Gasterophilus intestinalis, commonly known as the horse botfly, is a parasitic concern for horse owners globally. This insect is an obligate parasite of equids, meaning it relies entirely on horses, mules, and donkeys for its larval development. Understanding this parasitic relationship is important for maintaining equine health and implementing effective management strategies.
What is the Horse Botfly?
Gasterophilus intestinalis is an insect species within the family Oestridae. The adult flies are bee-like in appearance, measuring between 10 to 18 millimeters in length, with hairy bodies that are yellow and black. Adult botflies possess greatly reduced mouthparts and do not feed, focusing solely on reproduction.
Female botflies lay their small, pale yellow eggs, typically 1-2 millimeters long, cementing them onto the hairs of a horse’s coat. These eggs are most commonly found on the forelegs, shoulders, and belly regions. The larval stage, which develops inside these eggs, is the parasitic form that infests the horse.
The Life Cycle and Its Effects on Horses
The life cycle of Gasterophilus intestinalis begins when adult female flies lay their eggs on the horse’s hair. Each female can lay up to 1,000 eggs. These eggs require moisture and warmth, often triggered by the horse licking its coat, to hatch into first-stage larvae.
Upon hatching, the first-stage larvae burrow into the horse’s oral cavity, migrating into the tongue, gums, and interdental spaces where they remain for approximately a month. This oral migration can cause irritation, lesions, and stomatitis, leading to discomfort or difficulty eating. After this period, the larvae are swallowed and move to the stomach.
Once in the stomach, the larvae attach to the mucosal lining using oral hooks. They develop into second and third-stage larvae, remaining embedded in the stomach wall for about 8-10 months. This attachment can cause gastric lesions, inflammation, and ulceration of the mucosa, though clinical signs are not always apparent. Large infestations may interfere with digestion or nutrient absorption, and can result in colic, weight loss, or even gastric rupture.
In the spring, the mature third-stage larvae detach from the stomach lining and pass out of the horse with its feces. These larvae then pupate in the soil or manure for 3-5 weeks. Adult botflies emerge from the pupae, ready to lay new eggs on horses.
Controlling and Preventing Botfly Infestations
Identifying a botfly infestation involves visually inspecting the horse’s coat for the characteristic yellowish eggs. While eggs on the coat indicate exposure, internal larval stages can be confirmed through gastroscopy, which allows direct visualization of larvae attached to the stomach lining. Fecal examinations are not effective for diagnosing botfly infestations because the larvae are not passed in the feces until they are mature and ready to pupate.
Treatment for botfly infestations involves deworming medications. Macrocyclic lactones, such as ivermectin and moxidectin, are highly effective against both the oral and gastric larval stages of Gasterophilus intestinalis. Ivermectin has shown 100% efficacy against both oral and gastric stages in studies. Moxidectin is also highly effective, with studies showing 100% efficacy against second-instar larvae and 99.5% against third-instar larvae. Treatment should be timed appropriately, often after the first hard frost in autumn, as this kills adult flies and prevents new eggs from being laid, allowing the dewormer to target the existing internal larvae.
Beyond medication, physical removal of eggs from the horse’s coat is a preventive measure. Regular grooming with specialized botfly knives or stones can scrape off and destroy the eggs, reducing the number of larvae that can be ingested. Pasture management strategies, such as rotating pastures and harrowing to disrupt pupae in the soil, can help break the life cycle. General fly control measures around the stable environment also contribute to reducing the overall botfly population.