Bots are pesky creatures, capable of causing irritation and physical damage to horses. They aren’t categorized as being the worst of internal parasites, but they can cause problems externally and internally.

The external aspect is primarily one of irritation to the horse. The botfly is about the size of a honeybee, and its prime purpose in life is to lay eggs on the hairs of equine legs, necks, faces, and other parts of the anatomy. They don’t bite the horse, but they do create an irritating tickling sensation as they land to deposit their eggs.

And although we will talk later about “deworming” as a weapon against these parasites, they are not really worms, such as ascarids and strongyles. Instead they are flies, and like other flies their life cycle involves four distinct stages—egg, larva, pupa, and adult fly.

As is the case with other parasites, bots need a host to carry out their life cycle. They are specialists, in that they only attack horses, mules, and donkeys—perhaps zebras as well—and do not seek to use cattle or other livestock as hosts.

When attacking equids, the botfly is a pest supreme. Botflies generally lay only one egg at a time, but depending on the species, one female is capable of depositing 150 to 500 eggs. This means that the fly will be buzzing persistently around the horse’s head and legs, often causing it to become so irritated that it has trouble focusing on the task at hand, such as being ridden or driven. Instead, it will be busy tossing its head or stomping its feet.

We know a good deal about bots because research has unlocked most of their secrets, and agricultural extension agents stand ready to disseminate this information to the horse-owning public. Some of the information that follows comes from extension specialists at North Dakota State University, the University of Missouri, and West Virginia University.

What Are Botflies?

They tell us, first of all, that there are three species of bots that affect horses, mules, and donkeys. They are the common bot (Gasterophilus intestinalis), the throat bot (G. nasalis), and the nose bot (G. haemorrhoidalis).

As indicated, all three species are about the size of a bee. However, they do not bite or sting and, during their brief life, the adult flies do not feed. Basically, their mouthparts are very small and nonfunctional. They exist on stored energy until their demise.

The flies are active during the warm months all across the country. Generally speaking, they disappear after the first frost. It is impossible to devise a bot-control strategy that will be right for all parts of the country because of the broad variance in the occurrence of frost. Freezes come much sooner in Minnesota than in southern states, for example. This means horse owners should consult with their veterinarians to determine the type of bot control program—with correct timing being very important—that is right for their locales.

The eggs that are laid by the botfly are a familiar sight in late summer for most horse owners. They are yellowish in color and are firmly affixed to the hair. Just where they are found depends on the type of botfly laying them. In addition, there are some differences in structure.

According to Craig Reinemeyer, DVM, PhD, a parasitologist and the president of East Tennessee Clinical Research, Inc., bot larvae hatch from the eggs on the horse’s hair coat. “They emerge instantaneously in response to warmth, moisture, and carbon dioxide,” he says. “The first larval stage occurs in the tongue and around the check teeth.” The bots are in their first instar stage when residing in oral tissues, and they molt to second instars (or second-stage larvae) and travel to the stomach.

Here is how John F. Baniecki, PhD, professor of extension and plant pathology and specialist in plant pathology and entomology with the West Virginia University Extension Service, describes bot larvae:

“They are usually about three-quarters of an inch long and about three-eighths of an inch in diameter. They possess strong mouthparts by which they attach themselves to the stomach wall of the host. There are rows of spines on the larvae that run around the body like stripes and are very irritating as the larvae roll with the stomach movements. The number of larvae found in the stomach may vary from a few to hundreds.” Bots are either in the second or third instar stage when they’re in the stomach.

Baniecki points out that the bots’ presence can lead to intermittent colic, chronic indigestion, and an inability to maintain proper weight in accordance with feed intake.

Reinemeyer disagrees, believing that larval bots are relatively harmless, despite their fearsome appearance.

Types of Bots

Here is a description of the three species as passed along by North Dakota State extension specialists:

Common bot

“Eggs of the common bot are stalkless and are generally glued near the end of the hairs,” they note in an extension bulletin. “The eggs are grayish-yellow to yellow in color and about 0.05 inches long. Two flanges (projecting rims) along the lower half of the egg encircle the hair and serve to attach the egg to the hair. The nonflanged half extends from the hair at about a 30-degree angle.

