by Casie Bazay

Up-to-date advice about horse bot flies

Up-to-date advice about horse bot flies

Published: July 2021 | Updated: June 2023 

You’re grooming your horse for a ride when you spot it: a beelike insect swarming around your horse’s legs. Your horse stomps and tries to rid itself of the annoying pest, but often to no avail. What is it? A female horse bot fly. What’s it doing? Attempting to lay its eggs on your horse. 

What Are Bot Flies? 

Bot flies are a highly evolved group of parasites affecting mammals and they are classified into four distinct subfamilies. The interesting thing about this pesky parasite is that the adults do not feed or take in nutrients in any way. 

In fact, they only have rudimentary, nonfunctional mouthparts and exist for one purpose: to reproduce. However, they need a host animal in order to do so.

Different species of bot flies are specific to certain hosts, including horses. They do not typically affect people. The adults congregate in specific topographical areas in order to reproduce; favored sites include hilltops, cliff faces, steep slopes, prominent rocks, trees, and streambeds. 

Male bot flies remain in these locations during their brief lifespan, but females leave after mating in order to find suitable hosts or egg-laying sites.

Bot Fly Eggs on Horses

Bots are seasonal parasites, but they are not like other types of parasites (worms) that affect our horses. Instead, they are flies with four distinct life stages: egg, larva, pupa, and adult. 

Bot flies lay their eggs on the horse’s hair coat during the summer grazing months. In time, some of these eggs are ingested as the horse grooms itself or participates in mutual grooming with another horse. Other bot eggs hatch on their own and crawl to the horse’s mouth.

Once the bot eggs are inside the horse’s mouth, they hatch and migrate into the soft tissue of the oral cavity before migrating to the horse’s stomach or small intestine. Once inside the digestive system, they attach themselves to the lining (mucosa) and continue to develop. 

The larvae remain in the equine digestive system until spring or early summer of the following year when they detach and pass through manure where they will pupate into flies. This last process generally takes several weeks to months and then adult flies emerge as the cycle begins again. 

Types of Bots

Three types of bot flies generally affect horses, as well as mules and donkeys. These include the common bot, the throat bot, and the nose bot. All three are similar in appearance, with the adults resembling a bee. The life cycle of these three types of bots is also similar and lasts approximately one year.  

1) Common bot

The common bot fly (G. intestinalis) lays yellow to gray eggs on the hair of the horse’s forelegs, mane, and flanks where they will later be ingested as the horse rubs its muzzle or licks these areas. The common bot fly female is capable of laying some 500 eggs in about one week’s time!

2) Throat bot

Throat bot fly eggs (G. nasalis) are often laid closer to the horse’s skin and may be obscured by overlying hair. The eggs are whitish-yellow and hatch within three to five days. Once hatched, the throat bot eggs on horses (now larvae) crawl along the horse’s jaw to the mouth where they burrow into the gumline to start their journey through the horse’s digestive system.

3) Nose bot

Nose bot flies (G. haemorrhoidalis) lay their eggs on the fine hairs around the horse’s lips and muzzle. The eggs of the nose bot look a little different than other bot eggs on horses; they are brownish-black, have a stalk, and are shaped like barnacles. 

Nose bot eggs hatch in as little as two days. They then burrow into the lip and tongue membranes where they remain for 5-6 weeks before migrating to the stomach where they spend the winter maturing. Nose bots also differ from the other bots in that when they detach from the stomach lining in spring, they migrate to the rectum where they reattach near the anus before dropping to the ground in order to pupate. 

How Bot Flies Affect Your Horse’s Health

The number of bot flies in the horse’s stomach can range from a few to several hundred. 

For the most part, bot flies do not cause disease in horses but very large numbers of larvae may lead to gastric pain or inflammation of the small intestine. This, in turn, can obstruct gastric outflow. 

The main problem with bot flies is the irritation they cause to horses, often resulting in restlessness and stamping. Because adult bots don’t have functional mouths, they don’t bite, but they do create an irritating tickling sensation as they land to lay their eggs. 

The females often lay one egg at a time, but some species are capable of depositing between 150 to 500 eggs which means that the fly will be persistent in hovering around the horse’s head or legs, causing plenty of irritation. In some cases, they can irritate horses enough to create dangerous behavior, especially during training or riding. 

Throat bots can cause the horse further irritation as pus pockets in the gums are sometimes the result of a larval invasion.

Diagnosing and Treating Bot Fly Infection in Horses

Despite their worrisome appearance, bot flies do not typically cause serious problems for horses. In fact, finding some bot fly larvae in horses’ manure is a normal occurrence. However, in some cases horses can become ill from bot fly infection. This usually happens with older or immuno-supressed horses. 

Possible complications from bot fly infections include erosions and ulceration which can occur during the gastric stage. Gastric perforation, as well as esophageal impaction, have also been reported on rare occasions. Clinical signs of bot fly infestation include those similar to gastric ulceration or colic. Other signs associated with infection include:

  • Excessive salivation;
  • Headshaking;
  • Tongue irritation; and
  • Problems chewing.

In some cases, infection can be diagnosed during oral inspection by a veterinarian. Larvae may be found in the spaces between molars and premolars. Lesions at the base of the tongue may be visible as well. During the gastric stage, larvae can be seen via gastroscopy.

Preventing Bot Fly Infection in Horses

There are several things horse owners can do to prevent bot fly infection in horses. The first and most obvious step is to remove eggs from the horse’s hair coat on a regular basis. 

The following are several methods for bot fly egg removal:

  • Bot knives and stones have been specially developed to remove eggs. These products can be purchased from most equine supply companies.
  • Use a razor to shave off eggs.
  • Bathe affected areas in warm water.
  • Heat vinegar and wipe on eggs.
  • Apply a layer of Vaseline over eggs. Several hours later, the eggs should wipe off with a paper towel. 

Removing all eggs isn’t feasible because some will be hidden in the hair and the process of removing them regularly can be time consuming. This is where deworming can play a part in stopping the life cycle of bot flies. It’s best to deworm for bots after the first frost has killed the females. It’s important to note that only ivermectin and moxidectin are effective against bots. 

Sanitation and manure/pasture management on the farm can also help to play a part in preventing bot fly infection since bot fly larvae (as well as other parasite larvae) spend part of their life cycle developing in manure. 

Collecting and composting manure from stalls and paddocks, especially, is important, as is frequent mowing. Avoid overstocking pastures and employ rotational grazing, if possible. It’s also helpful to co-graze horses or rotate pastures with other livestock such as sheep or cattle as this helps to break the life cycle of not only bot flies but also some other types of parasites.

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