Desirai Schild


We usually get our first, hard freeze in October. Thirty days after that freeze is when fall worming is required in horses.

That freeze will kill the insects causing the worms so the month after worming should annihilate most of the beasties until spring worming in March or April.

“Use an ivermectin product 30 days after the first, hard freeze,” said Dr. Lonna Gerstner, Hawthorne Animal Clinic owner. “That wormer will work on a broad spectrum of worms and larvae.”

Ivermectin based wormers kill parasites in many stages of development. They target bots, strongyles, lungworm, etc.

To me, bot flies are the worst because of their creepy life stages. Bots look a bit like honey bees but hover around horse’s legs, flanks, shoulders and stomach to lay eggs. The eggs appear in those areas as tiny yellow dots attached to individual hairs.

The location of bot egg distribution is tactical because those are where horses can reach to nip or mouth.

The eggs develop into the first larval stage within five days. They hatch into a maggot with seven to ten days. The larvae are stimulated by the horse biting or licking the fully developed eggs.

“The larvae get into the horse’s mouth and bury themselves in the tongue, gums or lining of the mouth,” said Dr. Jason Moulton, Animal Health Clinic, Blackfoot. “They will be there for about 28 days.”

The burrowing can result in severe irritation, pus pockets and even loose teeth. Loss of appetite also is possible.

When the larvae molt into the second stage they move into the stomach, attach themselves and reside there and in the intestinal tract for nine to 12 months. This can cause blockages and colic. They damage the digestive tract lining and take part of the nutrients the horse consumes.

Once this developmental state is completed, the future bot fly detaches from the horse’s insides and is passed out in manure. This tends to occur in late winter and early spring. The insect gestates in manure for three to ten weeks and emerges looking for a mate when the weather gets warm.

There are knives made specifically for removing bots. Many people coat the eggs in petroleum jelly or baby oil because this prevents them from hatching. I’m lazy and prefer

to remove them with clippers. No matter how you combat bot eggs, they need to go before they are ingested if possible.

Dr. Rex Gillespie, Cottonwood Veterinary Clinic, Blackfoot, advises developing and sticking to a regular rotational worming schedule. Rotating types of wormer is important to prevent resistance developing in the worms, he said.

My trainer in Kentucky uses a regular feed through wormers in her horses’ rations. Gillespie said that is literally overkill in our area because winter temperatures discourage all types of insects and their offspring.

Valley Vet Supply recommends Pyrantal in Jan.-Feb., Fenbendazole/Oxibendazole March-April, Ivermectin, May-June, Pyrantal, July-Aug., Fenbendazole-oxibendazole, and Ivermectin Nov.-Dec.

All vets agree that a regular worming program will prevent a variety of health issues.

Desirai Schild has been involved in raising, breeding and showing gaited horses in eastern Idaho for more than 20 years. She may be reached at