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Australia

BOTS – More than a Pest

Flies are a common part of most stables, often swatted at but rarely hit. They are a pest poorly tolerated by horse and owner. Bot flies can be much more than just pests, however. Flies are also carriers of diseases that can seriously harm your horse’s health and performance.

The annoyance and distractions they cause can also interfere with feeding and affect nutrition. The migration of the bot larvae under the skin in mucous membranes causes lesions which may provide openings for infection. Without treatment, bots can cause severe damage in the stomach and intestine of your horse.

A long life cycle

Adult bot flies are brown, hairy and bee-like with one pair of wings and measure about three-quarters of an inch. The larva is also three-quarters of an inch long with a narrow hooked end and a broad, rounded body. In the warm summer months adult bot flies are a common sight around horses. Yet this adult stage is just a brief part of the bot fly lifecycle. Female bot flies have no mouthparts so they cannot feed. They live in stored reserves only long enough to lay eggs on the hair around a horse’s eye, mouth, nose or on the legs. Moisture from the skin or from the horse’s licking causes the eggs to hatch into larvae.

The Bot Life Cycle

After a three-week developmental period in the mouth, bot fly larvae of both species, Gasterophilus intestinalis and Gasterophilus nasalis migrate and attach themselves to the mucous lining of the horse’s stomach and remain there during the winter. After about 10 months they detach from the lining and are passed out of the body through the faeces. The larvae burrow into the ground and mature. Depending on the conditions, adults emerge in three to 10 weeks. Adult females deposit eggs on the horse’s legs, shoulders, chin, throat and the lips. Depending on geographic location, the lifecycle of bot flies is not fixed to only certain times of the year and bot larvae can be active in horses anywhere from August to May.

The Bot Life Cycle


Egg laying begins in early summer. Eggs of the two species differ in colour and placement. Gasterophilus intestinalis lays up to 1,000 pale yellow eggs on the horse’s forelegs and shoulders. Moisture and friction from a horse licking itself cause the eggs to hatch in about seven days. After hatching, Gasterophilus intestinalis larvae are licked into the mouth. Gasterophilus nasalis lays about 500 yellow eggs around the chin and throat of the horse. These eggs are not dependent on the horse licking them to hatch. Gasterophilus nasalis burrows under the skin to the mouth, wandering through it for about a month before migrating to the stomach for over wintering. Then the cycle begins again.

Close up image of bot fly larvaeSigns of Bot infestation

Horses that show no outward signs of illness can be severely infested, giving no clue of the potential damage occurring inside. However, some horses do show signs of infestation including an inflamed mouth area and stomach irritation. Infestation with the bot larvae may cause ulcers in the stomach lining. If the infestation is severe, the opening from the stomach to the intestines may be blocked with can cause irritation, ulcers and even colic. The burrowing larvae can cause small tears in the skin which can become infected.

Treatment for Bots

Traditionally horses are treated for bots at the end of autumn, after a frost that kills the adult fly, and again at the beginning spring to rid the stomach of all the larvae. In the past the treatment was worse than the disease, with extremely toxic chemicals given via a stomach tube to the horse. Modern parasiticides like Equimax, Equimax Elevation and Eraquell are extremely effective and safe in the treatment of bots, and have had an impact on lowering the number of bot flies in areas where a good parasiticide treatment is practiced. Where possible, bot fly eggs should also be removed from the horse’s coat daily.

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