Bots, Internal Parasites

The little yellowish dots on your horse's lips and front legs appear harmless at first, but several months from now those bot fly eggs will have developed into larvae in your horse's stomach. Your horse is sure to be infested with bots unless your area is entirely free of flies.

Bots are the larvae of the Gasterophilus fly. Since these flies can't live through the cold months of winter, they have developed a unique method of ensuring the survival of future generations. The small, yellowish flies (which look a little like bees) live only for a week or so as adults, spending most of their lives as immature bots in the horse's stomach.

Bot flies start laying their eggs in the spring and summer, with a peak egg-laying period in the fall. The season can extend almost year-round in warmer climates. The eggs hatch when the horse rubs or licks his legs with his lips or tongue. Tiny immature larvae latch onto the horse's mouth and burrow into the tissues. A large infestation may cause tenderness and ulcers throughout your horse's mouth.

The larvae spend a month or two maturing in the tongue before they emerge and are swallowed. Over the next several weeks, they grow and develop into stomach bots. Bots create deep pits in the stomach wall producing ulcers. severe cases may cause perforation of the stomach or blockage of its outflow into the small intestine. Bots can live attached to the stomach wall for up to a year before finally release their hold and are expelled in the manure. They spend another month or two on the ground and then hatch into adult bot flies.

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Diagnosis of Bot Infestation

There is no simple way to diagnose bot infestation. You can assume that horses with eggs on their legs and face are infested with bots. A bot treatment has traditionally been recommended after the first killing frost each year, when all adult flies have died. While this advice is well taken, you are leaving your horse open to infestation the rest of the year if you don't treat more often.

Bot flies begin to deposit their eggs in early spring. Studies show that the highest average number of bot larvae per infected stomach occurred in the winter and spring with about 44-94 percent infection rate. If you don't use a boticide (bot treatment) until November, you're leaving bots in your horse's stomach all summer long. Treatments in spring, summer , and fall are a good idea. The organophosphates and invermectin kill both bots at the tongue-migrating stage and bots in the stomach.

Gray and brown horses

Controlling Bots

Fly control is as important as regular bot treatments. Since fecal piles of the horse are used as the mating site for the newly hatched flies, regular manure cleanup will keep any hatching bot flies away from your horse. Remember to shave the eggs of your horse's hair after your last bot treatment in the fall.

Fly Control

Several products provide feed-through fly control. These products contain low levels of organophosphates, which remove the bots from the horse's stomach. The drug persists in the manure, killing the larvae of flies that lay eggs in compost.

The low levels of drug in feed-through products may not be enough to remove all the bots from your horse. If you're using a feed-through products regularly, be aware that you may overdose your horse by deworming him with organophosphates paste or tube product. Also be sure you're environmentally responsible in the disposal of manure that contains chemicals.

You should not take bot infestation lightly. There has been reported a case of septic peritonitis due to bot infestation. The larva was embedded deep in the horse's colon which eventually caused perforation of the colon and leakage of intestinal content into the gut. The horse developed acute colic and fever and was euthanatized.


  1. Comparison of the efficacy of inactivated combination and modified-live virus vaccines against challenge infection with neuropathogenic equine herpesvirus type 1 (EHV-1). Goodman LB, Wagner B, Flaminio MJ, Sussman KH, Metzger SM, Holland R, Osterrieder N. (2006)
  2. Septic peritonitis due to colonic perforation associated with aberrant migration of a Gasterophilus intestinalis larva in a horse. Lapointe JM, Celeste C, Villeneuve A.

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