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The larvae of the parasite gasterophilus, commonly known as the botfly, attaches to the mucosal lining of the equine stomach, which can cause gastritis and may lead to ulceration in horses.

There are three types of gasterophilus:

  1. G. intestinalis – the common bot. Adults leave eggs glued to horse hair, most commonly on the forelegs and shoulders. Eggs hatch after a week when stimulated by licking.
  2. G. haemorrhoidalis – the nose or lip bot. These attach their eggs to the hairs of the horse’s lips. They emerge after a few days and crawl into the horse’s mouth.
  3. G. nasalis – the throat bot. Lay eggs on the hairs of submaxillary region, which then hatch in approximately one week.

After crawling into the horse’s mouth or obtaining passage through licking, botflies burrow into the tongue and interdental spaces where they remain for approximately a month before making their way into the gastrointestinal tract. Gasterophilus larvae then attach to the mucosal lining of the stomach with oral hooks. G. intestinalis and G. nasalis are most common and attach near the margo plicatus or in the gastric fundus and duodenum, respectively. After developing for 8-10 months, they eventually pass in the horse’s feces, pupate in the soil for 3-5 weeks, and then emerge as adults.

Presentation and Symptoms of Gasterophilus Infection

The primary pathogenic effect of gasterophilus infection is ulceration at the site of attachment accompanied by a hyperplastic reaction around it. Oral stages may cause cause sinus tracts with mucopurulent discharge.

Clinical signs of gasterophilus include mild gastritis and gastric ulceration. However, a large parasitic infection may be present with no clinical symptoms. It may also cause stomatitis and pain on eating.

Symptoms typical of gastric pathology in horses, including gasterophilus infection, may include weight loss, lack of appetite, changes in body condition and hair coat, changes in behavior, difficulty maintaining condition, stereotypies such as cribbing or wood chewing, or underperforming. However, these symptoms may also be attributed to a wide variety of foregut and hindgut conditions, making differential diagnosis crucial.

Diagnosing Gasterophilus Parasite Infection

Specific diagnosis of gasterophilus infection is difficult. It it typically confirmed by visually identifying larvae as they pass in the feces. Environment and history are crucial to diagnosis; likelihood of infection may be deduced based on the horse’s individual history, the local seasonal botfly cycle, and the observation of bot eggs on horse’s hairs within a given herd.

Treating Gasterophilus in Horses

Ivermectin has typically been found to be effective against both oral and gastric stages of bots. When used as part of a routine parasite control program, appropriate ivermectin use has been shown to effectively control gasterophilus in horses.

However, updated AAEP guidelines for parasite control strongly recommend foregoing a routine approach in favor of regular fecal testing and monitoring, with therapy based on identified parasite load as determined by testing. This selective strategy is believed to provide more effective parasite control overall in the face of growing anthelmintic resistance.

While anthelmintics will treat the gasterophilus challenge, they will not aid the horse’s digestive tract to recover. Given that varying levels of damage to the digestive tract can occur, it would be prudent in this situation to ensure overall gastrointestinal function is supported throughout treatment and beyond. Clearly, the basic principles of feeding should apply but promoting a healthy absorptive surface, as well as a stable hindgut environment, should be key targets for additional intestinal support.

Equine GI Disease Topics

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