Dealing With Bots In Horses

A common parasite that is often forgotten when considering worming protection is the bot. In fact, this is not a worm but a form of insect in the Oestridae family, and is a parasite which can cause damage to the mouth, gums and stomach lining when its larvae develop inside the horse. It is normal to find bot flies on your horse’s droppings, but if you start to notice eggs on your horse’s body or in his droppings, it's time to talk to your vet about a targeted dewormer.

Lifecycle of the bot

Botflies (Gasterophilus nasalis), begin to lay eggs on the legs, chest, neck and abdomen of horses around May, depending upon your location, and continue to do so throughout the warm summer months. When your horse licks at the eggs, the motion and saliva trigger them to hatch and they enter the horse by burrowing into his mouth, usually at the base of the tongue or between the molars of the horse’s teeth. Throughout the United States, the widespread use of worming pastes and gels has greatly diminished the presence of bots.

Bot Fly Larvae
"Magendassel". Bot fly larvae*

The larvae remain within the mouth of the horse for between three to six weeks, after which time they migrate to the stomach. Upon reaching the stomach they attach to its lining, and that of the gut, and can remain there over winter in a dormant state. The rise in temperature and lengthening daylight hours brought by spring activate the next stage of the lifecycle, allowing the larvae to mature within the GI tract and loosen their hold on the lining. They are subsequently carried through the GI tract and compacted into the feces. Once droppings are deposited, the larvae burrow into the ground and pupate, eventually hatching into adult flies in late summer or fall.

Detecting bots

The bot fly is similar in appearance and size to a bumble bee. The eggs are ~2mm long and are creamy yellow in color, while the larvae are red, up to 2cm long and shaped like a cocoon.

As botfly larvae are not worms, they do not shed eggs inside the horse and so a fecal egg count will not tell you whether a horse is carrying this particular parasite. Therefore, it is important to inspect the body of the horse and his droppings for any indication of infestation. It is easy to see bots in manure, as they are large and easily recognizable. Bot eggs on the surface of the body appear as yellow specks and, as females can oviposit hundreds of eggs, they usually appear in clusters.

Treating and preventing bots

An appropriate dewormer will kill any larvae inside the horse, so be sure to select a brand designed to eliminate bots. The best time for bot deworming is mid-winter, while the bots are maturing in equine digestive tracts. Should you need advice, your veterinarian can assist you in selecting the best product for your horse.

Fort Dodge Equine Quest Plus Gel, for example, is highly effective against bots and widely available. One of the best defenses for preventing bots from entering your horse's digestive tract is to physically remove the eggs from your horse's coat, before they can be ingested. One popular method involves using a warm, damp cloth to wipe the bot eggs away.

Contact with the wet cloth may cause the bots to begin hatching, so be sure to thoroughly rinse the cloth when done, placing it in the sun to be sure it dries completely. This can also be done with a bot knife, featuring a serrated blade that scrapes the eggs from the hairs, or using a fiberglass bot block, which will remove eggs when wiped over them. A handy hint is to apply baby oil to the hair to prevent eggs laying on the coat, as this creates a slippery surface to which they cannot attach.

Remember that, in conjunction with a great bot prevention program, you should ensure pest flies do not irritate your horse. If your horse tries to itch areas of his body on which flies land, he could ingest bot eggs at the same time. Make sure you use Spalding Fly Predators early in the season to help eliminate local house fly and stable biting fly pests, appropriate fly trap solutions to address flies coming in from neighboring properties and UltraShield for when riding away from your property (when flies are a problem,) and outside the area you are administering fly predators. You should do everything possible to maintain proper horse manure sanitation, as well.

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*Image courtesy of Wikimedia Commons