Guides

Worming: the advice reversed, and most yards have not heard

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.

What changed

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.

What the modern routine looks like

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.

Three habits worth breaking today

• Dosing by eye. Eyeballed weights under-dose, and under-dosing breeds resistance. Use a weigh tape, and the British guideline adds a concrete number: add ten percent to a weigh tape estimate.
• Rotating brands. Ivermectin and moxidectin are the same drug class. Swapping between them rotates nothing.
• Egg counts as a sickness test. A count estimates egg shedding, not worm burden, and cannot tell you whether worms are why a horse is ill. That is a vet question.

Foals are the exception, in the other direction

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.

Why the record is the treatment plan

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.

What is worth writing down

• Every egg count with date and result, per horse
• Every treatment: product, active ingredient, dose, and why it was given
• The weight the dose was based on, and how it was measured
• The saliva test for tapeworm, with its score