Protect Your Horse From Botflies

The female equine botfly can lay 150 to 500 eggs, one to a hair.

Compared to mosquitoes, gnats, stable flies, deerflies and other insects that bedevil horses, botflies seem relatively benign. After all, they don’t bite–adult botflies do not even have fully developed mouth parts–and their main preoccupation in life is mating and laying eggs.

Yet, as any horsekeeper can attest, botflies are persistent and troublesome winged pests. Persistent, because to procreate they must reach their target species. Unlike biting flies, which feed on many species and can be deflected to other victims, nothing short of death will stop a female equine botfly from getting to a horse to lay her eggs. And troublesome, because the larvae that hatch from those eggs head toward the horse’s stomach or small intestine, where they implant themselves and spend most of their life span as internal parasites.

Botfly larvae are not among the most destructive of the internal parasites–nematodes such as strongyles and roundworms hold that distinction. But large populations of bot larvae in the gut have been implicated in mild colics, occasional blockages and–in rare cases–fatal perforations of the stomach. Fortunately, you can keep bots under control with judicious use of dewormers, along with a few preventive measures. Here’s what you need to know to protect your horse from botflies.

A Fly’s Life

Adult botflies are distinctive in appearance and behavior. “They resemble a small bumblebee–brown and hairy,” says Craig Reinemeyer, DVM, PhD, a parasitologist with East Tennessee Clinical Research. The females hover around horses in a somewhat C-shaped posture, ready to lay eggs as soon as they dart onto the target site.

Horses have no difficulty recognizing approaching botflies. “The horse is irritated or sensitized to the vibration or buzzing of the fly and reacts strongly,” says Dennis French, DVM, professor of farm animal health management at Louisiana State University. To dodge botflies, horses may run, seek protection in thick brush or stand in deep water. To protect their heads they may stand together and rub their chins on each other’s backs.

Three species of equine botflies are found in the United States, and you can tell them apart by observing where on the horse’s body they prefer to lay their eggs. By far the most common equine botfly is Gasterophilus intestinalis. These are the flies that lay pale yellow eggs on a horse’s legs as well as on the mane, shoulders and flanks. Each female can lay 150 to 500 eggs, one to a hair.

Another common botfly is G. nasalis, which lays its yellow eggs under the chin or lower jaw. These eggs are harder to see since they are deposited between the hairs, and you often have to part the hair with your fingers to find them.

“In the United States probably 95 percent of horses would have G. intestinalis if you didn’t treat them to get rid of bots. This species is ubiquitous,” says Reinemeyer. “By contrast, only about 30 to 50 percent of horses will have G. nasalis.” In fact, for a recent study he conducted, Reinemeyer needed to find horses infected with G. nasalis. “We needed 17 horses for the study, and we had to scope more than 50 horses to find enough,” he says.

A third type of bot, G. haemorrhoidalis, is very rare in the United States. This species lays clusters of black eggs on the muzzle near the horse’s lips.

Internal Migration

The egg-laying habits of botfly species may differ, but once hatched their larvae all have the same goal: to get into the horse’s mouth. Those whose eggs were laid on the head move there on their own; those on the legs and other parts of the body need to attach themselves to the horse’s lips or tongue whenever the muzzle is in the vicinity. It might sound like a tricky bit of timing, but in fact the G. intestinalis larvae are well adapted to perform this feat: As the horse licks or rubs his legs, the heat, moisture and carbon dioxide in his breath stimulate the eggs to hatch very quickly.

Researchers have witnessed this phenomenon using a microscope. “If you breathe on them, they immediately hatch,” says Jack Campbell, PhD, veterinary entomologist at University of Nebraska’s West Central Research and Extension Center. “I’ve had them in a petri dish, getting ready to photograph them, and if you happen to breathe on them, you can see the egg opening up as they come out the end of it.”

