best deworming practices


The best way to deworm your horse!


  • This used to be as simple as giving a different product every couple of months, willy – nilly, known at rotational deworming.


  • Now, we have learned that many horse parasites are actually resistant to the only dewormers that are available. This is bad. One more piece of bad news – there are no new dewormers currently being created, so what we have is what we have.


  • The American Association of Equine Practitioners (AAEP) has up-to-date guidelines for deworming protocols. These guidelines are long and involved, but give great detail in case you are into that sort of thing.



Why fecal egg counts and targeted deworming are best instead of rotational deworming.

  • It starts with drug resistance. In this case, resistance to anthelmintics which kill worms. Worms and parasites that are unaffected, and therefore are not killed, by these drugs are called resistant. In the case of horse parasites, anthelmintic resistance is a growing concern.


horse manure in a pile

A fecal egg count test is easy! A sample of your horse’s manure is collected and send via mail for testing. The lab does the rest! Always work with your veterinarian when creating a fecal testing and deworming program for your horse, it’s typically in the spring and the fall.


  • When resistant worms reproduce, they create more resistant worms. Logical and bad. But there is a segment of the worm population known as refugia. Refugia just mean the number of worms that don’t contact a dewormer when one is given.


  • Suppose you deworm your horse. There might be worms in the encysted stage of their life cycle in your horse’s gut. Those encysted buggers are in refugia. So are the worms that are still in egg form in your pasture. So are the worms that live in your barn mate’s horse in the next paddock over who did not deworm.


  • So now the worm population is a two-parter – the resistant ones, and the ones that are in refugia. We need the non-resistant worms from the refugia population to continue to reproduce with the resistant worms to dilute the resistance.


  • Imagine a scenario where every single non-resistant worm was eradicated. This could be on a farm level, a state level, or even larger. The only worms left are the resistant ones. You would be out of luck with any dewormer, and the populations could increase so greatly that horses are at risk of the complications of worms – colic, impactions, etc.


  • Most adult horses now have parasite loads that exist, but do not pose an immediate threat to the health of your horse. But if only resistant worms are left, those loads become harmful.



dewormer options at the store

Lots of dewormers create lots of confusion. Your vet and some fecal egg counts can make things easier to manage.


Now the fecal egg count (FEC) enters the picture.


  • The fecal egg count test tells you and your veterinarian how many eggs per gram (EPG) your horse is shedding via his manure. It’s not an indication of how many actual worms he has.


  • Horses are classified into high, medium, and low shedders depending on their FEC results. Generally speaking, low shedders have less than 200 EPG. Medium shedders are somewhere around 200-500 EPG and high shedders are over 500 EPG.


dewormers on store shelves

Pick wisely, with the help of your vet!


Fecal egg counts are not intended to replace deworming but should be a guide to indicate when to deworm and which horses to deworm.


  • It’s recommended practice to take a horse population as a whole on a farm or boarding facility and deworm the high and medium shedders. These horses shed so many eggs that the life cycles of the worms are fast and furious.


  • Most low shedders can remain as is.


  • It’s prudent to do a fecal egg count in the spring and fall, and ideally, all horses on the property should be tested to get an idea of the herd status.


Why doesn’t rotational deworming work?


  • Rotational deworming is when you give your horse a dewormer on a set schedule. I have heard that many people do this every 6 or 8 weeks, or every few months. Again – this is not recognized by the AAEP as an effective way to take care of your horse, and may, in fact, be harmful.


  • This overuse of medications to horses that may not need it is the perfect storm for creating resistance to dewormers.


  • It’s precisely this random dosing of meds that leads to anthelmintic resistance. Remember – there are NO NEW DRUGS on the horizon if all worms become resistant.


When you follow a rotational program without fecal egg tests, you are shooting in the dark.


  • You have no way of knowing what your horse’s parasite load is. And you have no idea if what you are giving is correct for that stage of his intestinal parasite’s life cycle. And you have no idea if it even works.


  • The only ways to monitor your horse’s parasite load are to do a FEC. Then, based on his status as a shedder (high, medium, or low), target his worm population.


  • About 2 weeks later, perform another FEC, this time it’s called a fecal egg count reduction test (FECRT) and the results will tell you and your Vet are on the right track with the dewormer that you used.



Time to stop rotational deworming and stick to the fecal egg count!


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You can actually do a mail order fecal egg count.