Editor’s Note: This is part eight of a 12-part series on internal parasites of horses.

The shelves at the local co-op or tack shop, and the pages of your animal health product catalog, all feature a bewildering array of dewormers. If you feel confused every time you have to make a decision about anthelmintic selection, it’s no wonder. How do you choose?

Parasite battle
What compounds work against what parasites, how effective they are, and how to use them wisely

In order to make an informed decision, you need to know something about the properties of the drugs. How do they act once they’re inside your horse? What about safety and efficacy? Here’s a primer on the active ingredients in various dewormers that are currently available.

How Do They Work?

All nematocides (drugs that kill nematode or roundworm parasites) essentially kill worms by either starving them to death or paralyzing them. Because worms have no way of storing energy, they must eat almost continuously to meet their metabolic needs. Any disruption in this process results in energy depletion, and interfering with feeding for 24 hours or less is sufficient to kill most adult parasit