What Are Bot Flies And What Should I Do About Them?


Botfly aka Gasterophilus intestinalis

Gasterophilus intestinalis, or horse bot flies, tend not to cause horses significant problems but are unsightly and can serve as a signal that there is a hole in your current deworming protocol. These flies lay their eggs on horse’s legs, girths, flanks and shoulder areas and the horses lick or bite them off and cause the eggs to hatch. The larvae are then able to tunnel into the horse’s mucous membranes where they reside for around 4 weeks (28 days).  After this time they molt and migrate to the stomach of the horse where they attach to the demarcation of the glandular and non-glandular stomach, the margo plicatus. They can cause mild tissue irritation here and may stay in this location for up to a year! Once the larvae detach from the stomach they migrate through the digestive tract and are passed in the manure where they pupate and eventually the next generation of bot flies emerge. (McLendon & Kaufman, 2007) (http://entnemdept.ufl.edu/creatures/livestock/horse_bot_fly.htm)

How do we treat them?

Currently, the American Association of Equine Practitioners recommends treating horses with a boticide dewormer product at least once a year in the late summer or fall regardless of their fecal egg count. These products include ivermectin or moxidectin as the active ingredient (Zimecterin, IverCare, Equimax, and Quest are a few products that would be appropriate). 





Bot Fly Egg Knife


Removing and disposing of manure appropriately is important in breaking the life cycle of this parasite. Additionally, making sure you are taking steps to control the fly population in your horses’ environment will also help decrease the amount of eggs they are exposed to. The best thing to do if you see eggs on your horse is to regularly remove them using a bot knife, a pumice stone, or a “Slick ‘N Easy” block available from most feed stores.




Additional notes about appropriate parasite control:

     Strategic deworming is important to prevent parasite resistance and to ensure your horses are adequately protected. It also ensures you are using the right product to target the most important parasites at their most susceptible stage during the year. The American Association of Equine Practitioners recommends fecal egg counts be performed and serve as the basis for parasite control recommendations for each individual horse.  It’s important to remember that there are over 150 parasites that affect horses (bots, small and large strongyles, tapeworms, pinworms, thread worms etc.) and the threat to each individual animal changes throughout its life based on its age, immune status and environment. Fecal egg counts only account for some of these parasites and therefore every horse should receive at minimum one or two deworming treatments every year which contain a boticide (ivermectin or moxidectin) and praziquantel to kill tape worms in addition to the other recommended treatments based on their egg burden. There is no easy recipe for an appropriate deworming strategy for every horse or every horse farm and it is important to work with a veterinarian to determine how to best meet your individual needs. More information is available at: http://www.aaep.org/info/horse-health?publication=876


McLendon, M., & Kaufman, P. E. (2007, December). Featured Creatures: Horse Bot Fly. (J. L. Gillett-Kaufman, Editor, & U. o. Florida, Producer) Retrieved December 20, 2016, from Entomology and Nematology Department, University of Florida: http://entnemdept.ufl.edu/creatures/livestock/horse_bot_fly.htm


Neilsen, M.K. et. al,. AAEP Parasite Control Guidelines. (2013). AAEP Parasite Control Subcommittee of the AAEP Infectious Disease Committee.