Autumn Horse Worming: Smarter Parasite Control (Not Just More Drenching)

Article by Veterinarian, Dr. Rebecca Walker

For years, horse owners were taught to worm “by the calendar”, every 6–8 weeks, rotating products, and blanket-treating the whole herd.
We now know this approach drives drug resistance and isn’t actually better for your horse.
Modern veterinary parasite management, including the work of Holly from Worm-Ed, focuses on targeted, data-driven worming in New Zealand that protects both your horse and the future effectiveness of wormers.
Autumn is a great time to review your programme and make sure you’re doing what actually works.

1. Every horse is different (and should be treated that way)

One of Holly’s biggest messages:

Not all horses shed worms at the same rate.

Horses fall into three main groups:

Why this matters:

Only 20–30% of horses shed most of the worms.

Research shows that only 20–30% of horses shed the majority of worm eggs. Blanket-worming every horse:

• Wastes product
• Increases resistance
• Treats horses that don’t need it

Best practice

• Use faecal egg counts (FECs)
• Identify which horses actually need treatment
• Target worming where it’s genuinely needed

2. You don’t need to rotate wormers

Older advice encouraged rotating wormers every few months.
This is now considered outdated and ineffective.

Why rotating wormers doesn’t work

Rotating products:

❌ Does not prevent resistance

❌ Can actually speed resistance development

❌ Makes it harder to track effectiveness

A better approach

• Use FEC results to guide treatment
• Reserve stronger products for when they’re truly needed
• Use wormers strategically, not routinely

3. Moxidectin: powerful – but only once per year

Holly’s key message: Only use a moxidectin drench ONCE a year

Why?

Moxidectin:

•  Is extremely effective
• Targets encysted larvae
• BUT it’s one of our last-line drugs

✔️ Once yearly
✔️ Strategic timing
✔️ Only when justified

Not “every season”, and not “just in case”.

4. Timing matters – but not by the calendar

Instead of “spring drench” or “autumn drench”, modern programmes ask:

• What does your FEC show?
• Is your horse a high or low shedder?
• What parasites are relevant on your property?

This is why Worm-Ed programmes are individualised – not one-size-fits-all.

5. Don’t move horses straight onto fresh pasture

Another great Holly tip: Don’t put your horse onto a clean paddock for at least 72 hours after worming

Why?

• Worm eggs continue to be passed after treatment
• Moving immediately contaminates your clean paddock
• You undo all your hard work

✔️ Leave horse on same paddock
✔️ Pick up manure if possible
✔️ THEN move once egg shedding has stopped

6. Harrowing can make things worse

Holly’s advice: Don’t harrow paddocks unless you can cross-graze afterwards

Harrowing:

• Spreads worm larvae everywhere
• Massively increases exposure
• Looks tidy… but isn’t horse-friendly

Only harrow if followed by cattle or sheep grazing
Otherwise, you’re spreading parasites.

7. What actually reduces worm burden?

The real game-changers in modern horse worming:

✅ Regular faecal egg counts
✅ Identifying high shedders
✅ Strategic (not routine) worming
✅ Manure removal
✅ Smart pasture management
✅ Avoiding unnecessary drenches

Autumn takeaway for horse owners

Autumn isn’t about asking “Which wormer should I use?”

It’s about reviewing your overall strategy:

• Do you know which horses actually need treatment?
• Are you protecting wormer effectiveness long-term?
• Are your paddock practices helping, or hurting?

If you’d like help building a personalised horse worming programme, talk to Dr. Rebecca at Vetpost or Holly from Worm-Ed.