Horse bot flies are Dipteran flies (two wings) of the species Gasterophilus that belong to the the family Oestridae. They occur worldwide. Most species affect horses, donkeys and mules a few species also wild equids (e.g. zebras). Some species may affect caribous as well.

Horse bot flies are all parasitic myiasis, i.e. the parasitic stages are not the adult flies but the larvae (maggots), which in this case develop inside the host gastrointestinal tract (stomach, throat, intestine, etc.) and feed on the tissues of the host. Very occasionally Gasterophilus spp may also cause ocular or skin myiasis.

Gasterophilus intestinalis, adult fly. Image taken from

They are obligate parasites, i.e. they can neither survive nor complete their life cycle without parasitizing their hosts.

There are about 25 different Gasterophilus species worldwide. About 10 species occur in Europe. The most relevant species for domesticated animals are in Europe and the Americas are:

  • Gasterophilus intestinalis (also called Gasterophilus equi), the common horse bot fly. This is the most frequent species in Europe and the Americas. It affects horses and donkeys.
  • Gasterophilus nasalis, the throat bot fly. Affects horses, donkeys and other equids (e.g. zebras).
  • Gasterophilus pecorum, the plant bot fly. Affects horses, donkeys and other equids (e.g. zebras). They are found mainly in Europe, Africa and Asia.
  • Gasterophilus haemorrhoidalis, the lip bot fly, the rectal bot fly. Affects mainly horses and donkeys.

Other relevant species in Europe are Gasterophilus inermis and Gasterophilus nigricornis.


Prevalence and incidence are regionally very different and depend strongly on climatic and ecologic conditions. In regions with moderate or cold climate (e.g. Europe, North America, etc.) adult flies are active (i.e. infective) only in the summer months. In regions with a warm climate they may infect horses the whole year through.

Horse bot flies are obligate parasites, i.e. they cannot complete their life cycle without parasitizing their hosts. Within the myiases, horse bot flies are so-called primary myiases, i.e. they do not need a previous injury of the host to deposit their eggs on it.

The disease caused by the larvae of these flies is called gasterophilosis.

Are horses infected with bot fly maggots contagious for humans?

  • YES, for people handling horses with their hair coat infected with bot fly eggs. Since these eggs are very small (~1 mm), they remain unnoticed by most people. Many of these eggs contain infective L1-larvae that are close to emergence. Although it only happens occasionally, such L1-larvae can get on the skin or the eyes of people handling the infected horses, and they can cause cutaneous or eye myiasis. In endemic regions, people working with horses during the fly season (usually early summer) should be aware of this and take adequate precautions, e.g. wash hands thoroughly after working, and do not rub the eyes after combing or washing potentially infected animals.
  • NO for people not involved in regular handling of horses. The reason is that the parasitic maggots that infect a particular animal cannot be transmitted to other animals directly. The eggs that will become parasitic maggots are deposited by the adult flies preferentially on or close to horses, not on humans. For additional information read the chapter on the life cycle below.

You can find additional information in this site on the general biology of insects and on the parasitic flies.

Biology and life cycle of horse bot flies

Adult flies are 9 to 16 mm long, with a hairy, bumble bee-like hairy aspect, with black and yellow hairs. The wings have brown patches. Adult flies do not feed at all and live only for a few weeks, enough to mate and reproduce. Female flies are oviparous, i.e. they deposit eggs, either directly on the host, or in plants close to their hosts. The larvae migrate to their preferential sites, where they attach thank to the hooked mouthparts. They feed on tissues they abrade with their mouthparts.

Gasterophilus intestinalis, early larva (top) and mature larva (bottom). Image taken from wikipedia.commons

Fully developed larvae have hooked mouthparts that help them attaching to the hosts tissues. Fully developed L3-larvae reach up to 2 cm length, are of a yellowish color, and have numerous hooks arranged in rows. Form and disposition of the hooks are species specific and are used for species determination.

After development inside the host, the larvae leave it with the feces. Outside the host they do not migrate away from the fecal piles, but burrow into the soil or in dry manure and pupate within 1 to 2 months. For this reason, in endemic areas the number of pupae in horse fecal piles may be very high.

Adult male and female flies mate within a few days after emergence. Females start searching for adequate hosts a few hours after mating and can travel several miles to find one. Most single flies lay eggs on more than one host, which increases their chance of survival and dispersal. Usually only one generation is produced each year.  Dispersal is also ensured through movements of hosts infected with larvae.

Most species lay eggs in early summer. The eggs show species-specific features and are rather small (0.9 to 1.5 mm). Further development and behavior are also species-specific.

  • Gasterophilus intestinalis lays 200 to 1000 eggs. The eggs are of a pale yellow color. The eggs are glued to single hairs in the front legs, shoulders, flanks and abdomen. L1-larvae (maggots) emerge out of the eggs with about 10 days after oviposition. Horse licking or biting stimulates larval emergence. After emergence the eggs migrate to the mouth or are ingested by the horse. Within the mouth larvae attach to the tongue, gums, or other mouth tissues where they remain for about 4 weeks. There they molt to L2 and migrate to the stomach, where they attach to the lining in the cardiac region. There they remain for about 12 months. 
  • Gasterophilus nasalis lays about 500 yellowish eggs on single hairs of the neck and in the intermandibulary space. Emerging of L1-larvae does not need stimulation by licking or biting. Hatched larvae migrate into the mouth where they attach and develop to the next stage. L2-larvae migrate further to the small intestine (duodenum) where they attach and complete development to L3-larvae.
  • Gasterophilus pecorum lays 1300 to 2500 black to dark brown eggs in food plants close to the hosts, occasionally on their legs. L1 larvae hatch in the eggs still attached to the vegetation, where they can survive for up to 9 months. Infection occurs mainly through ingestion of infected hay or other plants. After a period of attachment in the mouth region, L2-larvae continue migrating down the throat and molt to L3-larvae can occur already in the pharynx. L3-larvae attach mainly in the cardiac region of the stomach.
  • Gasterophilus haemorrhoidalis lays 160 to 200 dark brown eggs on single hairs of the lips and around the mouth. Emerging of L1-larvae does not need stimulation by licking or biting. After spending a few weeks attached to the mouth tissues, they continue migrating down the gastrointestinal tract. L2- and L3 larvae are found in the stomach, but some L3-larvae may attach for a long time in the large intestine (rectum) and around the anus.

