Horse Bot Fly

There are about 150 different varieties of bot fly. All of them are parasites, where the adult fly lays its eggs on a mammal and the eggs hatch into larvae which feed on the flesh of the mammal until the larvae are mature, at which point the larvae leaves the host mammal and hatches into an adult bot fly (which then lays its eggs on another host mammal, to complete the life cycle).

Each species of bot fly is specific to one type of mammal. For example, the Human Bot Fly lays its eggs on people, while the Equine Bot Fly lays its eggs on horses and other equines. There are nine different species of Equine Bot Fly, of which the most common is 'Gasterophilus Intestinalis'. This version is the main focus of this article, with the other eight species considered at the end of the article.

Infection and Life Cycle

The adult bot will fly up to several miles (kilometers) in search of horses to lay its eggs on. Each fly will lay between 150 and 1000 eggs, distributed over a number of horses. Each egg is laid on an individual hair, with the majority of the eggs laid where the horse can reach them with its mouth (legs, shoulders, flanks and abdomen). As the adult fly is active only during warm weather, in temperate climates this occurs only in summer and early fall. Within a herd of horses, it is common for some horses to be more targeted than others (this may be color related), with the result that some horses will have eggs laid on them more frequently and in greater numbers than others.

The eggs hatch into larvae (known as first stage larvae) within 7-10 days of being laid. They release a chemical which causes skin irritation, which results in the horse nibbling with its mouth at the irritated sites, allowing the larvae to enter the mouth. The larvae then burrow into the tongue, gums or mouth lining and remain there for approximately one month. During this month they develop into second stage larvae, at which point they exit the flesh of the mouth and are swallowed into the stomach and intestines. They then burrow into the flesh of the stomach or intestines, where they feed and develop into the third stage larvae. The third stage larvae then pass out of the horse in the feces. 

Upon passing out of the horse, the larvae remain in the feces or burrow into the ground, where it pupates over the next 1-2 months, developing into the adult fly. The adult fly then leaves the pupae and starts to look for a mate. After mating, the female starts searching for horses and laying its eggs on them, thereby completing the life cycle. The adult fly is unable to feed and consequently lives only until it uses up its store of nutrients (approximately 2 weeks).

The entire life cycle of the bot fly is approximately one year, with the majority of this time being in the larval stage, where it feeds on the horse and grows. The larvae which hatches from an egg is approximately one millimeter (tenth of a centimeter) in length, while the third stage larvae which emerges with the feces is 1-2 centimeters in length. The following two photos (courtesy wikipedia) show the adult fly and the larvae stage.

Adult Bot Fly Photo Horse Bot Fly Larvae

Horse Health Implications

The bot fly can cause damage at three stages:

  • Laying Eggs. The 'dive-bombing' action of the bot fly when it is trying to lay eggs on a horse can be very annoying to horses, sometimes resulting in severe fright. If the population of adult flies trying to lay eggs is high, this can interrupt the horse's grazing to the point that they lose weight.
  • First Stage Larvae. As the larvae burrow into the mouth, they can cause mild or severe irritation (depending on the number of larvae). They may also result in infections, development of pus pockets and loosened teeth. These issues can affect eating and cause weight loss.
  • Second and Third Stage Larvae. In this stage they attach to the stomach and intestine, where they feed. When present in large numbers, they can cause blockages and increase the incidence of colic. In large numbers, the drain in nutrients and damage to tissues weakens the horse. In severe infestations, other potential issues include chronic gastritis, ulcerated stomach, stomach rupture, anemia, peritonitis, tumors.

It should be noted that the severity of issues is directly related to the population; a few bots do not pose a serious health risk but large quantities do.

Human Health Implications

Although the Human Bot Fly can infest people, humans are largely immune to the Equine Bot Fly. However, there have been rare instances of individuals being infected from handling horses with bot fly eggs on their coats. When working with horses in these conditions, one should take care not to rub one's eyes (to prevent the larvae from infecting the eye) and should wash hands afterwards (to prevent larvae from burrowing under the skin). One may also wish to use rubber gloves, especially when removing eggs manually (see 'Prevention' below).


The both fly is normally treated with worming medication, since the medication used to treat intestinal worms is also effective against bots, so it is economical and easy to treat both types of parasite at the same time. Of the four types of worming medication, Ivermectin is effective against the oral and gastric stages, while Moxidectin is effective against the 2nd and possibly third stages of the most common variety of horse bots but is not effective against all species of horse bots. 

If the levels of bot fly are relatively low, a single treatment is sufficient. This treatment is best administered approximately 1 month after the end of the bot fly season (slightly later if Moxidectin is used instead of Ivermectin, as Moxidectin is not effective until the larvae have progressed from the oral to the gastric stage). Administering it later than this allows the bot larvae to start developing and damage, whereas administering it earlier will result in larvae infestation at the end of the season being untreated. Treatment prior to the bot season is ineffective for the next bot season and too late to prevent the damage from  the previous season.

Prevention (Horse Bot Fly Removal)

One of the most effective techniques is daily removal of the eggs from the coat, so that they do not have a chance to hatch and the larvae to enter the horse. They can be pulled off the hairs, using one's fingernails. However, it is usually faster to scrape them off with a sharp knife, being careful to scrape and not to cut the horse. Alternatively, one can use sandpaper, again being careful not to injure the horse. The eggs should not be removed in the horse's box, where they may fall into the hay or bedding.

One can buy a special 'bot fly egg removal knife', which has a serrated edge sharp enough to scrape off the eggs but designed not to cut the horse (unless you are really careless how you use it); following is a photo. Typical prices range from $3 to $6, depending on model and where you buy.

Bot Fly Removal Knife

One can also use warm water with appropriate insecticide, the warm water inducing the eggs to hatch and the insecticide then killing the larvae.

Removal or chemical treatment of the feces containing the 3rd stage larvae may help. However, as bot flies can travel several miles (kilometers) looking for host horses, such techniques are of limited value unless practiced by all horse stables in the area.

Other Species of Equine Botflies

There are nine species of equine bot flies, with the above discussion specifically describing the most common version (Gasterophilus Intestinalis). The above is applicable to the other eight species, with two exceptions:

  • Life Cycle. The life cycle for the other species is slightly different in terms of the way in which the larvae enter the horse. With Gasterophilus Haemorrhoidalis, the eggs are laid on the lips or nose (instead of the body) and the larvae migrate into the mouth on their own upon hatching. With Gasterophilus Nasalis, the eggs are laid on the throat and the larvae burrow into the horse, to the mouth region. The other six varieties are rare or unknown in the USA but likewise are basically similar.
  • Medication. Invermectin is used for treatment of all varieties of horse bots. Moxidectin, although effective for the 2nd stage of the most common variety of horse bots, is not recognized as effective against all species.