Even the millennials among us fondly remember the good old days of the rotational deworming chart. You could tear out a simple calendar from any horse magazine, hang it up in the barn, place one order from your favorite catalog (ahem), and BOOM—you’re set for the year! Alas, as is often the case with the “good old days,” those days are long gone. Nowadays, industry experts and veterinarians are increasingly focused on “resistance” among parasites, meaning that the worms are less and less sensitive to the active ingredients in the dewormers we’ve been using for the last ~40 years. That means we have to change up our strategy.

In this article, we’ll start with an overview of the parasites you should be aware of and the available active ingredients you can use to target them. Then we’ll take a look at the current state of resistance, broken out by parasite and active ingredient. Last but not least, we’ll do the most important part—help you understand how you can build a smarter deworming program for your horse in four easy steps!

Know the Enemy

Since horses are grazing animals, they can never be completely free of intestinal worms like dogs and cats. We protect them by deworming which does two things. First, it reduces the number of parasites within the individual horse, and second, it reduces the number of parasite eggs contaminating the ground and pastures.

Small strongyles
What they do
Encyst (burrow) into your horse’s intestinal wall then emerge, which damages tissue and can lead to difficulty digesting and absorbing food

Good to know
Currently one of the most problematic internal parasites affecting horses

Large strongyles
What they do
Migrate in the arteries of the horse’s abdomen, causing damage to blood vessels which can lead to colic

Good to know
Widespread ivermectin use had nearly eradicated them, but inappropriate deworming practices may be leading to a resurgence

Tapeworms
What they do
Attach at the junction of the ileum and cecum (in the hindgut), potentially blocking the GI tract and causing colic

Good to know
Difficult to detect with fecal tests so blood and saliva tests have been developed

Roundworms
What they do
Migrate from the intestine through the circulatory system into the lungs, where they’re coughed up, then re-swallowed, completing the life cycle; can cause GI problems as well as respiratory issues like pneumonia

Good to know
Primarily a concern for young horses (2–15 months)

Bots & pinworms
What they do
Bots burrow into the mouth and stomach while pinworms lay eggs around the anus, causing itching

Good to know
Nowadays, both are more of a nuisance than a major threat to your horse’s health

*Egg Reappearance Period (ERP)
This is the number of weeks it takes parasite eggs to “reappear” in a horse’s manure after a deworming. The ERP is different for every class of dewormer, with benzimidazoles having the shortest time frame (only four weeks) and moxidectin providing the longest protection (10–12 weeks). A product’s ERP is what you use to plan the date of the next deworming, so that you’re killing parasites again before they start shedding eggs.

Modern deworming is focused on managing the herd, not just the individual horse, and working to reduce the parasite load on the entire property. Instead of focusing on eliminating your horse’s parasite load completely (which is impossible), your entire barn should work together to reduce the parasites present on the property. This means deworming the high shedders most frequently and the low shedders as infrequently as possible (to help reduce further parasite resistance).

What do you mean “shedding”—like the hair coat?! A horse’s shedding status refers how many eggs are present in the manure when you perform a fecal egg count test. The higher a horse is classified, the more eggs are present in his manure, which means he’s contaminating the pasture at a higher rate, and therefore exposing his pasture/barn-mates to more risk of disease. Sound complicated? Don’t worry, it’s not! We’ll walk you through how to handle your horse, and then you should chat with your barn manager and vet to address the whole herd.

1. Every horse should get dewormed at the beginning of the grazing season

• Generally speaking, worms are dormant when it’s regularly under 40° F or over 90° F. Your vet will be able to give guidance on your individual climate.
• Use a product that works. How do you know it works? Use something that doesn’t have widespread resistance (refer to the chart above), and use a fecal test! Test your horse before, deworm, and test 10–14 days after. If the counts are reduced by a certain percentage (your vet can help you with this), you’ve got a product that works great for your horse.

Fecal test? That sounds hard (and gross).
Happily, it’s neither! If your vet does it, you don’t have to do anything but ask.

If you’re more of a DIY-er, our Equine Fecal Test Kit makes it easy & clean. Just wait for your horse to pass manure, use the scoop to drop a bit into the collection container, close it up, put it in the envelope, and drop it in the mail. You’ll have your results in less than two weeks!

2. Know your horse’s shedding status, and deworm the high shedders at every ERP (Egg Reappearance Period) throughout the season

• Use a fecal egg count test to determine which horses on the property are high (500 eggs per gram), medium (200–500 epg), and low ( • Any horse identified as a high shedder should be dewormed at every ERP (this is based on the type of product you last used to deworm that horse).

3. Use a product that’s effective against bots and tapeworms at least once annually

Look for products containing the active ingredient Praziquantel, such as Equimax, Zimectrin Gold, or Quest Plus.

4. Deworm all horses at the end of the grazing season

You can use the same product selection guidelines you used in Step 1!

These are just examples, and your horse’s situation and your vet’s recommendation will likely be different. We hope this article was helpful, and that you now feel empowered to join the resistance against resistance and start deworming smarter!