The Worm That Kills – And Why Only Two Worming Chemicals Can Stop It

This article details what can happen if you leave equine worms unchecked. Worms can kill or make your horse very ill. That is a fact. Your horse’s best health advocate is your vet. Be sure to turn to him (or her) for advice concerning your horses’ health and how to deal with worms. ~ HfH

From: The Horse’s Back


Guest Post: After worming with standard wormers, some horses become critically ill with colic. Some even die. This post by Ann Nyland explains why this can happen in horses that haven’t received chemical wormers for a long time. It is adapted from her book, Horse and Donkey Worms and Worming (details below), and also appears on her website.

New guy in town: the encysted small strongyle

The old idea of worming in rotation lingers on from the early days, when it was first put forward in 1966. In the 1960s, the dangerous worm was the large strongyle (Strongylus vulgaris) and worming treatment in the 1980s and 1990s targeted this worm.
Yet today, the problem worm is the small strongyle (cyathostome).

Rotation is no longer advocated by equine parasitologists. At any rate, no amount of rotating will help against encysted cyathostomes.

Unfortunately, most advice given today is, sad to say, still aimed at the old way designed to eradicate the large strongyle – even though this worm is no longer the biggest problem.

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  • Maggie Frazier

    WOW! Information that I never had – only used the Moxidectin one time on my horse. And, as she said – was worried that it would make him sick!! Had heard horror stories about it – apparently untrue! This is the kind of info all horse owners should have -especially the part concerning no eggs in the manure!

    October 7, 2015
  • Robynne Catheron

    I read this article with baited breath, because I’ve known several horses who colicked after being wormed with Quest (I’m guessing that’s the US equivalent to Equest in AU?). I was grateful for the education on the reasons why so many horses had problems. But after reading the comments, some of which are from equine vets, the question of encysted small stongyles is still unanswered.

    Have you and your vet at HfH found an effective but safe protocol?

    October 7, 2015
  • Margo Nielsen

    For those who haven’t gone to the original article in The Horse’s Back, there is a free download of Dr. Nyland’s book on worming, from which this information was taken. Here it is:

    October 7, 2015