I’ve been thinking a lot about parasites lately — partly because I’ve just brought home two foals, and partly because the more I read, the more I realise how much of our inherited “knowledge” about worms is decades out of date. With a background in microbiology, I’ve been doing my own faecal egg counts (FECs) for years, but with youngsters on the land the stakes feel different. I want to set them up well, not simply reach for a syringe because that’s what everyone else does, but also not put them at risk by assuming they can regulate parasite burdens alone.
As I’ve dug deeper into the research, a pattern emerges: we’ve been fighting parasites with the wrong weapons, at the wrong times, and for the wrong reasons. The good news is that the solutions are already in our hands. They lie in our grazing plans, our soil biology, our stocking density, our rest periods, and our willingness to see the land as an active part of the parasite cycle rather than a passive backdrop.
This is the story of that shift.
The Parasites We Fear… and the Ones We Don’t Need To
For decades, equine worming culture was shaped by the threat of Strongylus vulgaris, the “large strongyle”. It was once a major killer, causing thromboembolic colic and severe intestinal damage. But thanks to decades of anthelmintic use, large strongyles are now rare on most managed holdings (Nielsen & Reinemeyer 2018; Matthews 2014). Their near‑eradication is one of the great successes of modern parasite control — and ironically, one of the reasons we must now rethink our approach.
What remains in otherwise healthy adult horses are parasites that are ubiquitous, usually well tolerated, and rarely cause disease unless the horse is very young, very old, or immunocompromised. Cyathostomins (small redworms) are everywhere. Most horses carry them. Most horses cope with them. The aim is not eradication — it’s balance.
This is where modern guidance diverges sharply from the old “worm everything, often” approach (AAEP 2024; BEVA 2022). The goal is no longer to eliminate parasites, but to manage them in a way that protects both horses and the drugs we rely on.
The “First Frost” Myth — and Why It’s Backfiring
One of the most persistent pieces of inherited wisdom is: “Worm after the first frost.” It sounds sensible — frost kills things, right?
Except it doesn’t kill worm larvae. It sends them dormant.
Strongyle larvae survive freezing temperatures — and even repeated freeze–thaw cycles — by entering a state of arrested development (Nielsen et al. 2022). When you worm after the first frost, you kill the worms inside the horse, but the drug passes out in the dung. The larvae in that dung are now exposed to the drug, selecting for resistance. They sit dormant all winter, then wake up in spring — and your pasture is suddenly seeded with the survivors. Worse still, if you harrow or roll your fields at this point, you spread those resistant larvae even further.
Modern recommendations flip this on its head: worm (if needed) before spring turnout, not after the frost. This reduces the number of larvae emerging onto pasture when temperatures rise and avoids creating a “spring bloom” of resistant worms.
It’s a small shift with big consequences.
The 20% That Matter Most: High Shedders
I used to think that if my horse had a high FEC, it meant he had a high worm burden. But that isn’t what FECs measure. They identify high shedders — the horses that shed the most parasite eggs into the environment.
Across herds, parasite shedding follows a striking pattern: 20–30% of horses shed around 80% of the eggs (Kaplan & Nielsen 2010; Nielsen et al. 2022). These individuals maintain the parasite population on your land.
We worm them not because they are sick or carrying dangerously high burdens, but because they are the main source of pasture contamination. Treating them reduces infection pressure for the whole herd. It is a shift from treating individuals “just in case” to treating strategically for the health of the system.
The rest — the low shedders — are left untreated, forming part of the refugia, the susceptible worm population that slows resistance.
Refugia: The Most Important Concept No One Talks About
Refugia refers to the population of worms that have not been exposed to wormers, remain susceptible to the drugs, and slow the development of resistance (Matthews 2014). When you worm unnecessarily, you shrink the refugia. The only worms left are the resistant ones. They breed. They dominate. And suddenly your wormer doesn’t work anymore.
Maintaining refugia is now considered essential. It’s why we don’t chase a 0 epg, we don’t worm low shedders, we don’t worm on a calendar, and we don’t worm after the frost. Refugia is the safety net that keeps our drugs working.
The Lifecycle: Why Pasture Management Matters More Than Chemicals
Most equine strongyle larvae can survive on pasture for up to 90 days, sometimes longer in cool, moist conditions (Nielsen & Reinemeyer 2018). They climb up blades of grass in films of dew, waiting to be eaten.
This means that overgrazed paddocks force horses to graze closer to the ground, where larvae are concentrated. Poached areas become hotspots of contamination. Short rest periods don’t break the cycle. Rotational grazing — with long rest periods — dramatically reduces infection pressure. Cross‑grazing with cattle, sheep or poultry is the gold standard, because most equine parasites cannot complete their lifecycle in these species.
