Bot flies lay their eggs on a horse’s coat. When the eggs hatch, the larvae begin to crawl toward the horse’s mouth. The larvae embed in the tongue, then grow and migrate into the gums or cheeks. Eventually the larvae are swallowed where they attach to the walls of the horse’s stomach. If the infection is severe, those larvae can cause intestinal upset, mild colic, and nutrition deficits.
The best thing is to inspect your horse daily. If you find any of these tiny yellow eggs, remove them as quickly as possible. There are bot fly combs, which shave the eggs off the hair, you can simply pick them off (although it may take a while), or you can clip the hair. Removal is the best prevention.
Once in the stomach, bot fly larvae can be killed by using any dewormer containing ivermectin. I like to deworm in both the winter and summer to prevent any bot fly larvae. Of course, you can always deworm anytime you are seeing these eggs – Just be sure to remove all of the eggs before you use the dewormer.
Those of you in cold winter areas won’t experience bot flies in the winter, but in the south, they are a threat year-round.