Gastrointestinal worms in the stomach and intestine are without doubt one of the biggest threats to the health and welfare of grazing animals in New Zealand.

These internal parasites live in the digestive tract and lay eggs that are passed out in faeces and hatch into larvae. If these larvae are eaten by animals of the same species, they develop into adults and begin the life cycle again.

It’s a highly successful cycle from the worm’s perspective, but one that can cause serious harm to your animals if not properly managed.

The Impact of Worm Burdens on Animal Health

Unless action is taken, worm burdens in livestock can build to dangerous levels. Infected animals may:

• Develop diarrhoea

• Become unthrifty and pot-bellied

• Lose appetite

• Become anaemic

• Experience stunted growth or even death

Other diseases and conditions can cause similar signs, so if there’s no response to drenching, consult a veterinarian. For example, lush spring grass or intestinal bacterial infections can also lead to diarrhoea.

Each gram of faeces can contain thousands of worm eggs. While freezing and dry conditions can kill larvae, New Zealand's temperate climate means worm burdens often peak in autumn.

Young Animals Are Most at Risk

Young animals are generally most susceptible, but some species don’t build strong age-related resistance. Goats often remain vulnerable throughout their lives, and horses may not develop significant immunity either.

To understand parasite control specific to your animals, see our species-specific courses on animal care, which explore internal parasites in more depth.

How to Control Internal Parasites on Your Block

Drenching Programmes

When and how often you drench should be decided in consultation with your vet. Overuse of drenches can lead to drench resistance.

• Lambs often need regular drenching from weaning in December until May

• Goats may require annual drenching throughout life

• Horses may need frequent drenching during summer and autumn

• Young cattle usually need several drenches during their first summer and autumn

• Older stock may need strategic or targeted drenching

Some newer anthelmintics provide longer protection, so fewer treatments may be required. Always factor in withholding periods for meat and milk.

Natural and Pasture-Based Worm Control

Though not always practical on smaller blocks, natural control methods can play a vital role:

• Rotational grazing to rest pastures between grazings

• Cross-species grazing to break the worm cycle

• Lower stocking rates to reduce pasture contamination

• Grazing after hay or silage has been cut to reduce worm exposure

For example, grazing sheep after horses can reduce horse-specific worm larvae on pasture.

Combining Drenching and Grazing Strategies

The most effective worm control uses both strategic drenching and pasture management. A well-designed plan will:

• Improve animal welfare

• Increase productivity

• Help avoid drench resistance

Talk with your vet or an experienced advisor to develop a programme that works for your land, stocking rate, and species.

Choosing and Using Anthelmintics

Anthelmintics come in different forms:

• Oral drenches (most common)

• Injectables

• Pour-on treatments for cattle and deer

• Long-acting capsules that sit in the rumen for around 3 months

Some, like ivermectin, also kill lice and are known as endectocides.

All are effective when used according to instructions, but correct technique and dosing are critical.

Drenching Technique and Dose

Always follow manufacturer instructions and use the correct dose for the animal’s weight. Underdosing encourages the survival of resistant worms.

After drenching, move animals to low-risk pastures that haven’t been grazed recently by the same species.

Managing and Preventing Drench Resistance

Drench resistance occurs when worms survive treatment and continue to reproduce, making the drench ineffective over time.

To monitor for resistance:

• Test dung samples 10 days after drenching

• If worm eggs are present, resistance or drenching errors may be to blame

• Your vet can help interpret results and recommend next steps

Refugia, the deliberate preservation of some worm populations unexposed to drenches, can help slow resistance. Your vet can advise on how to apply this safely.

A Note on Alternative Remedies

Products like garlic or cider vinegar may reduce egg-laying or support overall health but do not replace proven anthelmintics. They should never be relied upon as your primary worm control method.