Botflies (Gasterophilus spp) are parasitic flies that affect the horse’s digestive tract and can cause negative health consequences.

Botflies lay eggs on the horse’s coat in the summer. Some of these eggs, known as horse bots, are ingested as the horse licks and grooms itself.

The bot eggs hatch and the larvae develop in the horse’s mouth before migrating to the stomach where they attach to the gastric mucosa.

Once mature, they detach and are passed through the manure. They pupate into flies, and the cycle repeats with new botflies seeking out horses to host their eggs.

Prevention and treatment of bot invasion are essential to keeping your horse healthy. If you suspect that your horse is affected by botflies, consult with your veterinarian for a diagnosis and treatment options.

What are Horse Botflies?

Botflies are flying insects that belong to the Oestridae family. These winged insects can be identified by their fuzzy yellow, brown or gray bodies that give them a bee-like appearance. [4]

Horses are susceptible to over 60 common parasites that can affect their behaviour, health and performance. [1][16]

Unlike other parasites, such as worms (helminths), bots are larvae that grow and develop within the horse. Botflies use horses as hosts for most of their life cycle. [16]

Botflies Gasterophilus in Horses

Where are Botflies Found?

Bots are common pests in North America, the United Kingdom, the Mediterranean and China. [2] It is believed that imported horses originally brought bots to North America from Europe. [4]

Horse bots have extremely strong host specificity and cannot survive in the digestive tracts of other livestock. [4] They will travel long distances to find an appropriate equid host. [17]

These seasonal parasites can be found at most farms where horses are present, making them pervasive pests for horse owners.

Types of Botflies

Three types of horse botflies can be found in North America. Each species has a similar life cycle, apart from where they attach to the horse.

The common botfly (Gasterophilus intestinalis)

The common botfly deposits her eggs on the forelegs, chest, shoulders, belly or flanks during late summer or early autumn.

These eggs incubate on the body for 1-2 weeks. The horse must stimulate the eggs to hatch by licking or biting the larvae during grooming. [15]

The throat botfly (Gasterophilus nasalis)

The throat botfly deposits her eggs on the hairs under the jaw, or near the throat latch during late spring and early summer. Each egg attaches to a long, singular hair.

Unlike the other bots, hatching can occur without friction from the horse’s mouth within 4-6 days. [15]

The nose botfly (Gasterophilus haemorrhoidalis)

The rarer nose botfly is smaller than the other two types. She deposits her eggs on the hairs of the lips and nose.

These eggs are black and have stalks. They hatch in response to moisture from the lips within 2-4 days.

Unlike the other species, nose bot larvae will attach to the rectum when excreted before dropping to the soil to pupate. [16]

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Botfly Life Cycle

Understanding the bot life cycle is important for prevention and treatment. Each species requires 10 – 12 months to develop within the digestive tract. [4][16]

Eggs (2-10 days)

During the summer months, the female botfly mates and then seeks out a host for her eggs. [5] Once a suitable host is found, the fly will hover over the horse and attach 150 – 1000 small eggs onto the horse’s coat.

These eggs range from cream and yellow to orange in colour and are extremely sticky. The flies and eggs cause irritation and itchiness, prompting the horse to lick and bite at affected areas to soothe the skin. [4]

Common botfly eggs require friction and moisture from the horse’s licking to hatch. [10]

Larvae (8-12 months)

The hatched larvae are ingested into the oral cavity where they develop over a period of three weeks to a month. [14]

During this time, the bots bury themselves in the gums, tongue and lining of the oral cavity using their strong mouth-hooks. [15]

Common bots migrate into the stomach and attach to the mucosa of the non-glandular region, near the esophageal entry site. The nose bot attaches near the pylorus at the exit from the stomach. [14]

Once attached, they become immobile for eight to twelve months while they grow. [4] This parasitic infestation is known as myiasis.

Pupal Stage (3-8 weeks)

After developing in the gut wall, the larvae detach from the lining and pass through the digestive tract. They are then excreted in the horse’s feces, where they enter the pupal stage.

In early summer, they burrow into the pasture for 3-5 weeks before emerging as an adult botfly. [4]

Adult (7-10 days)

The adult botfly does not have a mouth and is unable to feed. She only lives for a few days to reproduce, find a horse host and deposit her eggs.

Symptoms of Botflies in Horses

How do I know if my horse has botflies? Botfly infection can go undetected for long periods of time as most horses demonstrate no outward signs of illness.

Severe mouth or gastrointestinal tract infection due to bot invasion is rare. Large numbers of larvae can be present in the body without symptoms.

Adult bots tend to fly around and dive at the horse’s head and face, causing annoyance and stress. They have been known to prevent horses on pasture from grazing due to their aggressive flight patterns. [4]

Adult bots do not bite or sting, but they can cause significant distress and spook the horse.

Clinical Signs

In some cases, horses infested