Deworming with Bots in Mind

Even if you dewormed during the summer, fall is the time to deworm your horse with a product that specifically targets bots.

This parasite has several stages that affect horses and mid-summer is when horses typically get infested by bots.

What are bots exactly?

The adult female bot fly only lives for a few days. She doesn't feed on anything. She mates, glues her eggs on horses' hair and dies. Her short life is fueled by stored energy and during that brief lifespan, she can deposit as many as 1,000 eggs.

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 actually fly larvae, so while they are intestinal parasites, they are not worms.

Horses are the host that bots use to continue their life cycle, which primarily takes place in the horse's gastrointestinal tract. Unlike many other fly larvae that develop in manure, damp hay and garbage, bot fly larvae develop inside the horse.

There are three common species of horse bot flies in North America. The females lay their eggs on different areas of the horse's body:

• between the jaw bones
• on the short lip hairs
• on the front legs and shoulder

It's the most common species, Gasterophilus intestinalis, that is responsible for laying eggs on the horse's front legs and shoulder.

Those yellowish, cream-colored flecks laid at the ends of the hairs are typically seen on the horse in August and September.

Bot life cycle

Bot eggs are stimulated to hatch by the warm breath of the horse. This happens when your horse licks the eggs on his legs, or when eggs laid on the head crawl into his mouth.

Once in the horse's mouth, these (first stage) larvae attach, burrowing into the tongue, gums and lips. This can be highly irritating to the horse and may even cause pus pockets and decreased appetite.

After 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 GI disturbances.

After about nine months, the bots release from the stomach and pass out in the manure. They then burrow into the soil to develop as pupae, remaining in the soil for a month or two, depending on the season. Come spring, they emerge as adult flies and the cycle continues.

Fighting back against bots

Bot flies aren't active year-round. Adults are only active for a few months--from late spring to late fall, or when there is killing frost.

The whole life cycle of the bot fly takes one year, but you can interrupt this process by:

• being diligent about manure management,
• using fly repellent on horses to discourage female flies from landing to lay eggs,
• removing any eggs that are laid on the horse's hair, and
• strategically deworming with a product that kills bot larvae that get into the horse's body.

Manage manure

Manure management is a key tool in controlling bots, as this is where their final development occurs before the adult flies emerge in warm weather.

Remove manure from stalls daily and routinely from areas where horses are turned out.

Ideally, you should have manure hauled away or you can compost it. What you don't want to do is maintain a manure pile near the barn or spread that fresh manure on pastures where horses graze, as this will help the proliferation of parasites.

Fly control

Bot flies don't bite or sting, but some horses react dramatically to a bot fly persistently buzzing around trying to land and lay her eggs.

Proper use of a fly repellent or topical equine insecticide will 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.

Manually remove eggs

If bot eggs do show up on your horse, remove them as soon as you spot them.

Add a bot knife and a grooming block like Slick 'N Easy Horse Grooming Block to your supplies as these tools make it easy to remove the eggs.

“Removal of bot eggs by mechanical means can be effective in reducing the numbers of bots that enter the horse and cause damage before deworming treatment can kill them,” says Tom Kennedy, Ph.D., a veterinary parasitologist based in Westport, Wisconsin.

Humans are not the preferred hosts for horse bot flies, but these parasites are opportunistic. There have been reports of larvae burrowing under the skin and into the eyes of humans. The internet has enough disturbing photos of such infestations to inspire caution!

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. Wash your hands thoroughly and don't rub your eyes after removing bot eggs from your horse.

Deworm strategically

A deworming product that specifically targets bots, also known as a "boticide," is an essential piece of the puzzle in fighting these parasites.

“Check product labels carefully. All equine deworming drugs do not necessarily control horse bots," says Dr. Kennedy. "Before purchasing any product, check the list of parasites 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. An ivermectin product like Farnam IverCare® 1.87% ivermectin paste dewormer effectively removes both the oral and gastric stages of bot fly larvae in the horse.

Daily dewormers, such as Farnam PyrantelCare Daily Dewormer 2.11% (pyrantel tartrate) Equine Anthelminitic, work against many species of internal parasites, but are not effective against bots. If you're using a daily dewormer just remember, you still need to treat for bots in the fall, even if you used a boticide in mid-summer.

Timing of deworming

"Based on the larval life cycle, bots accumulate in the stomach over late summer and 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.”

If bot eggs are seen on your horse in early summer, schedule your first deworming with a product effective against bots within one month. Deworming with a boticide again in November or December will control any second and third stage larvae.

This timing of treatment should ensure your horse is unlikely to get reinfected until warm weather sets in next summer.

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

Download Farnam's deworming handbook to learn more.

Did you know?

Within hours of emerging as an adult, the female bot fly will mate and then begins seeking hosts on which to deposit her eggs. To increase the chance of larvae survival, she lays her eggs on multiple horses, if available. Those eggs hatch approximately 10 days after being deposited. One bot fly can lay as many as 1,000 eggs in her short life span of just 7 to 10 days.

Did you know?

There are nine different species of bot flies worldwide, but the three most common are found in North America and primarily affect horses, donkey and mules. Gasterophilus intestinalis is the most common horse bot fly in the U.S.