Bot flies of the genus Gasterophilus infest horses around the world, including Canada.


Bot flies of the genus Gasterophilus infest horses around the world, including Canada.  The adult flies lack mouthparts and so do not feed on the horses, but lay eggs on the haircoat.  The different species of Gasterophilus lay eggs on different parts of the body.. A larva develops within each egg, which then hatches spontaneously or in response to the warm breath of the horse.  The hatched larvae then enter the mouth of the horse either directly or following migration through the haircoat.  Once in the mouth the larvae wander in the tongue, gums and retro-pharyngeal tissues before emerging and moving to the stomach and duodenum.  After several months here the larvae leave in the faeces and pupate in the environment.  In Canada Gasterophilus has an annual cycle.

The buzzing around of the adult bot flies, and especially egg laying by the females, can be very annoying to horses.  The larvae of Gasterophilus in the stomach and intestines can cause quite severe erosions of the mucosa, but distinct clinical signs are rarely observed.  It is possible, however, that large numbers of these larvae could interfere with digestion, absorbtion, and growth and development.

Gasterophilus is very rarely zoonotic, although larvae sometimes develop in unusual sites, for example adjacent to the eye.


Phylum: Arthropoda
Subphylum: Uniramia
Class: Hexapoda
Subclass: Pterygota
Order: Diptera
Family: Gasterophilidae

The Order Diptera includes mosquitoes, midges, keds as well as the various types of fly. There are species of Gasterophilus in horses other than G. intestinalis, G. nasalis and G. haemorrhoidalis, but if they occur in Canada they are of minor significance. The closest relatives to Gasterophilus of importance in veterinary medicine are Hypoderma bovis and H. lineatum, the warble flies of cattle, and Oestrus ovis, the nasal bot of sheep.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.


Adult Gasterophilus species are large flies, measuring up to approximately 2 cm in length, and are bee-like, brown with a hairy head and thorax. The adult flies have greatly reduced mouthparts and do not feed.

Eggs of Gasterophilus species can be seen with the naked eye and are laid towards the tips of the hairs. After a few days a segmented first-stage larva can be seen within each egg. The species cannot be readily differentiated on the basis of the eggs.

Third stage larvae  of Gasterophilus species from the GI tract of the horse measure up to approximately 2 cm in length, are segmented and have spines. The exact arrangement of the spines can be used to distinguish between the various species, but this is rarely required.


Host range and geographic distribution

Gasterophilus species occur in horses and other equids around the world where climate will support the free-living stages of the life cycle. In some areas the prevalence and intensity of Gasterophilus infestations in horses may be decreasing, possibly because of the widespread use of very effective anti-parasitic compounds.

Life cycle - direct

The adult flies are around in the mid to late summer. Adults do not feed on animals and rarely survive for more than a few days. The adults mate "in a frenzied manner" around horse fecal masses. The adult females find horses by smell and sight, although horses may secrete attractants (kairomones). The females lay eggs attached to hairs, for G. intestinalis primarily on the forelimbs and sometimes the shoulders, for G. nasalis in the inter-mandibular region, and for G. haemorrhoidalis around the mouth and cheeks. Over approximately ten days a first-stage larva develops within each egg, which then hatches. Eggs of G. nasalis and G. haemorrhoidalis hatch spontaneously, while those of G. intestinalis require the hot breath of the horse. The newly hatched larvae then enter the mouth of the horse, either when this is brought close or following migration through the haircoat. Larvae of G. haemorrhoidalis may be able to penetrate the cheek and enter the mouth. What happens next depends on the species of Gasterophilus.

With G. intestinalis, larvae wander in the tongue and sometimes the gums for up to approximately four weeks, emerge, are swallowed and attach in the mucosa of the cardiac region of the stomach.

Larvae of G. nasalis follow a similar route, although they attach in the pyloric region of the stomach and in the duodenum.

Larvae of G. haemorrhadalis also follow a similar route but attach in the fundus of the stomach and in the duodenum. Third stage larvae of this species attach in the rectum for a few days before leaving.

After several months within the horse, the larvae detach, pass in the feces and pupate in the environment, releasing the adult flies. In Canada, these parasites have an annual cycle.

Life Cycle: Gasterophilus species

Pathology and clinical signs

Both male and female Gasterophilus buzz and this, together with the egg-laying by the adult female flies, can be very annoying for horses. The adults tend to be most active in the early afternoon on warm, sunny days.

Larval migrations in the mouth are rarely associated with detectable clinical signs, although there are reports of a dermatitis associated with larvae that penetrate the cheeks. Larvae in the stomach and intestines may cause ulcerations, and associated inflammatory response, but unless burdens are very heavy, or the larvae lead to perforation of the gut wall, or the horse is otherwise stressed, clinical signs are unlikely. It is possible, however, that the health impact of bots is underestimated.


Eggs of Gasterophilus species can be detected on hairs and can be differentiated from nits on the basis of their position on the hairs (nits tend to be at the base of the hair) and the presence of a first-stage larva. Third-stage Gasterophilus larvae can be detected in feces. Otherwise, diagnosis is at post mortem.

Treatment and control

There are several products containing either ivermectin (VARIOUS) or moxidectin (QUEST, QUEST PLUS) approved in Canada for treatment of Gasterophilus species in horses.  Most commonly, a treatment is administered after the first frost in the fall, when the adult flies are presumably all dead.

Additional information on the product mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Control of Gasterophilus species is difficult, other than by ensuring appropriate treatment and fly control. The fall treatments help to reduce adult fly populations for the following year, and may reduce the pathological effects of the larvae in the horses. 

Public health significance

There are very rare reports of first-stage Gasterophilus larvae invading people, usually around the face, but also in the lungs. Exactly how this happens is not clear.
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