New Zealand

Parasite Management

Parasite Management

When it comes to impacts on growth rates for stock over summer and autumn, parasitism is second only to nutrition as the most common reason for ill-thrift in young sheep1.

Effective parasite management on your farm is therefore crucial to maximise production and reduce the chances of developing drench resistance.

It is therefore important to consider the following factors:

Management Principles

To prevent significant production losses and death from occurring, sheep should be routinely monitored to assess worm burdens, to establish infection trends, and to assist with the selection of the most effective treatment. The following principles apply2:

Well fed animals are less affected by worms than those under nutritional stress.

  • Good nutrition is a key tool for managing gastrointestinal parasites.

Healthy animals harbour worms and always will – eradication is neither an appropriate goal nor achievable.

  • Healthy, good conditioned un-drenched animals are a valuable tool for slowing down the development of drench resistance on your property.

Use older animals or different species to decrease pasture contamination.

  • Good conditioned ewes are net removers of parasites from your pastures.
  • Cattle and deer will kill the majority of sheep parasites they ingest.

Each farm is unique and effective worm management may be different from farm to farm.

  • There are no set recipes, each farm needs to have their own parasite management program.

Animals can be selectively bred for resistance or resilience to roundworms.

  • Choose your stud breeder from those that carry out resistance testing on their rams.
  • Ensure the ram you buy has a good breeding value for this trait.

Long-acting drench formulations may hasten development of drench resistance.

  • Do not use long acting products on all your ewes.
  • Select animals for treatment depending on their need (e.g. poor body condition score ewes carrying twins).

Drenches (anthelmintics) used in combination can delay the onset of resistance.

  • Ensure you are using an effective combination drench.
  • Combinations are only an effective tool against resistance when the gene frequency for resistance on your property is low.

A population of un-drenched worms should be maintained on farm (refugia).

  • This is the most important tool for slowing down drench resistance.

Drenches are a finite resource. The way in which you use drenches and manage parasites can change the rate at which you select for resistant worms.

Worm Management

As most of the worm population lives on the pasture, rather than in the animal, it is important to address worm management by looking at both the animal and the pasture. The following can help you manage the worm burden on farm2:

Animal Management

Young stock (under six months) have little or no immunity to worms, so where possible, avoid exposing young stock to high levels of infective larvae.

Minimise nutritional stress on stock (ideally maintain stock at BCS3) to reduce susceptibility to infective larvae.

Use an appropriate drenching programme.

Incorporate stock bred for resistance to parasites.

Pasture Management

Keep pasture covers long where possible – most infective worm larvae are present low down in the pasture, towards the base.

Spelling pasture (for 3 months or longer) will reduce the level of eggs and larvae.

Cross grazing of stock species (e.g. cattle, deer) can reduce the number of worms on pasture (note, goats share the same worm species as sheep so are not suitable for cross grazing under this strategy).

Stocking rate influences risk: the higher the stock rate of a particular stock class on pasture, the higher the potential for worm problems.

Drench Classes

Whilst there are a large number of commercial drench treatments available in New Zealand, all products belong to at least one or more drench classes/families (i.e. as a single active or combination drench). Development of new drench families is a complex process, and so it is important we use best practice principles to ensure the longevity of the families that we have.

DRENCH FAMILY (and common descriptors)


Benzimidazoles (white drench) Oxfendazole, Albendazole
Imidazothiazole (clear drench) Levamisole, Morantel
Macrocyclic Lactones (ML’s) Moxidectin, Ivermectin, Abamectin
Amino Acetonitrile Derivative (AAD’s) Monepantel
Spiroindoles (SI’s) Derquantel

AAD’s and SI’s are considered ‘novel’ actives as they are the most recent drenches created. Strategic use of these drenches can help prevent and or delay the development of drench resistance on your farm.

Strategic Drenching

Strategic drenching is the planned use of drenches to manage or delay anthelmintic resistance, to make routine drenches work effectively for longer. A strategic drench plan could include a Knockout drench, Exit & Quarantine drenches.

