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By Tom Lenz, DVM, MS, DACT


Compared to most of the parasites that infect horses, botflies are relatively benign because they can't bite, but in large numbers, their larvae can cause problems.

Botflies resemble small bumblebees that have a somewhat C-shaped posture and are commonly seen buzzing around your horse's legs during the summer and early fall. Since they do not posses mouth parts, the botfly's only purpose in life is to lay yellow eggs (nits) that they attach to your horse's hair. The adult's only annoy your horse. However, the larvae that hatch from the nits can cause damage to the horse's mouth and intestinal tract.

You can tell the three species of equine botflies in the United States apart by observing where on the horse;s body they prefer to lay their eggs. Ninety-five percent of botflies in the United States, belong to the species Gastrophilus intestinalis (common botfly) that lay as many as 500 yellow eggs on a horse's legs, mane, shoulders and flanks.

Another common botfly is G. nasalis (throat botfly) that lays eggs between the hairs on the horse's neck and below its jaw.

A third, much less common botfly is G. haemorrhoidalis (nose botfly) that lays clusters of black eggs on the horse's muzzle. The egg laying habits of each botfly species differs, but once hatched, the goal of the larvae is to get into the horse's mouth. Those near the legs and other parts of the horse's body attach themselves to the horse's lips and tongue during grooming.

Once in the horse's mouth, the larvae burrow into fissures on the surface of the tongue and gums. After a month or so, the larvae emerge and are swallowed. They then attach themselves to the nonglandular areas of the horse's stomach or the first inch or two of the small intestine.

It's not uncommon for untreated horses to have hundreds of the larvae in their stomach that spend seven to 10 months growing and absorbing nutrition from the passing flow of food the horse has eaten. When spring and warm weather arrives, the orange-red botfly grubs that are just under an inch in length detach from the stomach lining and are passed out in the horse's manure where they are easily recognized. Once on the ground, they burrow in and pupate for two to four weeks before emerging as adult flies.

A small number of bot larvae in the horse's mouth or stomach usually do not cause much damage, but large numbers can make the horse's mouth sore, create damage to the stomach lining and block passage of food from the stomach into the small intestine.

Control should be a two-pronged approach, at both the adult and larvae. First of all, remove or destroy the bot eggs on the horse's hair before they have a chance to hatch. Bot knives work well, as do sandpaper or a stone grooming block. Rubbing the horse's legs down with baby oil suffocates the eggs. Sponging the horse off with a warm water/vinegar mixture encourages the eggs to hatch, and the larvae are then carried off with the sponge or water rinse.

These procedures need to be repeated at least twice weekly to prevent the new eggs from hatching and infecting the horse. Deworming is the best defense against the botfly larvae in the horse's mouth and stomach. Make sure the dewormer you use contains either moxidectin or ivermectin, as they are the only active ingredients that are effective against bots.

It's best to wait until the first killing frost (November or December) to administer the dewormer because the cold weather will eliminate the adult botflies and their laying of eggs. To catch survivors of the fall deworming, a second treatment is recommended in early spring before the grubs detach from the horse's stomach wall and pass out of the horse to start their life cycle.


Article provided courtesy of AAEP-Alliance Partner, AQHA.