Horse Stomach Bots- The Life Cycle in Pictures

You've seen the little yellow eggs on your horse's legs. . . but do you know how those eggs eventually turn into bots inside your horse's stomach?  It's actually pretty cool (if you like that sort of thing)!

Read on to see the complete bot life cycle!

The life cycle begins when an adult bot fly (which looks a bit like a bee) lays eggs on your horse's coat.

The eggs look like this:

The horse ingests these eggs when he scratches his legs with his mouth and teeth. The eggs hatch inside the horse's mouth, and small larvae burrow into the soft tissues around the teeth.

Here's a video of bot larvae crawling around in a horse's mouth:

This image shows a "bot nest" in between a horse's teeth. We commonly see this in the fall and early winter months. The red arrow points to the nest in a mirror, the green arrow points to the actual nest.

A single bot larva, recovered from a horse's mouth, is circled in red.

After about three weeks living the mouth, the larvae are swallowed by the horse. The bots then attach to the stomach lining where they live for 8-10 months.

Here are the bots inside the horse's stomach (seen with an endoscope):

After 8-10 months, the larvae pass out of the stomach in the horse's manure. They burrow into the ground, mature, and emerge as bot flies to begin the cycle again!

So how do you treat bots?

There are two methods of control, and both are important. When you see bot eggs on your horse's coat, you should use a bot knife or stone to remove them. This will reduce the amount of bots that complete their life cycle.

In addition, horses should be dewormed once yearly, after the first hard frost of the year, with an ivermectin product. At Foundation Equine, we recommend combining this treatment with the yearly treatment for tapeworms. This means that each horse should be dewormed with a product containing ivermectin + praziquantel (Zimectrin Gold or Equimax) in the winter. It's important not to deworm all the horses on a property (or in a pasture) at the same time, since this creates resistant parasites (the only ones that survive will have resistance genes). We recommend deworming half the horses in December or January, and the other half in February!