Botfly Infection in Horses

Jennifer Rice, DVM
By Jennifer Rice, DVM on Jan. 31, 2023
Horse bugged by flies

In This Article


What is Botfly Infection in Horses?

Parasitic infections are extremely common in horses since they naturally live outside. One of the most common parasite infections are bots. Horse bots are the larvae of botflies, Gasterophilus spp. There are several species of botflies that can affect horses. While this is one of the most common parasitic infections, it generally leaves horses unaffected and is easily treated.  

Types of Gasterophilus

There are three types of Gasterophilus species that can affect horses:

  1. G. Intestinalis: This is the most common and abundant horse bot to affect horses. It typically lays eggs on the horse's legs, abdomen, flank, and shoulders.
  2. G. Haemorrhoidalis: This botfly is often called the throat bot. It often deposits its eggs on the underside of the neck and lower jaw.
  3. G. Nasalis: This botfly is often called the nose bot. It typically deposits its eggs around the muzzle of the horse.

Symptoms of Botfly Infection in Horses

Common signs of botfly infection include:

  • Very small white/yellow/cream-colored eggs on hair

  • Licking or rubbing areas where there are eggs such as legs and stomach

The most pronounced clinical signs are associated with the oral stage infection, including:

  • Excessive salivation

  • Head shaking

  • Lingual (near the tongue) irritation

  • Chewing issues

  • Ulcerations in and around the mouth

Clinical signs of botfly infections in the stomach include:

  • Colic symptoms

  • Gastric ulcers symptoms

Causes of Botfly Infection in Horses

The timeline of botfly infection in horses includes the following stages:

  1. An adult botfly glues eggs onto hair of the horse

    • Location depends on type of botfly

  2. Eggs hatch

    • G. intestinal eggs hatch due to stimulation from the horse licking/grooming the hair with the eggs

    • G. Nasalis eggs hatch spontaneously

    • Larvae make their way to the horse’s mouth and enter the oral cavity

  3. Larvae migrate into the tissue of the tongue, periodontal pockets surround the molars and premolars, and stay there for up to 21 days

  4. Larvae travel to the stomach after about 4 weeks in the oral cavity

    • G. intestinalis attaches to a portion of the stomach

    • G. Nasalis attaches to the pylorus region (opening from the stomach to the small intestines) of the stomach

  5. The larvae continue to develop and are passed out into the feces

  6. Once in the soil, larvae develop further for about 3-5 weeks then emerge as adult flies generally in summer or fall months.

Keep in mind, after this last step, the horse can then be reinfected or infected for the first time and the cycle starts again. Botflies can spend a total of 8-10 months within the horse.

How Veterinarians Diagnose Botfly Infection in Horse

Veterinarians can diagnose botfly infections by physical exam, fecal exam, and gastroscopy:

  • Physical/Oral Exam: On a physical exam your veterinarian may be able to find botfly eggs attached to your horse's hair as well as ulcerations or sores within the horse's mouth on oral exam.

  • Fecal exam: Botfly larvae can sometimes be observed in feces.

  • Gastroscopy: Your veterinarian will pass a scope with a small camera through the horse nose down to their stomach will allow for visualization of the horse’s stomach and any botfly larvae that may be attached.

Treatment of Botfly Infection in Horses


A proper deworming protocol can prevent your horse from having any major internal disease caused by botflies. Most often, ivermectin is the dewormer of choice for botflies. Deworming in the fall with a boticide can help reduce the larval burden going into the winter. Discuss your current deworming program with your primary veterinarian to ensure you are providing your horse with the best prevention for botfly infection.

Removal of Eggs

In addition to a regular deworming schedule, botfly eggs can be removed regularly from the horse's body by using a bot knife or small comb such as a flea comb. Removing the eggs from your horse before he ingests them will ensure they do not gain access to his mouth and digestive tract to continue their life cycle.

Environmental Management

Manure management can also play a large part in any parasite management program including botflies. Removing manure regularly from areas where horses commonly stand, whether that is in a paddock, run-in shed, or stall can help reduce the spread of botflies. It is not recommended to spread manure on pasture that is being grazed by horses.

Recovery and Management of Botfly Infection in Horses

If your horse acquires botfly infection it is important to reach out to your primary veterinarian to determine the severity of the infection. Mild infections may be able to be cleared with management changes and deworming. More moderate to severe infections that involve ulcerations of the mouth and/or the stomach may require prescription medications.

Botfly infections can lead to the following conditions:

Preventive measures are similar to treatment for botfly infections, which include:

  • Maintaining an appropriate deworming protocol year-round for your horse

  • Inspection of your horse daily to watch for any signs of bot eggs

  • Keeping the environment free of manure

  • Proper fly control and prevention to minimize flies

Botfly Infection in Horses FAQs

How do I know if my horse has bot flies?

Monitor for any signs of eggs attached to your horse's hair on the belly, legs, and face during summer and fall. If your horse is showing any abnormal behaviors, it is always best to consult with your primary veterinarian.

What wormer kills bots in horses?

Ivermectin is generally the treatment of choice for bots. Your primary veterinarian may recommend moxidectin if resistance to ivermectin is seen. It is important to keep your horse on an adequate deworming protocol according to your veterinarian’s recommendations.


1. Kentucky Equine Research Staff. Kentucky Equine Research. Botflies and Horses. 2015.

‌2. Nielsen, Martin. Merck Veterinary Manual. Gasterophilus spp Infection in Horses. 2019.

Featured Image: Orlova


Jennifer Rice, DVM


Jennifer Rice, DVM


Dr. Jennifer Rice is a 2017 graduate from Purdue College of Veterinary Medicine where she specialized in Equine medicine. Since graduating...

Help us make PetMD better

Was this article helpful?

Get Instant Vet Help Via Chat or Video. Connect with a Vet. Chewy Health