Principles

• Healthy animals harbour worms and always will – eradication is neither an appropriate goal nor achievable.
• Well fed animals are less affected by worms than those under nutritional stress.
• Older animals are generally less susceptible to worms than younger ones, and, at times, can be used to reduce the number of infective larvae on pastures.
• Animals vary in their susceptibility to parasites (genetic variability).
• Animals can be selectively bred for resistance or resilience to round worms.
• When breeding for a characteristic, intensive selection pressure will result in more rapid change being made (applies both to livestock and parasites).
• Breeding for a single trait leads to more rapid change than breeding for a combination of traits.
• Most of the year there are more worms, in the various life stages, on pasture than inside the animals.
• Anthelmintic drench is a finite resource and should be used to achieve the greatest sustainable benefits for the farmer.
• The way in which you use drenches and manage parasites can change the rate at which you select for resistant worms.
• Each farm is a unique ecosystem and effective worm management is dependent on knowledge of the round worms present and their anthelmintic resistance status.
• Some drench formulations, by their very nature, are long-acting and may hasten the development of drench resistance if they deliver a sub-optimal level of active ingredient for an extended period.
• Once present on a farm parasite resistance to anthelmintics is permanent.

Key points

• Parasitism is a major cause of loss of production in livestock.
• Based on current information, it is clear that many New Zealand farmers are using anthelmintics in a manner that will result in drench resistant parasites and drench failure.

Animal health plan

In order to optimise animal production in the face of parasitism, each farmer should have in place, and implement, a parasite management plan as part of an overall animal health plan.

A parasite management plan, and its application, will require a thorough knowledge of the parasites present on the farm, their biology, and methods that can be used to manage them. Remember that each farm is different and plans need to be farm-specific.

The drenching component of the plan should strive to minimise both the impact of worms on production and selection of worms for drench resistance.

All factors that impact on parasite management and animal production are inter-related and need to be considered together when developing and implementing the plan. 

Feeding

All classes of stock should be provided with sufficient feed to enable them to attain specified targets. Forward planning is essential and monitoring of live-weights and condition should be a routine operation.

Manage the level of larval challenge

Production benefits arise from lower levels of challenge of young stock in particular.

Plan to create low levels of larval challenge – use stock movement, grazing history, pasture species, weather, and stock type and classes to match nutritional needs.

It is preferable, not to use the farm paddocks for the same purposes year after year - for example, to always fatten lambs. All paddocks should be exposed to all stock classes  during the course of a year so that the worm population on the farm is continually mixed up rather than having some paddocks populated with worms that have all originated from frequently drenched stock.

Use a paddock diary – stock class mapping as part of the planning process. This will enable you to identify where a challenge is coming from and anticipate how to deal with it.

Drenching

You should always know which drenches are effective on your farm. A comprehensive drench test (faecal egg count reduction test and cultures) should be carried out regularly, with frequency depending on drench resistance risk factors on your farm – and your drench chosen on the basis of the results.

The available evidence suggests that combination drenches have the ability to slow the development of a drench resistance problem on a property when used prior to resistance developing. 

Good drenching practice is essential. Animals should all receive optimal doses of the most efficacious drench of the chosen action-family/families – body weights need to be known and you need to be confident, by regular checking, that your drenching gun is delivering the correct dose.

Drenching strategies will need to be tailored to suit individual farms, so seek advice.

In the absence of any alternative parasite management strategy, a preventative drenching programme is recommended. A fall-back preventative drenching programme may involve a drench at or around weaning followed by four or five further drenches at monthly intervals. ithin young stock, faecal egg counts can be used as indicators of adult worm infection i.e. worms that have fully matured and the likelihood that production loss due to worms is occurring, and that drenching is required. An effective treatment in late winter is recommended to remove inhibited larvae in at risk cattle (refer type II ostertagiosis).

Drenching adult stock

A good condition adult ewe or cow that is on satisfactory feed does not need to be routinely drenched. However under certain circumstances it may prove to be necessary, depending on feed levels, age of animals, number of foeti, parasite burdens, and other factors. If there is firm evidence that parasites are affecting the stock a drench can be given and its effectiveness monitored. Hoggets and two tooths often need to be treated differently to other older ewes.

