Moving from routine dosing to smarter, evidence-based care

By Charlene Carroll 

For generations, horse owners were told to deworm every few weeks without question. Bottles lined the tack-room shelf, and a quick syringe seemed the responsible thing to do. Every horse owner knows the routine: the suspicious sideways glance, the acrobatic neck stretches that would make a yoga instructor proud, and the inevitable splatter of paste that somehow ends up everywhere except in the horse’s mouth. We’ve all spent a small fortune trying every flavour and formula in the hope that this one will go down without a fight.

But science has caught up with that tradition, and it’s changing how we protect our horses. Routine, calendar-based deworming has led to growing resistance among equine parasites, particularly small strongyles (cyathostomins) and roundworms in young horses. These worms have adapted to the drugs we once relied on, making many products far less effective than they used to be. 

The message from veterinarians and parasitologists worldwide is now clear: deworm only when needed, not just for deworming’s sake.

Why resistance matters

Each time a dewormer is given, the few surviving parasites, those naturally more tolerant to the drug, reproduce. Over the years, that means fewer worms can be killed by the same treatment. The result is resistance that can’t be reversed. Once a drug class stops working on your property, it’s gone for good.

What’s more, horses don’t benefit from constant chemical exposure. In adult horses, many carry low worm burdens that do little harm and may even help maintain natural immunity. Blanket treatments simply wipe out the “good competition,” leaving resistant strains behind.

The new approach: test first, treat smart

Modern worm control is about targeting, not timing. Instead of treating the whole yard every two months, owners now work with vets to run faecal egg counts (FECs). This simple test, done one to two times a year, measures how many parasite eggs a horse is shedding.

Most horses fall into one of three categories:

• Low shedders typically require only one or two treatments per year.
• Moderate shedders, who need monitoring and seasonal doses.
• High shedders who require more frequent control and follow-up testing.

This system keeps drug use to a minimum while maintaining healthy horses, and it slows the spread of resistance across entire regions.

Beyond the syringe

Good management still forms the backbone of worm control. Removing droppings from paddocks at least twice a week can dramatically reduce larvae on the pasture. Avoid overstocking, rotate grazing areas where possible, and don’t mix young, vulnerable horses with adults. Keeping grass shorter and avoiding damp, muddy patches where larvae thrive also helps.

When you do treat, make sure the dose matches the horse’s actual weight and check that the product you’re using targets the right worms for the season, for example, using praziquantel in spring for tapeworms or ivermectin after the first frost for bots.

A shared responsibility

In South Africa, resistance patterns are already emerging. The more each yard adopts targeted, evidence-based worming, the longer our dewormers will remain effective. It’s not just about one horse’s health; it’s about protecting an entire population.

By rethinking old routines and leaning on science instead of habit, we can keep horses healthier for longer and make sure the tools we rely on still work for the next generation of riders.