Daily dewormers ineffective in horse parasite resistance battle


Horse owners are always looking for ways to keep their horses healthy, but parasitologist Martin Nielsen says that using a daily dewormer is not the solution.

“There’s so much drug resistance out there to these daily dewormer products that they actually don’t work so well anymore,” Nielsen says.

“Horses are supposed to have parasites. It is a normal state for them. There’s absolutely no way we can prevent them from acquiring these parasites.”

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3 thoughts on “Daily dewormers ineffective in horse parasite resistance battle

  • November 25, 2019 at 12:20 pm

    No longer physically around horses anymore. But this is interesting – not surprising really. I remember when I first got my horse – we were worming every 2 months, with alternating various types of wormers. I remember a friend who took a manure sample to her vets every so often & then wormed or not depending upon what was in there – which really made more sense. Like anything else, using the same thing over & over would seem to cause immunity to that particular product. I wonder how many barns will look at this and make a change!

  • November 25, 2019 at 12:22 pm

    I have 2 horses that are very high shedders with egg counts of 1,200 and 2,500. I did a 5 day fendendazole wormer on the one with 2,500 and 6 months later his count was 2,500. We were advised to put these two horses on daily wormer. After 6 months on the daily wormer the egg count on the one at 1,200 was zero. The horse with the 2,500 had a count of 25.

    It seems counts that high are very bad for the horse so I was willing to go the daily wormer. Should I stay with this? There is so much conflicting information.

    The rest of the herd is wormed based on egg counts which are usually very low. After the first cold snap usually in January they all get Quest plus.

    • December 4, 2019 at 1:15 am

      Hi Pat,
      I don’t think there is any justification for a daily dewormer on the long term. I am glad that it seems to have worked in your case, but it is likely that any effective dewormer would have accomplished the same reduction. The 5-day fenbendazole (I have another video on that one) is highly unlikely to work well anymore – anywhere – due to all the resistance, so that is probably why it didn’t work for you either. I would not continue with the daily dewormer if I were you.

      And I also have a video on deworming after the first frost. That is another misunderstanding. Not that it hurts the horses, but it doesn’t really do anything for parasite control, and we shouldn’t let our parasite control strategy guide by when we get the first frost. The frost doesn’t kill the eggs and larvae anyway.

      Martin K. Nielsen


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