Once in the mouth, the larvae of all three species undergo several stages of development. “First, the tiny larvae of G. intestinalis burrow into fissures in the surface of the tongue,” says Reinemeyer. “Later on, as they grow, they often migrate out of the tongue and crawl between the gum and the upper cheek teeth.”

After a month or so, the larvae emerge from the mucosal tissues and are swallowed, then they attach themselves to the walls of the stomach or small intestine. “G. intestinalis, despite its name, is usually found in the stomach. It’s not uncommon for a horse to have hundreds of these,” says Reinemeyer.

The lining of a horse’s stomach is divided into two regions–one area is made of soft glandular tissue, which secretes the acids, enzymes and other fluids used in digestion. “The other area is very tough, with a keratinized epithelium not unlike skin. This is where the forage is churned up,” Reinemeyer says. G. intestinalis larvae attach to the nonglandular part of the stomach lining.

G. nasalis typically passes through the stomach to lodge in the first inch or two of small intestine, called the duodenum, just past the stomach, while G. haemorrhoidalis settles around the pylorus, the lower end of the stomach just before the duodenum.

Once in place, the larvae spend seven to 10 months growing and absorbing nutrition from the passing flow of food the horse has consumed. Come spring–and researchers do not know how the larvae know when the weather has warmed up outside the horse–they detach and pass out in the manure. The orange-red grubs, just under an inch long, are clearly visible in the manure. Once on the ground they burrow into the top layer of soil and pupate for two to four weeks before reemerging as adult flies to start the cycle all over again.

The timing of botfly season varies around the country, depending on when the warmer weather begins. On average, most adult botflies emerge during the summer months–July through September–but in northern areas they may not appear in force until August, and in hotter climates such as southern Florida, a small number may continue to emerge year-round.

Collateral Damage

Pictures of an equine stomach with a severe botfly infestation are quite horrific–hundreds of ugly larvae protrude in a thick layer that hides every square inch of the lining. It’s easy to blame the invaders for everything from weight loss to ulcers to ruptured stomachs (deep ulcers that perforate), but in truth, says Reinemeyer, bots probably don’t do as much damage as previously thought. A ruptured stomach, for example, is extremely rare in horses. “If a common infestation, such as stomach bots, happens to be present when a rare event such as a stomach rupture occurs, that’s not proof of a cause-and-effect relationship,” he says.

Nor is it certain that bots contribute to severe ulcers, says French. Although the front hooks the larvae use to attach themselves to the stomach lining do leave visible lesions, he says, this part of the stomach has strong, keratinized skin, so it’s questionable how severe a problem they cause.

Nonetheless, bots can cause harm, especially as their numbers increase. Normally, in a herd of horses that carries bots, “you might find one with 10 [larvae in the stomach], one with 50, and you might find three or four horses that have none,” says Reinemeyer.

Fewer than 50 bots are not likely to produce any clinical signs in a horse, but if the number rises, they may cluster and block the passage of food from the stomach. Bot infestation has also been associated with anemia, irritation of the stomach membranes and other ills.

During the phase when the bot larvae reside in the mouth, they can irritate and inflame local tissues, causing discomfort and possibly leading to secondary bacterial infections–pus pockets.

“There’s actually some thought that bots may cause more harm in their first two stages, in the mouth, than they do in the stomach,” says Reinemeyer. The horse’s mouth probably won’t be sore enough to make him go off his feed, but if there are dozens or hundreds of larvae in there at one time, there must be some discomfort, he adds.

And, in at least a few cases, bots have been shown to cause a horse’s death. This risk seems to increase if they end up in unusual places within the body. In one mare, a G. intestinalis larva lodged itself deep within the wall of the colon–not a site where it would normally attach, but perhaps it came unmoored from its usual site in the stomach. The errant bot penetrated deeply enough to puncture the gut wall, leading to peritonitis.

