With the first frost of the year right around the corner, fall poses an excellent time of year to discuss the Bot fly, its impact on the horse and best means of control.
The Bot fly resembles a honeybee in appearance and is often observed flying around the muzzle and legs of horses during the warmer months of the year and in particular, late summer. The adult fly is of little harm to the horse but is often a nuisance and may create a significant distraction as it furiously attempts to deposit its eggs along the legs and under the jaw of the horse. These eggs are ‘glued’ to the hair of the horse as they develop into their larval stage and await the perfect opportunity to migrate into the horse’s mouth and make their way to the stomach. Owners may notice these tiny little yellow eggs attached to the lower legs and along the underside of the mandible of their horse.
The larval stage enters the horse’s mouth either by migrating along the jaw or by being picked up by the horse as it scratches its legs with its mouth. The larval stage typically burrows into the mucosa of the oral cavity where it matures and then later migrates to the stomach where it will overwinter in the horse. In spring or summer, the larval stages release to be passed in the manure where they pupate, mature to adult flies and begin the cycle over again. A good frost in the fall of the year often kills any remaining adult flies leaving the larval stages within the horse to carry the population to the next year.
The bot larvae can cause irritation to the mucosa of the mouth and to the lining of the stomach but with the exception of severe infestations; bots generally do not cause significant harm to the health of the horse and are mostly regarded as a nuisance parasite. Some veterinarians, however, believe that bots can contribute to poor digestion, inflammation within the stomach and stomach ulcers and most would agree that eliminating or minimizing the presence of bots in the horse is in the horse’s best interest.
Understanding the life cycle of the Bot fly allows for strategic intervention on the part of the horse owner or veterinarian. Deworming horses with products containing ivermectin or moxidectin at the proper dosage are effective in killing the Bot larvae present in the horse’s stomach. This is best administered after a good frost, once all of the adult flies are eliminated and no longer available to lay additional eggs. Furthermore, removing any eggs present on the horse using an instrument such as a bot knife, stone or washing with warm water will help to minimize re-infection. Good manure management and maintaining a sanitary environment, particularly during the warmer months, will make for an environment less conducive to Bot fly emergence.
Talk with your veterinarian this fall about implementing a plan to get Bots under control on your farm.
Dr. Darren Loula, DVM, is owner of Christian County Veterinary Service, LLC, a mobile large animal vet clinic.

OFN Site ManagerOn-Call / VetMissouri,On CallWith the first frost of the year right around the corner, fall poses an excellent time of year to discuss the Bot fly, its impact on the horse and best means of control. The Bot fly resembles a honeybee in appearance and is often observed flying around the...The Ozarks' most read farm newspaper, reaching more than 58,000 readers in Missouri, Arkansas and Oklahoma