Bot Flies and Your Horse

Nov 09, 2011
Preventative/Preparedness bot flies | parasites

Equine Parasitology - Gastrointestinal Parasites. Have you ever seen your horse itch his tail or rump on the fence or scratched your head in frustration over mysterious skin lesions that just don't seem to go away? You and your equine partner might be dealing with parasites! In this series of articles I'll explain the life cycles of the most common gastrointestinal parasites, how they can affect your horse and how to prevent and treat parasitic infestation.

Gasterophilus intestinalis – Common Horse Bot

  • The parasite commonly known as the 'Bot' is actually a brown, hairy fly in its adult form (look like brown honey bees).
  • Bots have one life cycle per year.
  • Most active in the Fall and early Winter.
  • Adult flies do not bite or feed on horses, but rather, the female flies seek an equine host and will deposit about 100 pale yellow eggs on the forelegs, belly and flanks. The horse then licks or breathes on these parts of their body and the warmth will cause the eggs to hatch.
  • Larval or juvenile forms of the Bot Fly are known as 'instar larvae'.
  • 1st instar larvae invade the oral tissues and can develop under the mucous membrane of the tongue or burrow into the gums and lining of the mouth where they remain for 3 – 6 weeks.
  • Larvae then migrate to the horse's stomach and overwinter as mature '3rd instar' larvae. The 3rd instar larvae are red in color and look like a little 'grub'. They attach to the stomach lining and often cause a mild inflammatory reaction. Most of the time 'gastric myiasis' (bot infestation of the stomach) causes no clinical symptoms at all, although large numbers of attached larvae can cause hemorrhage, ulceration and intestinal blockage at the pylorus (exit region of the stomach). Larvae can perforate the stomach, resulting in peritonitis and death.
  • 3rd instar larvae leave deep pits in the gastric mucosa at the sites of attachment when they mature and detach after 8 – 10 months in the stomach, are passed with the horse's feces in the Spring or early Summer and pupate in the ground for 3 – 10 weeks before adult flies emerge to complete the life cycle.
  • In the past, deworming was recommended after a 'hard freeze' because adult bot fly activity usually stops when temperatures drop below 55 degrees and bot larvae overwinter in the stomach where they can then be killed by dewormer. However, if you use Ivermectin and Moxidectin early in the fall, they will eliminate all instar larvae stages including the 1st instar larvae in the oral cavity so that they never make it to the stomach.
  • Washing horses' legs of Bot fly eggs once weekly with warm water to hatch the eggs will often help to eliminate Bots as will fly control!
Jessica Marsh

Written by Jessica Marsh, DVM