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Deworm for Horse Bots Now

Dr. Frederick Harper, Animal Science Department University of Tennessee

Bots are honey bee-sized and shaped flies that greatly annoy horses in the summer and autumn. Female bot flies persistently cements their eggs to the hairs of the horse’s chin, shoulders, lower legs and mane. The egg-laying process greatly annoys horses, causing them to stomp their feet and shake their heads to discourage bot flies.

The resulting eggs are small, yellow-orange capsules that contain bot larvae. When the horse licks the eggs, larvae emerge and attach to the tongue. Larvae of certain types of bots naturally migrate from the chin into the horse’s mouth. Within the month, bot larvae burrow into the mucous membranes of the tongue and into the gums around the cheek teeth.

After living in the mouth for several weeks, the bot larvae migrate to the horse’s stomach, where they reside through the winter, possibly for as longs as eight months. Ultimately, bot larvae pass into the environment in manure during spring or summer. The larvae burrow into the soil, pupate, and emerge during mid to late summer as adults that produce the next generation of bot eggs.

Because cold, winter weather kills adult bot flies, December and January are ideal times to deworm your horse for bots. The bot larvae present in the tongue and stomach are susceptible to certain dewormers, and many of the eggs remaining on the hair coat are sterile and represent a minimal source of infection.

Only a few equine dewormers are effective against bots. These compounds are known as boticides because they kill bot larvae in the tissues of the mouth as well as in the stomach. At the present time, only dewormers containing ivermectin or moxidectin are effective as boticides. All horses in the herd should be treated for bots, not just those with bot eggs visible on their hair coat. Ivermectin and moxidectin are also effective against a wide range of internal parasites, so they are excellent deworming products to use in winter.

Bots in the mouth and stomach normally do not cause problems, but on rare occasions they may contribute to colic or even rupture of the stomach wall. By deworming for bots during winter, the number of adult bot flies next summer will be much lower.

You may want to physically remove the bot eggs from the hair coat of your show horse. Warm water may stimulate hatching of the bot larvae, which can be washed or wiped away. Empty egg capsules will remain attached to the hair, however. Grooming helps remove the egg capsules, as does fine grade sandpaper. Some commercial products are available to remove bots egg capsules. On other horses, the bot eggs are only unsightly and probably are not a problem at this time of the year.

Do not use sharp objects, such as knifes or razor blades. You could injure your horse while attempting to remove egg capsules.

Additional deworming information can be obtained from your county Extension office by requesting, “Internal Parasite Control in Horses.”

Reprinted with permission from the University of Tennessee Animal Science Department