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.