Good grazing is parasite control. Poor grazing is parasite amplification. Modern guidance is unequivocal: pasture management is the foundation of parasite management. Wormers are the backup plan.
FECRT: The Test That Tells You Whether Your Wormer Still Works
Most horse owners know about FECs. Very few know about FECRT.
A Faecal Egg Count Reduction Test measures how effective a wormer is on your yard. It involves taking a FEC before worming, worming the horse, then repeating the FEC 10–14 days later. If the egg count drops by more than 95%, the drug is working. If it drops by less than 90%, resistance is likely present. Anything in between is a warning sign (WAAVP 2013). FECRT should be performed on horses shedding at least 200 epg to ensure meaningful interpretation.
Without FECRT, you have no idea whether your wormer is doing anything. On yards with multiple horses, this is critical. One resistant population can spread across the whole grazing system. FECRT is now recommended annually for each drug class used on a property (AAEP 2024). It is the difference between parasite control and parasite guesswork.
The Environmental Cost: The Part We’ve Been Ignoring
Macrocyclic lactones such as ivermectin and moxidectin pass through the horse largely unchanged. Once in the dung, they continue to act as potent insecticides. They kill dung beetle larvae, reduce adult beetle survival, slow dung breakdown, disrupt nutrient cycling, and remove a vital food source for birds, bats and small mammals (Strong et al. 2022). Dung beetles are ecosystem engineers: they aerate soil, cycle nutrients, improve water infiltration and support a vast web of soil microbiota. When we lose them, we push our soils into a downward spiral of compaction, reduced fertility and reduced resilience to drought and heavy rainfall.
And the effects don’t stop underground. Dung beetles and other invertebrates form the base of above‑ground food webs. When their populations collapse, insectivorous birds, bats and small mammals lose a key food source. Wormers don’t just affect worms in your horse’s gut — they affect everything that depends on the tiny lives in the soil.
Water contamination is another overlooked issue. When manure containing wormer residues is left on pasture, rainfall washes those chemicals into streams, ponds, wetlands and groundwater. In many parts of the UK, this includes highly sensitive habitats such as chalk streams, peatlands, upland catchments and Sites of Importance for Nature Conservation. These ecosystems rely on invertebrate life that is extremely sensitive to macrocyclic lactones, so even low‑level contamination can have disproportionate ecological effects. For those managing herds on common land, uplands or nationally and internationally important conservation sites, the implications are especially significant.
What we already know from companion‑animal parasiticides makes this even more concerning. Studies of flea treatments such as fipronil and imidacloprid have found these chemicals in over 90% of tested UK rivers, often at concentrations exceeding ecological safety thresholds. They persist through wastewater treatment and accumulate in sediments, exposing aquatic invertebrates to chronic, biologically active doses. The environmental pathways for equine wormers are the same — but the quantities involved are far greater.
These contaminated watercourses ultimately feed into our drinking water supply, creating a plausible route for human exposure. Yet water companies do not routinely test for equine wormer residues. The absence of data is not evidence of safety; it reflects a monitoring gap. Given what we already know from pet parasiticides, this blind spot deserves far more attention than it currently receives.
Wild and Semi‑Feral Herds: A Different Logic
On commons, moors and large rough‑grazing systems, horses live in a more natural parasite ecology. They range over large areas, graze at low stocking densities, share land with sheep or cattle, move constantly, and rarely poach the ground. Under these conditions, parasite burdens tend to remain low and stable without routine worming (Nielsen 2025).
The risks arise when natural patterns are disrupted — when horses are brought into smaller areas, when supplementary feeding concentrates them in one place, or when new horses are introduced without testing. For semi‑feral herds, the priority is monitoring, strategic testing, avoiding unnecessary worming and protecting refugia. Blanket worming of wild or semi‑feral herds is not only unnecessary — it is ecologically damaging.
Where This Leaves Us
For my own foals, this deep dive has changed everything. Responsible parasite control no longer looks like a calendar and a row of syringes. It looks like understanding the lifecycle, protecting refugia, identifying high shedders, using FECRT to check what works, designing grazing that breaks the cycle, and treating only when the evidence says “yes”.
Parasites are part of the system. Our job is not to eradicate them, but to manage the ecology so that horses, land and wildlife can all thrive.
Managing the System: Practical, Evidence‑Based Parasite Control
The evidence is clear: a good grazing plan is the most effective parasite control tool we have. In many studies, well‑designed grazing systems outperform routine worming. And the beauty is that these same practices also support soil health, forage diversity and overall equine wellbeing.
