The Resistance - Equine Parasites Rise Up

Horses grazing in a pasture.
Horses grazing in a pasture. Lubos Chlubny

Newsdate: Monday, September 9, 2019, 7:00 am
Location: SANTA FE, New MexicoM

Equine parasites are becoming resistant to our de-worming compounds and this is a HUGE concern, not only for your horse, but for the entire horse industry.

Horse cribbing on a fence.

Feeding time for horses

Parasite resistance is a serious problem and will require that the whole horse industry change its way of doing things.
© 2017 by Sedin

Recently I had to euthanize a world-class 4-year-old Quarter Horse mare, a real tragedy. She had badly damaged intestines, and after 2 colic operations, we decided to put her down. Her problems were partly brought on by a severe infestation of large roundworms (Ascarids).

This type of parasite is common in young horses but VERY rare in adults. My first thought when I saw all the large adult parasites at surgery was “Why isn’t this mare being de-wormed?” When I asked the trainer about her parasite control program though, she said she had been wormed every 5 weeks!

While this mare probably had a very rare “immune tolerance” to these parasites, what was most alarming was that the parasites were able to survive in the face of repeated dousing in de-worming chemicals. These worms were resistant to the de-wormers being used!

Resistance to our common de-wormers is a rapidly-growing problem that you should be aware of. Excessive and indiscriminate worming- people simply grabbing random products off the shelf and giving them to horses without having any idea of why they are doing it, or what they should really be doing- has led to the problem. Outdated parasite control theories and practices continue to perpetuate it.

In the last ten years, there has been increasing evidence for worm resistance to common de-worming chemicals. The scary thing is that if worms develop resistance to our available de-worming medication classes, we will no longer be able to protect our horses from worm-associated diseases.

Parasite resistance is a serious problem and will require that the whole horse industry change its way of doing things. It is critical that you, as a horse person, understand the nature of the problem, and help solve it. Simply de-worming your horses with a rotation of deworming pastes is no longer the best approach. Yes, you will need to de-worm, in order to keep your horses’ parasite numbers controlled. But the desire to eliminate worms has to be balanced with intelligent, targeted methods.  To slow the onset of resistance, what will be required is actually LESS, but SMARTER deworming.

Without Change – The future is full of worms in horses!

In the past, we assumed that if we rotated compounds, that parasites that were not killed by one class would be killed by the next.  This idea worked pretty well when the 3 main chemical classes each killed the majority of parasites.

But it is a bad idea now, for these reasons:

Two out of the three main classes of chemical no longer kill parasites adequately.  Parasites have become resistant to them. Their continued indiscriminate use will quickly result in COMPLETE resistance to these compounds.

  • Rotation using ineffective compounds ensures complete resistance to them while creating a false sense of security. The potency of the effective de-wormers “covers up” the inadequacy of the others in the rotation.
  • We are already seeing pockets of resistance to ivermectin and moxidectin (our last remaining effective class) and it is inevitable that this will increase. Use of these compounds without fecal testing will ensure a short effective life for them.
  • There are no new chemical compounds in the works right now. Research and development is costly and takes time.  Our emphasis should be on extending the effectiveness of the drugs we have.

To read the entire article regarding worms and their resistance, including what we as horse owners can do, read our comprehensive article here:

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Doug Thal DVM DABVP, Creator of Horse Side Vet Guide

Dr. Thal has been a horseman his entire life. His father was a steeplechase rider and polo player. He grew up on a family cattle and horse ranch in Mora County, New Mexico and has been riding and training horses since early childhood. He has been an equine veterinarian since 1993 working on a mix of performance and pleasure horses. His special interests are lameness and surgery. Thal is board certified by the American Board of Veterinary Practitioners, in Equine Practice. He continues to manage his equine veterinary hospital, Thal Equine, outside of Santa Fe, New Mexico.

Press release provided by Doug Thal DVM DABVP

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