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It’s a Bee…It’s a Fly… It’s Stomach Bots!

Horse Bot Fly (Gasterophilus intestinalis)

The horse bot fly or stomach bot is an internal parasite of the horse’s gastrointestinal tract. Geographical location influences seasonality of bot flies. In north Florida, the flies are more active in late spring through early winter. Horses are the most common host for this pest; however, mules and donkeys can be infected as well.

Bot eggs on the front leg. Photo provided by Courtney Darling.

The adult bot fly itself causes minimal damage by simply annoying the horse. More seriously, the horse may experience indirect damage from being severely spooked or a lack of appetite if annoyance is great enough. Direct damage caused by this species occurs when it is in the larvae stage. Larvae burrowing into the mouth of the horse could result in infected pockets or loosened teeth.

The female lays her eggs on the horse’s hairs. Eggs can be found on front legs (most common), abdomen, flanks, and shoulders. As the larvae age and make their way through the horse’s gastrointestinal tract blockages can cause colic. Large larvae populations will damage the stomach tissue and gut. Larvae will steal necessary nutrients from the horse. The figure below describes the life cycle of a bot fly.

Cultural and Mechanical Control
  • Remove feces away from the horse so it does not reinfect itself when eating
  • Remove the eggs from the horse’s hair with a sharp-edged tool or sandpaper block

Bot Block. Photo provided Courtney Darling

Bot knife. Photo provided by Courtney Darling.

Dewormers
  • Ivermectin
  • Moxdectin

Remember to always follow label instructions when administering any chemical treatment. Work with your veterinarian or local county extension agent to develop a deworming protocol best suited for your farm.

UF/IFAS is an equal opportunity institute.

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