• 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.
  • Internal parasites live in the stomach and intestines, and they lay eggs, which are passed out in faeces and hatch into larvae.
  • If the larvae of worms from one particular species of animal are eaten by animals of the same species, they grow into adults and begin the life cycle again.
  • It’s a good lifestyle as far as the worms are concerned because they are very successful and there are lots of them! But it’s not so good for the animals or their farmers.
  • Unless farmers take steps to prevent it, worm burdens in livestock can build up to huge levels.
  • The animals will develop diarrhoea, they will become unthrifty, pot-bellied, anaemic, lose their appetite and may die because of hundreds of thousands of worms in their digestive tract.
  • There are conditions and diseases other than worms that can cause these signs, so if there is no response to drenching, consult a veterinarian. For example, lush spring grass can cause diarrhoea, and so can some bacterial infections in the intestine.
  • The worms can produce thousands of eggs in every gram of faeces passed.
  • Freezing and drying out tend to kill larvae, so pasture contamination can drop during cold winters and dry summers, but generally, worm burdens build-up to their highest levels in animals in autumn.
  • By the time worm numbers in the intestine have become large enough to cause diarrhoea, body condition and growth rate will have been adversely affected.
  • Animals are generally most susceptible to worms while they are young, but fortunately, as they age they tend to build up a natural resistance.
  • However, goats seem to remain susceptible throughout their lives, and horses too may not develop much age-related immunity.

Control of internal worms


  • When and how often you drench should be decided in consultation with your veterinarian.
  • Regular drenching can lead to drench resistance (see below.)
  • Lambs may need to be drenched (given a dose of anthelmintic by mouth) at intervals from weaning in about December until May.
  • Some goats may require this type of drenching treatment every year of their lives.
  • Horses too may require relatively frequent drenching each summer/autumn.
  • Young cattle may require several drenches during their first summer and autumn. Older sheep and older cattle may require strategic drenching too.
  • However, there is no general rule that can be used as a guide for all livestock on all farms, and this is why it is very important that you talk to your veterinarian or advisor to devise a worm control programme that will be cost-effective for your farm.
  • Note that some ‘third generation’ oral anthelmintics have a longer duration of effect so fewer drenches are needed, with a longer period between drenching.
  • There is a withholding period after giving any anthelmintic, as there is for most registered animal remedies, and this will have to be taken into account if milk or meat is to be sold.

‘Natural’ methods

  • Most farms, and certainly most small farms, usually don’t have the space or flexibility to be able to rely on natural parasite control methods, although natural methods are an important component of every worm control programme.
  • Natural methods include for example grazing paddocks in rotation, allowing time for some or most of the worm eggs and larvae to die off on the pasture between periods of grazing.
  • Grazing pasture with more than one species of farm animal can help reduce pasture contamination because most types of worms are specific for sheep and goats or for cattle or horses.
  • So grazing with one species of animal will generally reduce the number or infective eggs and larvae for other species.
  • For example, the larvae of horse worms will be killed when eaten by a sheep or a goat; so by following horses with sheep, or following cattle with goats, the numbers of worm eggs and larvae on pasture can be reduced.
  • Low stocking rates tend to mean lower pasture contamination too.

Combine drenching and pasture management

  • The best worm control programme for your farm will incorporate rotational grazing, mixed-species grazing and strategic dosing with reliable anthelmintics.
  • You will need a veterinarian or an experienced and competent farmer to help you devise this programme.
  • A well-designed programme will definitely pay dividends in terms of improved stock health and welfare, and productivity.

Various types of anthelmintic

  • Anthelmintics now come in various formulations.
  • There are long-acting capsules that sit in the rumen-producing anthelmintic for around 3 months.
  • But considerable care and skill are required to make sure they are given correctly without damaging the animal’s throat.
  • Most anthelmintics are given by mouth in a ‘drench’, but there are also injectable anthelmintics, and for cattle and deer there are anthelmintics that are poured onto the skin along the middle of the animal’s back (pourons).
  • Some of the ‘third generation’ anthelmintics like ivermectin can also kill some types of lice. These are called ‘endectocides’.
  • If used according to the manufacturer’s instructions, these are all reliable and effective worm treatments with no side effects.

Drench correctly with the proper dose

  • It is very important to follow the manufacturer’s instructions carefully so that you get the drenching technique and the dose right.
  • Make sure that a full dose of drench is given.
  • Under-dosing encourages the survival of worms that are resistant to the anthelmintic used, and drench resistance is an increasing problem on New Zealand farms.
  • Immediately after drenching, the animals should if possible be moved to pasture that has been not been grazed by that species for some time, eg after hay or silage has taken off and pasture has regrown.

Drench resistance

  • Drench resistance means that some of the worms have developed resistance to a particular anthelmintic, so drenching with that anthelmintic is not effective in getting rid of these worms.
  • With the help of your veterinarian, it is easy to test for drench effectiveness (and hence drench resistance).
  • Refugia can help with drench resistance.

Test drench effectiveness

  • Tests on dung samples 10 days after drenching will tell if the drench was effective or not.
  • The tests will show how many eggs there are per gram of faecal sample (epg).
  • If the drench was effective, there should be no worm eggs present.
  • If there are worm eggs present, the worms may be resistant to the drench used.
  • Another explanation might be that the drenching technique was faulty, for example, the drench gun may not have been working properly or insufficient drench may have been given for the size of the animal.
  • Your veterinarian or advisor will explain how to collect the dung samples and will arrange for laboratory testing, which is inexpensive and quick.
  • If drenching was not completely effective, your veterinarian or advisor will help resolve the problem.

Beware of alternatives to anthelmintics

  • It is important to know that alternative remedies like garlic and cider vinegar can sometimes help suppress the egg-laying of worms, and may even remove a few worms, but they do not get rid of them and they cannot be relied on as a means of worm control.