Click here to learn more about the general biology of insects.

Harm and economic loss due to horse bot flies

Adult flies do not directly harm their hosts, but female flies approaching a host for oviposition can frighten the horses significantly. This can disturb grazing, which leads to weight losses. Stampeding may also happen. Injuries can aso occur as the horse tries to avoid hovering flies.

In most cases, infections do not go beyond a mild gastritis, and even relatively large number of larvae in the stomach may cause no clinical signs. But massive infections can also cause significant damage, particularly to young foals.

Larvae attaching inside the mouth can cause major harm. Depending on the number of larvae, more or less severe swellings, irritation and inflammation of mouth lining, tongue, and related structures can occur. This can cause disturb mastication, pain on eating and swallowing, strongly restrict the tongue movements, etc., which can lead to weight loss, reduced performance and even deaths, particularly in young foals.

L2- and L3-larvae attaching to the stomach or intestinal walls can cause swellings, erosions, and other damages on the affected tissues. This can disturb the proper functioning of these organs (secretion, motility, etc.), with subsequent impairment of digestion. Massive infections can lead to diarrhea, colic, weight losses, and even perforations and peritonitis. Occasionally the feeding activity of L2 and L3-larvae may even break larger blood vessels in the gut's lining with subsequent blood loss and anemia. 

Adult horses usually develop a certain level of immunity to Gasterophilus infections after repeated exposure. But unprotected young foals are at risk of suffering severe damage and even death after massive infections.

Diagnosis is usually difficult, because many of the symptoms caused by Gasterophilus infections (diarrhea, colic, gastritis, anemia, etc.) are not specific, and can be due to other reasons. The only reliable diagnosis requires demonstrating larvae in the feces. However absence of larvae in the feces is not always conclusive, because release may be intermittent. Finding the yellow to whitish eggs on the animal's hair can be helpful. Knowing the local seasonal behavior of the flies can also help to estimate whereas the observed symptoms could be due to these flies or to other unrelated factors.

Prevention and control of horse bot flies


Removing and disposal of feces contaminated with larvae reduces the number of larvae that infect horses, pupate and subsequently hatch. This can significantly reduce the fly populations, especially if it is done by most horse keepers in large areas.

Another useful measure is to remove the fly eggs deposited on the host's hairs. However, since they are glued to the hairs, it is not easy to remove them with a brush.To this aim special combs or fine-toothed blades are available for scraping the hair coat. An alternative is to apply warm water (~45°C) to affected areas, which induces larval hatching. Afterwards these larvae can be easily brushed away or the warm water is mixed with an adequate larvicide that kills the hatching larvae. Adequate protection (e.g. rubber gloves) is highly recommended to prevent larvae from infecting the operator. However, this procedure is labor-intensive and usually unfeasible if many horses has to be protected.

There are also fly masks that fit around the face of the horse and prevent flies from depositing eggs on the animals' head. But several fly species lay eggs not only on the head, but also elsewhere (flanks, legs, etc.).

Repellents provide usually only a few hours protection, if at all, and may not properly protect the whole body surface of the horses. For these reasons it is seldom a suitable perventative method in most places.

For the time being there are no vaccines that will protect horses by making them immune to bots. And there are no traps for specifically catching horse bot flies.

Click here if you are interested in medicinal plants for controlling external parasites of livestock and pets.


Nowadays very effective chemical control can be achieved with macrocyclic lactones, among which ivermectin and moxidectin are approved for use on horses in most countries. Other macrocyclic lactones (abamectin, doramectin, eprinomectin, milbemycin oxime, selamectin) are usually not approved for use on horses. These compounds are used mainly as gels or pastes that are administered orally to the horses. Both are indicated for the control of L2- and L3 larvae in the gastrointestinal tract. Some ivermectin formulations are also effective against early stages (mainly L1-larvae) in the mouth.

In regions with cold or moderate climate one treatment at the end of the fly season in late summer or fall may be enough to kill the maggots inside the host body. In regions with a warmer climate where flies can occur throughout the year additional treatments may be needed.

In some countries a few organophosphates that are available for oral administration (e.g. dichlorvos or trichlorfon) may also be indicated for the control of gastric stages of horse bot flies. However these compounds have a lower safety margin than macrocyclic lactones.

There is also additional information in this site on the general features of parasiticides and ectoparasiticides, as well as on parasiticidal chemical classes and active ingredients.

Resistance of horse bot flies to parasiticides

So far there are no reports on resistance of horse bot flies or their larvae to parasiticides.

This means that if a particular product has not achieved the expected control, it is most likely because the product is not adequate or it was not used correctly, not because the parasites have become resistant.

Learn more about parasite resistance and how it develops.

If available, follow more specific national or regional recommendations or regulations for the control of horse bot flies.