24/09/2025
Spring is here! Therefore, it is time for your annual reminder that
you👏do👏not 👏need👏to👏automatically👏rotate👏wormers👏
There is nothing quite like people saying “I rotate my wormers!” to get my eye twitching. It is mostly the way people tell me, half proud of themselves for doing what they have been told is the right thing. So I cannot slap them as Batman is slapping Robin because it isn’t really their fault.
But the truth is, rotating actives is not always the right thing; you’ve been lied to, with the idea that it will prevent drug resistance and keep your horses healthy.
The “rotating” wormers regime puts people into a system of deworming for the sake of deworming: Oh, I used a “red” wormer in autumn, now that it is spring it’s time for a “green” one.
I have nothing against changing actives when worming, however swapping actives because the wheel on the stable fridge tells you to is very unscientific. And it does not prevent drug resistance.
If you want to prevent drug resistance there are two things you need to do:
1) Get a FEC prior to every deworming treatment. Let the FEC results guide you to choosing what dewormer to use – if you need to use one at all. The choice should be based on age, weather, climate, what worms are present, what drug resistance you have on your property, paddock situation/herd mates. You should always choose your dewormer based on the CURRENT infection, not a past treatment.
2) Find out what drug resistance you have on your property by getting FECs done AFTER treatment as well. If it turns out that the classically labelled “green” drenches (the benzimidazoles) do not work well, then rotating to them in spring is worse than useless.
It does not have to be your job alone to decide if your horse needs to be dewormed, or with what it should be treated with if it does. That’s the job of people like me (a parasitologist!) and vets. That is what we are here for, and I would much prefer you to ask rather than to continue putting unnecessary or inappropriate drenches down your horses’ throats.
Lastly, make sure to get your horses checked via FEC this spring. All details on how to get samples to me are up on the website (link in comments).
NB: This is a respost from Spring 2024.