Many experts consider that faecal worm egg counting has a less important role to play during winter-time, bearing in mind that the key parasite being looked for, the small redworm, will almost certainly be encysted in the gut wall. Indeed, a minimum of 90% of these parasites will under normal circumstances be lying dormant over winter. In cold months, the low temperatures cause the parasite to seek a way of surviving, so once small redworms are ingested off the pasture, they encyst; once encysted, they can stay in the gut wall for up to two years.

Encysted small redworm, one of the most common and harmful worm species found in horses, is the larval stage of the small redworm. The small redworm burrows its way into the gut lining where it stays dominant and thus can evade detection by the traditional faecal worm egg count. If left untreated these parasites have the potential to emerge, generally in early spring, in vast numbers. This mass emergence can have serious and even fatal consequences, a fact which is dangerous to overlook. For best results against encysted small redworm, we recommend that horses should be treated in late autumn or in early spring with a moxidectin, or with a five-day course of fenbendazole.