Wormer Resistance in Horses Australia | Causes & Prevention
Horse Wormer Resistance: Why It’s a Growing Concern in Australia
Most Australian horse owners have heard the term wormer resistance by now. It describes worms that have evolved to survive chemical wormers that once killed them. Much like antibiotic resistance in people, wormer resistance limits the tools we have to keep horses healthy. And with no new worming drugs currently in development, the threat is very real: if resistance spreads too far, some properties may struggle to keep horses at all.
What Is Wormer Resistance?
Wormer resistance develops when a few worms naturally survive treatment. Those resistant worms reproduce, and over time, the balance tips until most worms on your property can withstand the chemical. Unlike seasonal worm cycles, resistance is permanent — once it develops, the affected worm population won’t “revert” back to being susceptible.
In Australia, resistance is most common in:
- Small strongyles (cyathostomes): The most widespread parasite in adult horses. Encysted larvae can cause diarrhoea, weight loss, and colic when they emerge en masse.
- Roundworms (Parascaris equorum): Particularly dangerous for foals and yearlings, causing intestinal blockages and respiratory issues. Resistance is rising rapidly in this group.
- Pinworms: While less life-threatening, resistant strains contribute to persistent tail rubbing and discomfort.
Why Resistance Matters
Unchecked resistance puts horses at risk of serious illness. Problems include:
- Colic and diarrhoea caused by strongyles or tapeworms.
- Poor growth and pot-belly appearance in foals from roundworm burdens.
- Reduced performance and condition in adult horses.
Without effective wormers, the risks of parasite-related disease increase — and with no new worming classes being developed, preserving the tools we already have is essential.
How Wormer Resistance Develops
Several common practices speed up resistance:
- Over-worming: Frequent, unnecessary treatments expose worms to chemicals too often, killing off only the susceptible worms and leaving resistant ones behind.
- Underdosing: Giving too little product (by underestimating weight or poor application) allows some worms to survive and adapt.
- Overstocking pastures: Forcing horses to graze close to manure increases re-infection and spreads resistant worms faster.
- Worm-and-move strategies: Treating horses and immediately shifting them to clean pastures may unintentionally leave only resistant worms to populate that new ground.
How to Slow Wormer Resistance
The good news: smart management can protect the effectiveness of wormers for years to come. Key strategies include:
1. Worm Smarter, Not More Often
Most adult horses only need 1–2 foundation treatments per year (spring and late autumn). Young horses under three years old need more frequent treatment, as they are more vulnerable. Avoid unnecessary worming — low parasite levels are normal and even help stimulate immunity.
2. Always Dose Accurately
Use a weigh tape or scale and dose for the heaviest horse in the group. Add a small buffer rather than risk underdosing. Wormers have a wide safety margin, but underdosing encourages resistance.
3. Rotate Actives Sensibly
Rotate by chemical class annually, not every dose. For example, use mectin-based products one year, then switch to a different class such as pyrantel or benzimidazole the next. This helps maintain refugia — the population of worms that remain susceptible.
4. Manage Pastures Effectively
- Pick up manure at least twice per week, year-round.
- Rest and rotate paddocks where possible.
- Cross-graze with cattle or sheep if available — most horse worms don’t survive in other species.
- Keep muck heaps well away from grazing areas.
5. Quarantine and Treat New Horses
New arrivals should be wormed with a broad-spectrum product before turnout, and kept separate for a few days. This helps prevent resistant worms being introduced to your property.
Choosing Wormers That Support Resistance-Aware Programs
Delzani supplies a carefully selected range of wormers ideal for strategic parasite control in Australian horses:
- Equimax Plus Tape – Treats bots, strongyles and tapeworms in one dose.
- Virbac Ultimum – Dual-active broad coverage, ideal for resistance-aware programs.
- Virbac Equimax – Reliable all-rounder covering strongyles, bots and tapeworms.
- Strategy-T – Important rotation option containing pyrantel and oxfendazole.
Final Thoughts
Wormer resistance is already present in Australia, but it’s not too late to act. By worming strategically, dosing accurately, and managing pastures well, horse owners can protect both their horses and the effectiveness of wormers for years to come.
Shop Delzani’s complete range of horse wormers today and build a resistance-aware program that keeps your horses healthy, season after season.
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