Guides
If your routine is a tube of wormer every eight to twelve weeks for every horse on the yard, you are following the textbook of twenty years ago. Every current guideline, British, American, Dutch and Australian, now says the opposite.
The old logic was simple: worms are bad, wormers kill worms, so worm everything on a calendar. The problem is that worms evolve. Every unnecessary dose selects for the worms the drug cannot kill, and there are only four drug classes for horses, with nothing new arriving.
The results are in. In the Netherlands, 80 to 90 percent of yards now carry benzimidazole-resistant small redworm. Ivermectin used to suppress egg counts for eight to ten weeks; current studies measure four to five. Moxidectin has fallen from twelve to sixteen weeks to the same. The American guideline puts it in one line: stop deworming on fixed intervals, and stop rotating products blindly.
Test first, treat the few. A faecal egg count from a dung sample, two or three times over the grazing season, tells you which horses are actually shedding eggs. Only about a third of adult horses shed enough to justify treating, the common threshold being 200 eggs per gram. The rest are left untreated on purpose: their untreated worms dilute the resistant ones, which is what keeps the drugs working for the yard as a whole. That is also why "worm everyone on the same day" is not hygiene but harm.
An egg count of zero does not mean a clean horse. Encysted small redworm larvae sit in the gut wall producing no eggs at all, and their mass emergence is a genuinely dangerous disease. Tapeworm sheds eggs too irregularly to count, so it has its own saliva test. This is why the modern routine is testing plus a considered autumn decision, made per horse with your vet or advisor, not a calendar and not a guess. Young horses of one to three, and older horses at higher risk, are the ones where a routine autumn larvicidal treatment still earns its place.
Everything above applies to adult horses. In foals, testing-first does not work: roundworm larvae do damage before any egg appears in the dung. Foals get planned treatments at set ages, all youngstock counts as high risk, and several products carry age restrictions. Breeding is the one place where the calendar survives.
The modern approach runs on memory: which horse shed what in spring, what was given and why, what the reduction test showed last year. The British CANTER guideline devotes an entire chapter to record keeping and asks for the date, the product and active, the test results and the rationale for every treatment. In the UK the medicine record for a horse not signed out of the food chain is a legal requirement, kept five years. In the Netherlands wormers are prescription-only, so the vet's records exist, but yours are what connect them.
Or put practically: the yard that writes nothing down cannot do targeted worming at all. It has nothing to aim with.
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