“Females lay their eggs along the forelegs and flanks, where they can be reached by the horse as it rubs its muzzle and tongue over the area,” they continue.

The hatched larvae are picked up by the tongue and transported to the horse’s mouth, where they invade the mucous membranes.

Larvae remain there for several weeks before migrating to the stomach.

In the stomach, they attach to the lining and remain there until spring or summer. The larvae have become well adapted to life in the equine digestive tract. They are equipped with mouth hooks, setae (hair-like projections), and spines that allow them to attach themselves firmly to mucous membranes and later to stomach lining.

If left undisturbed, the larvae spend the winter maturing while attached to the stomach lining. Once mature, the larvae are passed out of the stomach with the feces in spring or summer. The next stage is pupation, which lasts for about one month and takes place in dry soil or dry feces. In about one month, adult flies emerge and soon thereafter they mate. The female is capable of laying some 500 eggs in about one week’s time.

Throat bot

“Eggs of the throat bot are also stalkless and are usually laid near the skin,” note the North Dakota extension specialists. “For this reason, they are often obscured by overlying hair. The flanges, which attach the egg to the hair, extend almost the entire length of the egg. The color is whitish-yellow, and the egg is approximately 0.05 inches long. The long axis of the egg extends parallel to the hair.”

The female throat botfly deposits her eggs under the jaw or throat area. She hovers in the air, often causing consternation to the horse, then darts in to quickly deposit the eggs. As with the common bot, the throat bot female is capable of laying about 500 eggs.

Larvae hatch within three to five days. Once hatched, they crawl along the jaw, enter the mouth, and burrow into the gumline. This can cause the horse further irritation as pus pockets in the gums are sometimes the result of the larval invasion.

Later the larvae make their way to the stomach, where they overwinter and mature. They are passed out with the feces to begin the life cycle all over again.

Nose bot

This fly’s eggs are stalked and are generally shaped like barnacles. “The connecting flange extends from the stalk upward toward the top of the egg,” say the North Dakota extension specialists. “The general color is brownish-black, and the egg is about 0.06 of an inch long.” Nose bot females lay their eggs on the very fine hairs around the lips, usually the upper lip. The females can cause extreme distress to a horse during this egg-laying period because they dart in to lay one egg at a time. Nose bot females are capable of laying about 160 eggs apiece.

The eggs hatch in a short time—as little as two days—and burrow into the lip and tongue membranes. They remain there for five or six weeks, then migrate to the stomach, where they spend the winter maturing.

In the spring, they detach and migrate to the rectum, where they reattach near the anus before dropping to the ground and pupating.


Historically, it has been reported that large quantities of bot larvae in the stomach can impede digestion, and outward signs of this infection can include loss of condition, increased body temperature, restlessness, kicking at the belly, loss of appetite, and intermittent diarrhea or constipation. Veterinarians have said the larvae can cause gastritis (inflammation of the stomach), stomach ulcers, and, in severe—though relatively rare—cases, perforation of the stomach, which can lead to fatal peritonitis (inflammation of the membrane lining the abdominal cavity).

But Reinemeyer isn’t convinced the larvae of G. intestinalis (the most common botfly) can be this destructive to the stomach, as they attach to the nonglandular portion of the stomach wall. He says, “The nonglandular stomach is squamous epithelium, so it’s more similar to the lining of a cow’s rumen or a calloused human palm than the mucosa found throughout the rest of the equine gut.”

Also, he suggests obstruction would be unlikely, since G. intestinalis bots don’t attach at the pylorus, which is the narrowest exit point from the stomach. He considers the clinical signs attributed to bot infection nebulous; such signs can occur for a variety of reasons. And, he has difficulty attributing catastrophic disease to an organism that is as prevalent as bots. “Stomachs rupture rarely, and then only for extreme reasons. If bots can be found in 95% of horse stomachs, they likely will be present in the rare stomach that does rupture,” says Reinemeyer. “That’s not very convincing evidence for cause and effect, and I still consider bots in the stomach to be harmless.”