What is this? Why Benefits
Knockout Drench
(fig. 1)
The substitution of a routine lamb drench with a novel active drench (e.g. Startect®), prior to optimal climatic conditions for larval survival and development on pasture. To remove parasites which have survived routine drenching and prevent an autumn larval peak of resistant parasites. Knockout drenching has been shown to delay the onset of resistance to the existing routine drenches used.3
A Knockout drench can be given to lambs at any time.
Exit Drench (fig. 2) Use a short acting treatment with an active ingredient from a different family, after a long-active drench or capsule. To remove parasites which have survived (resistant to) the long acting drench or capsule treatment. To maintain the efficacy of long acting products, keeping them working effectively for longer.
Quarantine Drench Drench any new stock brought onto the farm with a novel active drench and hold off pasture for 24 hours. To prevent the introduction of resistant parasites onto your farm through stock. Reduces the risk of drench resistance developing on your farm.
Knockout Drenching Example

Fig 1: An example of using a drench including a novel active, Startect, to ‘knockout’ resistant worms before they can contribute significantly to the autumn larval contamination on pasture.

Knockout Drenching Example

Fig 2: An example of using a drench including a novel active, Startect, to “exit” a long acting product and so kill any resistant worms that may have survived the long acting product.


Refugia is arguably the most effective way we can slow down the development of drench resistance. The aim of refugia is to have a large enough pool of susceptible parasites that are not exposed to drench and which can dilute the resistant parasites. The larger population of susceptible parasites therefore reduces the chances of the resistant parasite genes being passed on to the next generation, hence slowing down the development of resistance.

You can implement refugia on your property:

Don’t drench everything

  • Drench ewes based on BCS/feed levels/number of lambs carried.
  • Leave 5-10% of lambs un-drenched.
  • Extend the drenching interval of lambs using faecal egg counts as a trigger for drenching.

Grazing management

  • Run older stock with lambs to dilute the resistant worm population (e.g. run un-drenched cull or poor conditioned ewes with the lambs).
  • Don’t drench lambs onto clean paddocks or new pasture (e.g. move lambs onto these areas before drenching, and drench 1-2 weeks later).
  • Rotate un-drenched ewes behind lambs.


It is important to understand if the strategies you have implemented for parasite management are working to both control the effects of parasites on animals and to delay the development of drench resistance.

Faecal egg count monitoring

  • Ewe sampling: Sampling ewes at key times during the year (pre-mating, scanning and pre-lambing) can help you determine not only if your ewes might benefit from a drench but also how wormy your farm might be. Finding consistently high counts could indicate you have a wormy farm and that the ewes may be under nutritional stress.
  • Lamb sampling: Routine sampling of lambs at drenching can indicate how contaminated their grazing area is. The results can also be used as an aid to determine if you need to drench or not. This can be particularly helpful in a period of drought.

Drench Check

A drench check is a simple way to check the efficacy of a given drench, and can be performed by you with the help of your veterinarian. Collect 10-15 fresh faecal samples, 7-12 days after drenching with an oral drench. If eggs are present, this could indicate either an issue with administration of the drench, or the presence of resistant worms.


If eggs are present in a drench check, it is a good idea to do a Faecal Egg Count Reduction Test (FECRT). FECRT’s are also useful if you suspect drench resistance at any time, or want to test the efficacy of a particular drench.

You should contact your local veterinarian when planning this test to ensure that the work involved yields the most useful results possible. They will give good advice on mob and drench selection for the test and other local requirements such as the number of animals and faecal sample collection techniques that are necessary.


  2. Adapted from Wormwise National Worm Management Strategy Handbook July 2019.
  3. Leathwick DM and Hosking BC (2009). Managing anthelmintic resistance: Modelling strategic use of a new anthelmintic class to slow the development of resistance to existing classes. NZ Vet J; 57(4): 203–7.