Horse Bot Flies: Welfare and Control

Bot flies deposit eggs on horses' legs and mane in late summer. Larvae are ingested and develop in the stomach, causing welfare harm that is easily preventable through targeted treatment.

Key Facts

Horse bot flies (Gasterophilus species) lay yellow eggs visible on horse hair in late summer and autumn
Horses lick or bite the eggs, triggering larval hatching and ingestion
Larvae attach to the stomach lining and oral mucosa, potentially causing gastric ulceration and irritation
Heavy burdens can cause colic signs, weight loss, and anal irritation as larvae pass
Treatment with ivermectin or moxidectin after the first frost kills all larvae effectively
Manual removal of eggs with a bot knife or warm water is labor-intensive but reduces larval numbers ingested

Welfare Considerations

Bot fly larval attachment to stomach and oral mucosa causes irritation and potentially contributes to gastric ulcer formation in heavily infected horses. The egg-laying behavior of adult flies causes behavioral disturbance in summer — horses stamp, swish tails, and show agitation when bot flies are active. Welfare-positive management includes one effective anthelmintic treatment after the first killing frost (when adult flies have died and all eggs would have been laid), along with regular egg removal during fly season. The welfare harm of bot flies is modest compared to other parasites but is easily prevented.

What You Can Do

Apply ivermectin or moxidectin paste after the first autumn frost to kill all larval bot flies
Remove bot fly eggs from legs, mane, and shoulders regularly during summer using a bot knife or warm cloth
Avoid treating before frost — treating too early leaves horses vulnerable to later egg batches
Monitor horses for colic signs during summer if bot fly pressure is high in your area