Horse Botfly Welfare: Parasite Prevention and Management

Bot flies are a seasonal parasite welfare concern for horses, with larvae causing gastrointestinal irritation and management requiring timing-appropriate antiparasitic treatment.

Key Facts

Horse bot flies (Gasterophilus spp.) lay eggs on horse legs and faces which are then licked and ingested hatching in the mouth and stomach
Bot larvae attach to the stomach wall and small intestine causing irritation, ulceration, and potentially pyloric obstruction in heavy infestations
Horses show behavioral responses to bot fly egg-laying activity including leg stamping, skin twitching, and attempts to avoid the flies
Ivermectin and moxidectin are highly effective bot treatments that should be administered in late autumn after the first frost kills adult flies
Removing bot eggs from leg hair using a bot knife or sticky tape reduces larval burden from ingestion

Welfare Considerations

Bot flies cause both behavioral stress during egg-laying season through their persistent buzzing and landing on horses, and gastrointestinal welfare harms from larval attachment in heavy infestations. The behavioral disturbance during bot fly season can interfere with riding and training as horses react to fly activity. Autumn ivermectin treatment timed after the first frost eliminates the annual larval cohort before significant gastrointestinal attachment causes welfare harm. Regular bot egg removal from leg hair during fly season reduces the next generation of larvae ingested.

What You Can Do

Administer ivermectin or moxidectin treatment in late autumn timed after the first killing frost for effective bot control
Remove bot fly eggs from horse legs and faces regularly using a bot knife or adhesive tool during fly season
Provide fly protection during peak bot fly activity to reduce behavioral stress from egg-laying behavior
Include bot control in annual parasite management programs developed with equine veterinarians