Rotational grazing is the foundation. Moving horses every few days — or at least weekly — prevents overgrazing and reduces exposure to larvae concentrated at the base of the sward. Resting each paddock for a minimum of 30 days, and ideally 60–90, allows larvae to die off naturally. Longer rest means lower parasite pressure. Cross‑grazing with cattle, sheep or poultry is even more effective, because most equine parasites cannot complete their lifecycle in these species.
For horses kept on track systems, regular poo‑picking is essential. Research shows that removing manure at least twice a week significantly reduces parasite transmission, even in high‑traffic areas where grazing is limited.
Diagnostics should guide treatment. Quarterly FECs help identify high shedders, while annual testing for tapeworm and encysted cyathostomins ensures targeted, evidence‑based decisions. Worming should only occur when necessary — ideally before spring turnout, when larvae begin to migrate up the grass. And whenever wormers are used, a FECRT should be performed at least once a year to monitor drug efficacy and detect early resistance.
When worming is necessary, environmental protection matters. Horses should be kept off pasture for at least three days after treatment, with all manure collected and hot‑composted separately for at least a year. Ideally, manure should be kept off grazing land for seven days, especially in wet or warm conditions when larvae are most mobile. This protects soil life, water quality and the wider ecosystem.
Special Considerations for Foals, Elders and Compromised Horses
Foals sit in a category of their own. Unlike healthy adults, they do not yet have the immunity needed to regulate strongyle burdens, and they are uniquely vulnerable to Parascaris (ascarids). Because ascarids can cause life‑threatening impactions and because resistance to ivermectin and moxidectin is now widespread, modern guidelines still recommend interval worming during the first year of life, using products known to be effective against ascarids. This is one of the few situations where routine treatment remains justified. After twelve months, they can transition to the same evidence‑based, FEC‑led programme used for adults.
It’s also important to be clear about what we can’t rely on. Many people still look for a “pot belly”, a dull coat, or a tucked‑up appearance as signs of worms in youngsters. While heavy burdens can cause these signs, they are far from specific. A pot belly may indicate worms, but it may just as easily reflect diet, growth stage, gut microbiome changes, forage quality, or simply normal conformation in native foals. Likewise, a rough coat or vague ill‑thrift can stem from nutritional imbalance, stress, dental issues, or underlying disease. These visual cues are prompts to investigate — not diagnostic tools. Only diagnostics can confirm or quantify parasite burden.
Older horses and those with compromised health also require more individualised care. Some maintain excellent parasite immunity well into their twenties, while others begin to shed higher numbers of eggs as their immune function declines. Horses with chronic illness, metabolic disease or poor condition may also struggle to regulate burdens. For these individuals, a tailored plan developed with a vet who is familiar with the latest guidance is essential — and it’s worth saying explicitly that not all vets are fully up to date with the shift toward diagnostic‑led, pasture‑based parasite control.
Across all these groups, the principle remains the same: visual signs alone are not reliable indicators of parasite burden. They may raise suspicion, but they cannot confirm anything. Even in higher‑risk horses, the goal is still to use wormers judiciously, guided by evidence, and supported by strong pasture management.
Disclaimer
This resource is for educational purposes only and does not replace veterinary advice. Always work with your vet to develop a parasite‑control plan tailored to your horse, your land and your management system.
References
AAEP (2024) Internal Parasite Control Guidelines. American Association of Equine Practitioners.
BEVA (2022) Anthelmintic Resistance Toolkit. British Equine Veterinary Association.
Kaplan, R.M. & Nielsen, M.K. (2010) ‘An evidence-based approach to equine parasite control’, Equine Veterinary Education, 22(6), pp. 306–316.
Matthews, J.B. (2014) ‘Anthelmintic resistance in equine nematodes’, Veterinary Parasitology, 204(1–2), pp. 8–16.
Nielsen, M.K. & Reinemeyer, C.R. (2018) Handbook of Equine Parasite Control. Wiley Blackwell.
Nielsen, M.K. & Matthews, J.B. (2014) ‘Equine parasite control: A review’, Veterinary Parasitology, 204(1–2), pp. 8–16.
Nielsen, M.K. et al. (2022) ‘Evidence-based equine parasite control: Revisited’, Veterinary Parasitology, 302, 109–123.
Nielsen, M.K. (2025) Parasite Control in Horses. MSD Veterinary Manual.
Strong, L. et al. (2022) ‘Environmental impacts of macrocyclic lactones’, Veterinary Parasitology, 305, 109–145.
WAAVP (2013) ‘Guidelines for evaluating the efficacy of equine anthelmintics’, Veterinary Parasitology, 194(1), pp. 84–97.
