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Gasterophilus spp Infection in Horses


Martin K. Nielsen

, DVM, PhD, DACVM, DEVPC, Maxwell H. Gluck Equine Research Center, Department of Veterinary Science, College of Agriculture, Food and Environment, University of Kentucky

Reviewed/Revised Dec 2019 | Modified Oct 2022
Topic Resources

Horse bots are the parasitic larvae of the botflies, Gasterophilus spp. Adult females deposit their eggs onto hair shafts of horses. Bot larvae are eventually ingested through grooming and can cause inflammatory reactions during migration within the oral cavity and by attachment to the stomach wall. In general, bot larvae are considered benign, even though some pathology is observed. Bot larvae can be identified during oral inspection or gastroscopy. Treatment of choice is ivermectin (0.2 mg/kg), which has documented efficacy against both oral and gastric stages.

Botfly larvae are extremely common in horses, and several species can infect horses. The two most common species are Gasterophilus intestinalis and G nasalis. Other species are known to infect working, feral, and wild equids but are rarely, if ever, encountered in sport and leisure horses. By far, the most common and abundant species is G intestinalis.

The adult botfly glues eggs onto hairs on the body of the horse. Eggs of G intestinalis tend to be found on the distal legs and around the neck and withers, whereas eggs of G nasalis are typically found in the submaxillary region.

In G intestinalis, hatching is stimulated by licking/grooming, whereas G nasalis eggs hatch spontaneously. In both cases, larvae find their way to the mouth and enter the oral cavity. They first migrate within the tissues of the tongue for up to 21 days and can subsequently be found in periodontal pockets surrounding molars and premolars.

After about four weeks in the oral cavity, they relocate to the stomach as second instars, where G intestinalis attaches in the nonglandular portion surrounding the margo plicatus, whereas G nasalis can be found in the pylorus. The larvae develop into third instars before they are passed out in the feces. Altogether, they can spend up to 8–10 months within the horse. Outside, they pupate in the soil for 3–5 weeks before the adult flies emerge.

Clinical Signs of Gasterophilus spp Infection in Horses

The gastric stages can be numerous but rarely cause clinical signs, although erosions and ulcerations occur. Gastric perforation and esophageal impaction have been reported on rare occasions. The most pronounced clinical signs have been associated with the oral stages of infection and include:

  • excessive salivation

  • head shaking

  • lingual irritation

  • chewing problems

Clinical laboratory findings are typically unremarkable.

Diagnosis of Gasterophilus spp Infection in Horses

Gasterophilus spp infection can be diagnosed during oral inspection, where larvae can be observed within the diastemas between molars and premolars. Lesions at the base of the tongue can also be observed. Gastric stages are easily visualized with gastroscopy. In addition, bot larvae can sometimes be observed in the feces, but fecal floatation and fecal egg counts are not useful to diagnose this parasite.

Observation of bot eggs glued onto the hair coat can be viewed as indicative of infection. However, this only documents general exposure of the herd, because horses are likely to acquire infection from each other while grooming.

Treatment of Gasterophilus spp Infection in Horses

Gasterophilus spp are susceptible to ivermectin (0.2 mg/kg), which has high efficacy against oral as well as gastric stages. Moxidectin, in comparison, has more variable efficacy against these parasites, and activity against oral stages has not been investigated.

Prevention of Gasterophilus spp Infection in Horses

Gasterophilus spp are not considered primary parasitic pathogens, but an ivermectin treatment applied during autumn should help reduce larval burdens going into the winter. It is not necessary to wait until the first frost to get a significant reduction. Concerned horse owners could be instructed to manually remove bot eggs from the hair coat as they occur.

A detailed presentation of current recommendations for equine parasite control can be found in the American Association for Equine Practitioners (AAEP) Guidelines.

Zoonotic Risk of Gasterophilus spp Infection From Horses


Key Points

  • Gasterophilus spp infection is very common in horses across the world.

  • Horses are usually unaffected by infection.

  • Oral stages can cause clinical signs.

  • Diagnosis is by oral inspection or gastroscopy.

  • Ivermectin is the treatment of choice.

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