Got Bots?

Got Bots?

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.

Photo: Jennifer Whittle, Web Producer

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.

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?

There are multiple bot species of bots that affect horses, mules, and donkeys, but only two are significant in North America, says Craig Reinemeyer, DVM, PhD, a parasitologist and the president of East Tennessee Clinical Research, Inc., in Knoxville. They are the common bot (Gasterophilus intestinalis), which is responsible for about 98% of infections, and the throat bot, (G. nasalis), which is relatively uncommon, he said.

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.

Reinemeyer says 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. Those molt into a second stage, which is found in the gingival (gum) pockets around the check teeth." The bots are in their first or second instar stage when residing in oral tissues, and the second instars emerge and travel to the stomach.

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 third instar larvae as " 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.

Bot Relatives

Bot flies (Gasterophilus spp) are the sole equine examples of a large family of flies known as Oestridae. Oestrid flies are all parasitic, but the various species only attack very specific types of hosts. This entire family shares a common survival strategy. Rather than leaving the fate of the next generation to the vagaries of climate, Oestrids ensure the survival of their offspring by parking them inside a nice, warm mammalian host during the winter months.

The means by which bot fly larvae enter the host, and the sites they occupy during the winter vary by species. Some examples: Hypoderma spp, also known as “warbles” or “cattle grubs” lay their eggs on hair shafts of the cattle’s rear legs. The eggs hatch, and larvae penetrate the skin and begin a long migration to the back of cattle. During this migration, one species passes through the spinal canal, and another invades the esophagus. If certain parasiticides are administered during this migration, bot larvae could be killed while in these organs, causing inflammation which could result in paralysis or bloat, respectively. If the larvae survive their migration, they form large cysts under the hide on the topline of cattle, causing severe local damage which reduces the value of the leather.

Other bot flies deposit larvae, not eggs, directly into the nasal passages of sheep and reindeer. These larvae overwinter within the sinuses of the host. Still other Oestrids lay their eggs around rodent burrows, and are commonly found under the skin of squirrels and rodents during the colder months. These latter parasites (also known as “warbles” or “wolves”) commonly cause abscesses in kittens and sometimes adult pets during late summer and autumn.

Gasterophilus spp only occupy the alimentary tissues of equids, and probably cause less damage than most of their cousins because they don’t actively penetrate any tissues.

Craig Reinemeyer, DVM, PhD

Here is a description of the two significant 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.

“Modest numbers of eggs are also laid on the mane,” Reinemeyer adds. “It’s fascinating to think that perhaps they use this site because mutual grooming among horses is so common.”

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.


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. Further, they attach in a single layer, and don’t decrease the diameter of the stomach by more than maybe 5%. He considers the clinical signs attributed to bot infection dubious; 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 the two bots significant in North America is approximately one year, and, Reinemeyer adds, “unlike most parasites, bots apparently produce only a single generation within an annual cycle.” 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. Older horsemen might remember using organophosphates such as dichlorvos and trichlorfon, but these haven't been marketed for horses in decades.

“Current discussions about equine parasite control have also identified another potential concern related to bot control,” Reinemeyer notes. “Modern approaches to deworming emphasize minimal use of anthelmintics, and then only during seasons when parasite transmission from pastures is actively occurring. The standard bot treatments of ivermectin (IVM) and moxidectin (MOX) are also effective against equine strongyles. So, using these drugs for bots in late autumn/early winter comes at a time when treatments for strongyle control, using the very same drugs, should be avoided. Winter treatments with IVM or MOX won’t adversely affect a horse’s health, but might help to select for anthelmintic resistance among strongyles present at that time.”

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.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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