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SESK: Stuff Everyone Should Know · sesk.info/horse-overdeworming-resistance

What can happen if horses are dewormed too often?

Over-deworming can promote resistant parasites.

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  1. Worms can become drug resistant ✔
  2. The horse loses its appetite for hay
  3. Hooves grow too fast

Why: UMN Extension warns that over-deworming can promote the growth of resistant parasites. That is why many vets use fecal egg counts and targeted plans instead of treating everything on a fixed schedule.

Huh, didn't know that: Merck says mowing and breaking up manure in pastures limits the buildup of parasite eggs.

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References

Answer: Worms can become drug resistant. UMN Extension warns that over-deworming can promote the growth of resistant parasites. That is why many vets use fecal egg counts and targeted plans instead of treating everything on a fixed schedule.

  1. Deworming your horse (UMN Extension) extension.umn.edu/node/1456
    Over deworming can promote the growth of resistant parasites.
    Checked 2026-10-10.
  2. Horse Parasites and Flies (Merck Veterinary Manual) www.merckvetmanual.com/horse-owners/routine-care-of-horses/horse-parasites-and-flies
    Mow and break up manure in pastures to limit the accumulation of parasite eggs passed into feces on pastures.
    Checked 2026-10-10.

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