The Oft-Forgotten Horse Fly Bot

Give that bot fly some credit. She’s no slacker. In fact, she can travel several miles in the quest for a host and that host is your horse.

Life Cycle

There are three species of horse bots and despite having similar life styles, the females deposit their eggs in different locations on the horse. The most common is Gasterophilus intestinalis, which attaches her eggs primarily to hairs on the horse’s front legs and sometimes on the flanks and mane. Gasterophilus nasalis places her eggs on hairs beneath the horse’s jaws, while Gasterophilus haemorrhoidalis attaches her eggs to hairs found on the horse’s lips.

An adult female bot fly may only live for a week to 10 days, but during that brief lifespan, she can deposit anywhere from 150 to 1,000 eggs. You’ll find these yellowish, cream-colored flecks at the ends of the hairs in specific places on your horse, typically around August and September.

The ultimate goal is for those eggs to reach your horse’s gastrointestinal tract so the life cycle can continue. In order to accomplish this, they must first hatch into larvae, which either crawl to the horse’s mouth, or are ingested when the horse licks or bites at its body where the eggs are attached.

Once in the horse’s mouth, these first instar (first stage) larvae burrow into the tongue, gums and lips. As you can imagine, this can be highly irritating and may even cause pus pockets and decreased appetite. After literally incubating in the horse’s mouth for three to four weeks, the larvae molt into their second stage (second instar) and migrate on to the horse’s stomach.

Horse bots have three larval stages; both the second instar and third instar stages are highly adapted for life in the equine GI tract. Thanks to their hooked mouthparts, larvae are able to fasten themselves securely to the lining of the stomach and the intestinal tract.

At this point (third instar), the larvae are as long as one-half to three-quarters of an inch. Research has shown that the horse can tolerate an infestation of about 100 larvae. But in great numbers, larvae can cause various GI disturbances, including chronic gastritis, stomach ulceration, squamal cell tumors, anemia, and loss of functional stomach lining.

Thanks to their hooked mouthparts, larvae are able to fasten securely to the lining of the stomach and the intestinal tract.

After making their home in the horse’s GI tract for about seven to nine months, the third stage larvae finally mature and detach from the lining of the stomach and intestine, passing out of the horse’s body in the manure. At this point, they bury themselves in the soil for one to two months, depending on the season. Once the larvae develop into the pupal stage, the adult bot fly emerges and the cycle begins anew.

Flies, Not Worms

Because most of the bot’s life cycle is spent inside the horse as an internal parasite, we tend to think of them in the category of worms, but this is incorrect.

“Bots are fly larvae, so while they are intestinal parasites, they are NOT worms. Understanding that bots are fly larvae and the life cycle of these flies is important to the control of the flies. It is also important in understanding the pathology associated with the migration of the fly larvae through the animal's tissues before reaching the stomach,” notes Tom Kennedy, Ph.D., a veterinary parasitologist based in Westport, Wisconsin.

You should also realize that although bot flies vastly prefer horses, donkeys and mules as hosts, there have been occasional reports of bots burrowing under the skin and into the eyes of humans. That should be enough reason to remember to wash your hands thoroughly and don’t rub your eyes after handling your horse during bot fly season.

Control of Bots

Fortunately, there are definite ways to control and kill bots. For starters, be diligent about manure management. Remove manure from stalls and horse areas routinely. Ideally, have it hauled away, but if this isn’t possible, maintain a concentrated manure pile away from the barn and not in an area where horses are turned out. Do NOT spread manure on pastures where horses graze, as this just helps the proliferation of parasites.

Add a bot knife or a grooming block to your supplies and put it to use as soon as you spot bot eggs on your horse. These tools make it easy to remove the eggs, but for safety’s sake, slip on a pair of rubber gloves so there’s no chance of any hatched eggs getting into your own skin.

“Removal of bot eggs by mechanical means can be effective in reducing the numbers of bots that enter the horse and cause damage before treatment can kill them,” says Dr. Kennedy.

You should also use a topical equine insecticide or fly repellent to discourage bot flies from depositing their eggs on the horse. Always follow label directions to be certain you are applying the product correctly for best results.

Even with these two external methods—grooming tools and equine insecticide—you need one more piece of the puzzle: a deworming product that specifically targets bots.

“Using a boticide is the most effective way to make sure some did not ‘make it through,’” says Dr. Kennedy. “Check product labels carefully. All equine deworming drugs do not necessarily control horse bots. Before purchasing any product, read the pest list on the label and note any precautions regarding product use.”

Classes of dewormer products that are effective against bots include avermectin/milbemycins (ivermectin) and moxidectin. You may already be using a daily dewormer and that’s fine, but remember, you still need to treat for bots seasonally. Daily dewormers, such as pyrantel tartrate, are not effective against bots.

When should you deworm with bots in mind?

“Bot flies have only one life cycle per year. It usually occurs after the appearance of warm weather for emergence of adults. Horses become infested mid-summer in most areas. Based on the larval life cycle, bots won't be present in the stomach until late summer or fall,” explains Dr. Kennedy. “Application of effective treatment in mid-summer can control first instar (larvae), and then treatments later in the fall remove the second and third instars in the stomach.”

As with any type of internal parasite, you should speak with your veterinarian and develop an appropriate control program, based on your horses and your specific region.