Stop Worming Your Horse Willy Nilly, Please

I learned a long time ago, parasites are not managed in the EQUINE world like they are in the livestock world. They aren’t managed correctly.

Livestock have far more economic value in the United States and Worldwide, and really, that is probably why so many farmers have more knowledge on parasites and how to effectively handle them than do horse owners.

It may also be that horses, thankfully, tend to handle parasite burdens better and be less plagued by them, but that is no excuse to not educate yourself on how to most effectively treat your horse’s parasite load in a way that works best for the long term.


  • Do not Rotate Wormers

Ideally, you want to worm with the wormer that will best address the parasites you have. You want to use that wormer for as long as it works well before changing wormers. It is better to worm twice with a single type of wormer to kill the needed parasite load than to switch to another wormer. Building resistance is a dangerous thing. Switching around wormers willy nilly builds resistance. Thanks to parasite resistance, MANY parasites that are carried by sheep and goats are 100% resistant to ALL known approved wormers in the world, and this is starting to pop up in the equine world. What a scary thought!

  • Do not Worm Routinely

You do not need to worm a horse that isn’t carrying a notable worm burden. Fecals are, again, your friend. Find out who needs wormed, what worms they carry and then worm with whatever wormer will be effective. Stick with that WORMER as long as you can.

  • Do not worm all horses the same

It comes back to fecals and knowing what you’re dealing with, and in all herds, you have a horse or two that are your carriers. You need to know which horses and worm based on who actually has the worm load.

  • Do not Worm Daily with Feed Through varieties  

Feed through wormers are a combination of all the bad things that happen from willy nilly worming. They work on routine, worming horses all the same, they discard the value of fecal egg counts and while they do not work by using rotation, they cause resistance by their consistent and long term use.

Worms do not usually make horses emaciated, but they can lead to anemia, rough coats and pot bellies. Parasites can cause emaciation in livestock, so horses are fortunate that is rare in equines.

Look at Doc prior to worming:

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(This was Doc right before coming to HOP)


This is Doc less than a week after being wormed effectively (notice the belly change)


Luna after her first effective worming, but her parasite load, based on fecals, was so high, it took time and repeated worming, for us to address her parasite burden correctly


ONE MAJOR exception to HOW YOU WORM comes when we speak of horses that have EXCEEDING high fecal egg counts and/or are in poor body condition.

A very effective wormer can mean a mass die off of parasites and causes impaction colic.  In these cases, using a gentle and marginally effective wormer is best. Our approach is a dose of 10% Fenbendazole. We wait and usually refecal, and then we will do another dose and then in 7-14 days, we will use Equimax.

Remember, wormers like Fendedazole, Pyrantel Pamoate and Ivermectin have high resistances or limited effectiveness on parasites. The last two are inexpensive, so they are commonly purchased, but they do not usually address the full needs of a horse with a high parasite burden.

So, what should you be doing?

We suggest fecals in the spring and fall, at least. Knowing the signs of parasites not detected by fecals, as well. Fecals are one tool, along with monitoring your horse’s condition, manure, looking  for tail rubbing, pasture management and the like, to avoid worming “Willy Nilly.”

You can have your vet do these or use MidAmerica Lab, via mail.   Once you know what each horse is fighting, then you can worm effectively. Call your vet and ask what wormer worms best for the parasites your horses are carrying, and then stick with that wormer after you fecal 7 days post administration to be sure the wormer WAS effective.

Additional reading on Parasites:


Medical Animal on Fecals

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