Bot Worms in Horses | Botfly Larvae, Not Worms

Those tiny yellow eggs stuck to your horse’s shoulders and forelegs are the start of what owners call bot worms in horses. The name is misleading: these are botfly larvae, members of the Gasterophilus genus — insects, not worms.

Most horses pick them up at some point, which is why bots show up so often in deworming conversations despite not being worms. Adult botflies lay eggs on the coat, the horse swallows the eggs while grooming or licking, and the larvae spend months living inside the digestive tract before passing out in the manure.

What Are Botfly Larvae Doing In Your Horse?

Botfly larvae follow a slow, deliberate route through the horse’s body. Once swallowed, the larvae first spend time in the mouth, then move to the stomach, where they attach to the lining and feed there for roughly 8–10 months over winter. When development is complete, the larvae detach, pass out in the manure, pupate in the environment, and emerge as adult flies to begin the cycle again.

The species most often named in veterinary references are Gasterophilus intestinalis and Gasterophilus nasalis, with Gasterophilus haemorrhoidalis appearing less commonly. Whatever the species, the life cycle runs through the same four stages:

How Much Harm Do Bots Actually Cause?

Most infected horses show no signs at all, and even large burdens can be present without any visible problem. When burdens get heavy, the larvae can irritate the mouth, inflame the stomach lining, and occasionally contribute to ulcers, colic, or a drop in condition — but those are exceptions, not the usual outcome.

Direct diagnosis is the genuinely hard part. There is no practical test for larvae inside a living horse, and larvae are rarely seen in the manure, so most bot “diagnoses” are really seasonal assumptions: if flies were active and eggs appeared on the coat, bots are almost certainly present. The Merck Veterinary Manual classifies bots among gastrointestinal parasites of minor clinical importance, while noting that heavy burdens can bring on gastritis and stomach ulcers.

Treating Bots In Horses: Timing That Matters

Ivermectin at the standard deworming dose of 0.2 mg/kg by mouth once is highly effective against the oral and gastric stages of Gasterophilus species, according to the Merck Veterinary Manual. Timing matters as much as the drug itself: veterinary guidance calls for treating after the first frost, or in early to mid-winter, once the season’s exposure to adult flies has ended. Treat earlier and the horse can simply pick up new eggs while flies are still active.

Prevention is straightforward and worth doing through fly season:

1. Remove bot eggs from the coat with a bot knife or grooming block whenever you spot them, before the horse swallows them.
2. Pick up and compost manure regularly so pupating larvae never mature into adult flies.
3. Keep a regular deworming program and schedule the bot treatment for late fall or early winter.

Kansas State University’s entomology extension echoes that routine in its horse bot factsheet: egg removal, manure management, and a timed winter treatment together break the cycle. If you’re picking a product for that winter dose, our tested roundup of the best dewormers for bots in horses lines up the leading ivermectin and moxidectin options side by side.

FAQs

Can bots cause colic in horses?

Rarely, but heavy bot burdens are a recognized contributor to stomach irritation and ulcers, and severe cases have been linked to colic. Most horses with bots show no signs at all, so colic from bots is the exception rather than a common outcome of an ordinary infection.

How do I know if my horse has bots?

There is no practical way to test for larvae inside a live horse, and larvae are rarely seen in manure. Bot eggs on the coat during fly season are the clearest visible clue, and after a season of exposure, a winter deworming with ivermectin is the standard safeguard.

Is ivermectin enough to kill bots?

Yes. Ivermectin at the standard dose of 0.2 mg/kg by mouth once is highly effective against both the oral and gastric stages of Gasterophilus species. Moxidectin is also effective, which makes either a solid choice for the post-frost treatment window.