Historically most horse owners have wormed their horses every few months, (so called ‘interval dosing’), without checking the worm burden of the horse first.
However evidence is increasing all the time that we should change our policy:
Worms are becoming ever more resistant to all the groups of wormers, primarily due to the overuse of wormers. There are no new wormers on the horizon, so we need to use existing wormers as efficiently as possible to minimise the development of resistance.
Horses do not need to have a zero worm burden. A low worm burden helps stimulate immunity and is probably beneficial. A faecal worm egg count (FWEC) of 100-200 eggs per gram (epg) is considered acceptable by most clinicians.
In several recent surveys 85-90% of faecal samples had an egg count below this threshold and therefore worming was unnecessary.