Bots Gasterophilus intestinalis and G.nasalis

Bots are flies rather than worms. In addition to the usual nuisance flies cause, bot fly larvae burrow into the horses skin and cause lesions into which infection can occur.

Size Appearance

Adult bot flies are brown, hairy and bee like. The bot fly larvae are 2cm long, reddish orange in colour. The eggs are pale yellow and are laid on the horse’s legs, shoulders, chin, throat and lips.

Location In Horse

The larvae migrate from the mouth to the stomach and remain there for many months.


Some horses can show no signs of infection and so it is difficult to detect.

Symptoms that are shown include:

Ulceration of the mouth, bit shyness, sensitivity
Stomach irritation and ulceration
Tears in the skin that can become infected

Life Cycle

12 months


Ivermectin and moxidectin wormers will protect the horse against bot fly.
The dose should be given in winter after the first frost. This is to ensure that all of the adult flies are killed by the frost and cannot lay any more eggs.

If the frost is after December then treatment should be given anyway to prevent the larvae starting to burrow in the mouth.

Ivermectin is more effective than moxidectin.

Preventative Measures

To Use a bot knife to remove bot fly eggs from the horse’s skin. The eggs are visible and easy to remove.