Horse worming in Australia and NZ: why targeted worming has replaced the calendar

For decades, horse owners were told to worm every horse every six to eight weeks, rotating brands. That advice has changed. Worms in Australian horses are now resistant to every class of worming drug, and the routine, calendar-based approach is a major reason why.

In 2025, Australian researchers and vets published the Australian Guidelines for Equine Internal Parasite Management in the Australian Veterinary Journal (Australian Veterinary Journal, 2025). This guide summarises what they mean for everyday owners. Your vet should tailor a plan to your horses and property.

Some older guidance, including Agriculture Victoria's basic horse care page, still describes worming every 6–8 weeks (Agriculture Victoria). The 2025 guidelines say calendar-based interval treatment is no longer recommended for adult horses. Talk to your vet if you're unsure which approach to follow.

Why the advice changed

The 2025 guidelines report that resistance among small strongyles (cyathostomins) and roundworms (Parascaris) is widespread on Australian horse properties, and is increasing across all three main drug classes: macrocyclic lactones (such as ivermectin and moxidectin), benzimidazoles and tetrahydropyrimidines (such as pyrantel). Frequent, indiscriminate dosing is described as a major driver (AVJ 2025).

The key idea is refugia – a population of worms that haven't been exposed to drugs. Those worms still respond to treatment and dilute the resistant ones. Worming every horse often wipes out refugia and leaves only resistant worms to breed.

Not every horse carries the same worm burden

Worms aren't spread evenly. The guidelines note that a minority of adult horses – roughly 10–30% – shed most of the worm eggs onto pasture. In one sample of 729 adult Australian Thoroughbreds, 67% were low egg shedders (AVJ 2025).

So most adult horses don't need frequent treatment. The aim is to find the high shedders and focus on them.

Faecal egg counts: the foundation of a worming plan

A faecal egg count (FEC) measures how many worm eggs are in a gram of manure (eggs per gram, or epg). Your vet or a lab can do this from a fresh sample.

The 2025 guidelines use these categories for strongyle eggs in adult horses (AVJ 2025):

FEC result	Category
Under 200 epg	Low shedder
200–500 epg	Moderate shedder
Over 500 epg	High shedder

Whether moderate shedders are treated more often depends on risk on your property. When stocking rates are high, pasture is short, or conditions suit worm larvae (the guidelines note 15–25°C), a more cautious threshold may be appropriate.

How often to treat

Once horses are categorised, the guidelines suggest (AVJ 2025):

• Low shedders: at least one foundation treatment a year.
• Moderate to high shedders: two to four treatments a year.

Foundation treatments cover parasites that FECs don't measure well, such as bots, tapeworm and large strongyles. Ask your vet which product and timing suit your area.

Limits of the FEC

FECs don't reliably detect bots, pinworms or tapeworms. The guidelines note tapeworm infection in Australian horses appears to be low (under 5%), and that many Australian products already include praziquantel (AVJ 2025). Your vet may recommend different tests if they suspect these parasites.

Is your wormer still working?

A faecal egg count reduction test (FECRT) checks whether a drug works on your property. A sample is counted before treatment and again about 14 days after. The guidelines recommend using horses shedding at least 200 epg for the test (AVJ 2025). If egg counts don't drop enough, the worms on your property may be resistant to that drug. Ask your vet to set this up.

Foals, weanlings and young horses

Young horses are the exception to the "treat less" rule. The guidelines say Parascaris (roundworm) is the main concern in foals and horses under 12 months, and that three to four treatments a year may be warranted for young horses, timed for when transmission is favourable, such as spring and autumn (AVJ 2025). Work with your vet on a youngstock plan, because roundworms have also developed resistance.

New arrivals

A new horse may bring resistant worms with it. The guidelines recommend keeping incoming horses in a separate quarantine area and treating them with a combination covering all three drug classes, as advised by your vet (AVJ 2025). Victoria's Code of Practice also recommends quarantining new horses for 10–14 days (Code of Practice). See biosecurity and isolating new horses.

Paddock management: the other half of worm control

Drugs are only part of the plan. The worm lifecycle happens on pasture, so paddock hygiene matters.

Pick up manure

The 2025 guidelines describe removing manure as perhaps the most effective way to break the strongyle lifecycle. In warmer months, manure should be removed two to three times a week (AVJ 2025).

Be careful with harrowing

Some Australian pasture guides recommend harrowing paddocks after grazing (Healthy Land and Water – horse management factsheet). The 2025 worm guidelines take a more cautious view: they say spreading manure isn't generally recommended, except in very hot, dry weather when eggs and larvae dry out, and that horses should then be kept off the paddock for a few weeks (AVJ 2025). If you harrow, do it in hot, dry conditions and rest the paddock afterwards.

Rest and cross-graze

Victoria's Code of Practice suggests spelling paddocks for at least six weeks, preferably 12, or grazing them with other species such as sheep and cattle (Code of Practice). Horse worms generally can't complete their lifecycle in non-equine hosts. The 2025 guidelines cite one field study in which horses grazing with cattle shed 50% fewer strongyle eggs (AVJ 2025).

Avoid overstocking

Crowded, short paddocks mean horses graze closer to manure. See paddock management for stocking and rotation.

Building a plan with your vet

A practical yearly plan might look like this – your vet will adjust it:

1. Do FECs on all adult horses before treating, ideally at times of year when worms are active.
2. Categorise each horse as low, moderate or high.
3. Give every adult a foundation treatment at least once a year, and treat moderate and high shedders more often as advised.
4. Run an FECRT to check your drugs still work.
5. Treat foals and youngsters on a separate plan.
6. Quarantine and treat new arrivals.
7. Pick up manure, rest paddocks and avoid overstocking.
8. Keep records of each horse's FEC results, drugs used and dates.

Dosing properly

Underdosing helps resistance develop. Estimate weight accurately (see how to measure a horse's height and weight) and follow the label. Victoria's Code says that if response to routine treatment is poor, effectiveness should be checked by a vet (Code of Practice).

At agistment

Worm control is harder on shared properties. Ask the agistor about their parasite plan, and include it in your agreement – see our agistment guide.

In New Zealand

NZ owners face similar resistance issues. The Code of Welfare for horses and donkeys covers parasite control as part of health care (MPI). Ask your vet about local FEC services.

Find an equine vet near you on horse.work to set up FEC testing.

FAQ

How often should I worm my horse?

For adult horses, the 2025 Australian guidelines recommend basing treatment on faecal egg counts rather than the calendar. Low shedders get at least one foundation treatment a year; moderate to high shedders get two to four (AVJ 2025). Foals and young horses need more frequent treatment.

Is worming every 6–8 weeks bad?

The 2025 guidelines say calendar-based interval treatment is no longer recommended for adult horses, because it drives resistance (AVJ 2025).

What is a faecal egg count?

A test that counts worm eggs per gram of manure. It shows which horses are shedding the most eggs, so you can target treatment.

Should I rotate wormers?

Rotating brands doesn't help if the active ingredient is the same, and routine rotation isn't the main strategy anymore. Use FECs and FECRTs to choose drugs that still work on your property, with your vet's advice.

Do I need to worm for tapeworm?

Tapeworm prevalence in Australian horses appears low (under 5%), and FECs don't detect it reliably (AVJ 2025). Your vet may include a product that covers tapeworm in a foundation treatment.

Should I harrow my paddocks?

Only with caution. The 2025 guidelines say spreading manure isn't generally recommended except in very hot, dry weather, followed by keeping horses off for a few weeks (AVJ 2025). Picking up manure is more effective.