Worms in Depth Series: Bots

What is a Bot?

Bots are the larvae of the Bot Fly (Gasterophilus spp.). The bot fly larvae are approximately 2cm long and reddish/orange in colour.

How do horses become Infected?

Not only are bot flies (small brown, hairy flies) a common irritant to grazing horses during the summer months, the female lays her eggs on your horse’s coat. Tiny yellow stick eggs can often be seen on the forelegs, shoulder or abdomen. As the horse licks itself, or is groomed by another horse, the eggs hatch and the larvae are transferred into the horse’s mouth. Here they can burrow into the tissues of the tongue or mouth before being swallowed. Once in the stomach the larvae attach themselves to the gut lining to continue their development through the winter.


Bot Life Cycle

Bots Lifecycle


The effects of Bots

Bot flies in themselves are an irritant, however the bot larvae can cause actual damage. Symptoms include:

  • Ulceration of the mouth and cheeks, sometimes leading to head shyness and sensitivity
  • Inflammation of the stomach lining (gastritis)
  • Intestinal obstruction
  • Rectal prolapse
  • Colic


How to test for Bots

Bots are visible to the naked eye, therefore daily grooming is a great way to spot them. Removal of the bot eggs can be done easily with a bot knife and this will help to disrupt the life cycle of this parasite.

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