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.
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:
Clinical laboratory findings are typically unremarkable.
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.
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.