A Measure of Control

Eliminating bots from your herd is probably not possible, but it’s not necessary either. “The objective is to keep the population under control,” says Reinemeyer. Controlling bots requires a two-pronged approach, aimed at different stages of their life cycle:

Treat the horse with a dewormer that acts against bots. A well-planned deworming program is your best defense against bots. “There are several brands of dewormer that control bots, but there are only two active agents with label efficacy against bots: ivermectin and moxidectin,” says Reinemeyer. Both agents paralyze the immature forms of the flies wherever they may be, either in the mouth or the gastrointestinal tract. Unable to move, they cannot eat and soon die.

To be effective, the timing of the treatment needs to correspond to the times when the larvae are most likely to be present within the horse. “The usual advice is to wait until after the first killing frost because that will eliminate one source of infection–the egg-laying adult flies,” says Reinemeyer. “We recommend treatment in late November or early December. By that time most regions will have experienced a frost.”

Any bot eggs that remain on the horse’s legs after the first frost, however, may still be able to hatch and infect him, although their viability does diminish significantly over time, and additional adult flies could still potentially emerge to lay new eggs on warmer days of early winter.

To catch survivors of the fall deworming, a second treatment is recommended in early spring, before the grubs have begun to detach and pass out of the horse to pupate.

Of the two deworming agents useful against bots, ivermectin is the more effective. Even a partial dose will kill most bots present. In contrast, says French, “If you are using moxidectin in your deworming program, keep in mind that bots are the rate-limiting parasite for this drug. This means you have to get at least 95 percent of the total dose into the horse to kill the bots.” Any underdose will not have a very high kill rate for bots.

Remove or destroy the eggs before they hatch. Of course, the more you can kill before they get into the horse, the better. “Anything you can do to physically detach the eggs while grooming the horse is probably a good idea,” says French.

  • Pull them off – Simply pulling them off with your fingers works well, but this method may be too labor-intensive if a horse has hundreds or if he resists. Tools such as a small shaving razor or a bot knife made specifically for egg removal can help, as can sandpaper or a synthetic stone grooming block.
  • Wash them off – “Some people bathe the horse with a sponge and warm water to try to hatch the eggs and carry them off on the sponge,” says Reinemeyer. “This may not be entirely effective; warmth and dampness are prerequisites to hatching, but there also seems to be a carbon dioxide component that helps these eggs hatch.”
  • Suffocate them – Rubbing the legs down with a clean rag moistened with baby oil will coat the eggs, blocking their air pores and smothering them. This tactic won’t remove the eggs, but it may prevent them from hatching.

Eggs laid under the horse’s jaw are harder to remove because you must part the hair to find them. Rubbing the area with a hot, wet washcloth may encourage some of these eggs to hatch so the larvae can be carried away on the cloth. Smearing the area with petroleum jelly may also help kill eggs by smothering them.

Timing is key to controlling bots by destroying the eggs: The laying season may last for months in warmer climates, and, says Reinemeyer, “it takes only a few days from the time the eggs are laid until they are ready to infect the horse, so you need to get rid of them fairly quickly–at least twice weekly. It’s a relatively high-maintenance procedure.”

It may be tempting to just let the eggs go and rely on the deworming treatments, but there are drawbacks to that approach. “It’s not ideal to let infection take place and then kill them after they’ve been in the horse awhile and have already caused damage or irritation,” says Reinemeyer.

The bot is unique among the horse’s internal parasites in that it’s a fly larva rather than a worm. While the horse must actively pick up flat- and roundworm eggs by grazing, botfly eggs come to him. And keeping your pasture free of other parasites does little good against botflies, which can blow in from anywhere. “Your bot-control program is only as good as that of the guy across the fence,” says Reinemeyer. Even a single pile of manure left by a horse on a nearby trail can bring botflies to your farm.

Fortunately, a few simple steps, timed to correspond with the botfly’s life cycle in your climate, are all that’s necessary to protect your horse.

This article originally appeared in the May 2006 issue of EQUUS magazine.

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