. ABVP


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