Most adult roundworms live in the gut of grazing animals, usually the small intestine, where they suck blood, reproduce, and shed eggs. These eggs pass out in faeces and hatch into larvae, which climb up pasture plants. When animals graze, they ingest the larvae and the cycle begins again.

Some worms affect other organs:

• Lungworm lives in the lungs

• Liver fluke lives in the bile ducts of the liver

• Tapeworm attaches to the gut wall and sheds egg-filled segments into the environment

What Is Drench Resistance?

Drench resistance occurs when internal parasites develop the ability to survive a chemical drench. The drench fails to kill them, allowing them to continue damaging the host animal and reproducing.

Is Drench Resistance Becoming More Common?

Yes, and it is now widespread across New Zealand. Overuse and indiscriminate use of drenches, often without confirming that parasites are the issue, has accelerated resistance. Some worm species have developed resistance to multiple drench families.

What Are the Main Drench Chemical Families?

Main broad‑spectrum drench families

• Benzimidazoles

• Levamisole/Morantel

• Macrocyclic Lactones (avermectins/milbemycins)

• Amino‑Acetonitrile Derivatives (AADs), e.g., monepantel

• Spiroindoles, e.g., derquantel

These families are not always obvious on the product label. Always read the fine print to identify which active ingredient is being used.

How Do You Know If Resistance Is Present?

The only reliable method is to test. If your animals show signs of parasitism such as scouring or poor growth after drenching, it’s possible the drench is no longer effective.

How to Test for Drench Resistance

• Do a faecal egg count (FEC) before drenching.

• Drench as normal.

• Repeat the FEC 7 to 10 days later.

If egg counts haven’t dropped significantly, the parasites may be resistant. FECs are inexpensive and available through your vet or DIY kits.

Slaughter and worm counts can also confirm resistance but are not practical for most lifestyle farmers.

Can Resistance Be Introduced onto Your Block?

Yes. New or returning animals can carry resistant worms and re-contaminate your pastures.

How to Minimise the Risk

• Drench all introduced or returning animals on arrival with a moxidectin-based drench.

• Quarantine them if possible.

• Avoid buying animals from farms with known resistance issues.

How to Reduce Drench Resistance on Your Block

• Work with your veterinarian to develop a targeted drench plan.

• Drench based on FEC results, not the calendar.

• Rotate between drench families each year, and confirm effectiveness before switching.

• Avoid under-dosing. Dose for the heaviest animal in the mob.

• Make sure your drench gun is calibrated and functioning.

• Don’t drench animals straight off lush pasture. Allow some time off feed to improve efficacy.

Which Animals Are Most at Risk?

• Young stock are most vulnerable to internal parasites.

• Older animals generally develop some immunity and may not need regular drenching.

• Goats and horses tend to remain more susceptible throughout their lives.

Support and Further Learning

For practical strategies tailored to your livestock, our species-specific courses provide detailed guidance on parasite control, including sustainable drench use and resistance management.