Regardless of their impact on the stomach, as indicated earlier, bots in the mouth, tongue, and lips can cause irritation and other problems as they burrow into the tissues.


The life cycle of all three types of bots is approximately one year. In order to control bot infestations, it is necessary to break this cycle.

The first step in this effort, once the botfly has emerged, is to remove eggs from the horse’s hair on a daily basis. Special tools, such as bot knives and stones, have been developed to facilitate this process. Bathing the affected areas with warm water also helps remove the eggs. It goes without saying that if the eggs are not allowed to hatch, the larvae will be unable to attack.

Unfortunately, removing all of the eggs often isn’t possible, either because of time constraints on the part of the owner, or because some of the eggs are hidden beneath the hair.

The next weapon in the campaign against bots is deworming, often after the first frost has wiped out the egg-laying females. This is where the horse owner should form a partnership with her or his veterinarian. The veterinarian will know what type of dewormer is appropriate, as well as the correct timing for administering it.

The goal of a deworming program is to kill the maturing larvae while they are attached to the stomach lining, so that they can’t leave the horse’s body in a viable state to start the cycle all over again.

However, it should be noted that just simply administering a dewormer might not get the job done, and it is here where a veterinarian’s help and advice are so important. For one thing, the dosage must be correct. The dewormer administered to a pony might be very effective, but if the same amount is administered to a large horse, it might have only a limited effect.

Timing and correct dosage are necessary elements in an effective deworming program. Only ivermectin and moxidectin are efficacious against bots. Researchers have used organophosphates such as dichlorvos and trichlorfon, but these haven’t been marketed for horses in the past decade.

Management to Belay Bots

Sanitation and proper management of horses and the premises on which they are kept can figure strongly into the equation. Parasites such as the bot often spend part of their life cycle developing in manure. Thus, proper management of manure, both in pastures and corrals, can go a long way in helping break the fly’s life cycle.

Extension specialists at the University of Missouri suggest the following steps:

  • Proper manure disposal Stable manure should be composted before spreading on pasture or cropland. The heat generated during composting destroys the parasite in the pupa stage. When horses are kept in small corrals or paddocks, the manure should be picked up regularly and composted.
  • Pasture management Horse owners should practice frequent mowing and chain harrowing and should avoid overstocking enclosures with horses.
  • Rotational grazing Grazing should be rotated as much as possible, and young horses should be kept separate from older animals. If possible, it is a good plan to follow horses with cattle or sheep before putting horses back on the same pasture (to reduce the likelihood that equine parasites will persist in the pasture).
  • Feed Provide mangers, racks, or bunks for hay and grain, rather than feeding on the ground.
  • Water Clean water, free of feces contamination, should be provided year-round.
  • Egg removal Hairs with attached eggs should be clipped or sponged with warm water as the eggs appear—before they have opportunity to hatch.
  • Deworming Regular deworming should be practiced under the supervision of a veterinarian familiar with area geographic and meteorological variances. Unfortunately bot eggs, particularly of the common bot, can remain viable long after flies have disappeared, further emphasizing that an ongoing deworming program is important. Applying fly repellents during botfly season can help prevent the female botfly from laying her eggs in the first place.

It is also strongly recommended, as a normal part of parasite control in general, that you have your horse’s fecal samples examined regularly so that both the veterinarian and the owner know precisely which parasites are present and in what quantity.

Reinemeyer says these recommendations are good management methods to avoid parasite problems, but they aren’t directly applicable to bot control. “The bottom line is a producer can do an excellent job of removing eggs and deworming horses, but botflies can travel for miles. So, your bot control program is only as good as that of every horse farm within a decent radius.”

Take-Home Message

Bots are annoying flies, and their larvae can cause myriad internal problems from nose to tail. Watch for telltale signs of irritated lips, gums, and tongue during bot season. Then, understand if you have botflies and bot eggs, you have internal bot larvae in your horses, and you need to treat accordingly.