The aim should be to achieve a balance between maximising animal production and minimising selection pressure on the worm population. The decision to drench is best made in conjunction with your parasite control advisor.

Refugia

To reduce the chances of breeding resistant worms you need to make sure that they are overwhelmingly outnumbered by drench-susceptible worms on your farm (concept of refugia). When susceptible worms breed with the resistant ones they slow the development of resistance by diluting the gene frequency for resistance.
 
Quarantine procedure when introducing new stock to the farm
 
Check the worm status of stock that you are buying and are going to introduce to your farm. Seek providers who can document their animals as being “free” of drench resistant worms or are of lesser risk.

On arrival on the farm, drench stock with a combination drench (containing at least three broad spectrum actives) for sheep, or an effective combination for cattle. Hold stock in a quarantine area for 24 hours. Do not put stock onto “clean” pasture but on pasture that is likely to have susceptible infective larvae on it.

Ten days after arrival of the stock check, via faecal egg-counts, that the drenching was effective.

Using stock that are resistant to worms or resilient in the face of challenge.

You may choose to buy-in or breed your own stock that are resistant to parasites or are resilient as a component of your parasite management plan.  Sheep breeders have made progress with both of these approaches but at this stage it remains unclear which approach will result in the best long-term outcome. Whether ram breeders are recording resistance or resilience traits or both, their sale rams are best described by an economic index of production.

Things to avoid doing in order to slow the development of resistance

• Avoid using an ineffective drench.
• Avoid drenching young stock onto clean pasture/low challenge grazing.
• Avoid drenching more frequently than every 28 days.
• For sheep, avoid whole flock treatment pre-lambing with a long-acting anthelmintic product.
• For cattle, avoid using ML only in younger cattle up until about 9 months of age.  At least some treatments over this period should include levamisole to remove Cooperia.
• Avoid drenching immune adult animals, as this is highly selective for drench resistance (because they are not readily reinfected).
• Avoid using some long-acting formulations that can result in an extended period of sublethal dosing.
• Avoid importing drench resistant parasites onto your farm.

Handbook

A Handbook of Sustainable Worm Management for Livestock Farmers was published in December 2007.

You can download the entire handbook (1.02 MB) by clicking here. Alternatively, you can go straight to the topics that interest you by clicking on the links below.

Chapter 1: What effects do worms have and how do they limit production?
Chapter 2: Worm biology
Chapter 3: Principles of worm management
Chapter 4: Drenches and drench resistance

If you'd prefer to receive a hardcopy, email wormwise@beeflambnz.com with your contact details, and we'll send one out to you.

R&D Briefs

R&D Brief 136  Using undrenched ewes as a source of susceptible worms
R&D Brief 130  Worms in refugia 
R&D Brief 124  Ill thrift
R&D Brief 115  Managing worms
R&D Brief 77  Selecting highly productive sheep that require less less anthelmintic treatment
R&D Brief 76  The effect of drench capsules on the selection for drug resistance
R&D Brief 67  Sustainable internal parasite control for sheep
R&D Brief 34  Breeding sheep with resistance to nematode infection
R&D Brief 33  Breeding for reduced worm drench requirements in sheep
R&D Brief 6  Evaluation of an organic anthelmintic for farmed livestock
R&D Brief 1  The effect of pasture species on lamb parasitism

Sheep Improvement Ltd website

The SIL website http://www.sil.co.nz/ lists some technical information relating to internal parasite resilience in rams. Additionally the website explains the difference between resilience and resistance, and the protocols used to measure rams.

Other research reports

Strategies for managing internal parasites and drench resistance

Managing anthelmintic resistance – A role for undrenched adult ewes as a source of unselected parasites and in suppressing parasite populations?

The Wormwise^® national spokesperson is Trevor Cook.

Trevor is a well established vet at Totally Vets Manchester St, Feilding. He facilitates several Beef + Lamb New Zealand Monitor Farms in the central districts and has played an important role in the development of Wormwise^®.

Trevor was named 2008 Landcorp Agricultural Communicator of the year, an award recognising excellence in communicating agricultural issues, events or information.

He can be contacted on mobile 027 446 3616 and email trevorc@totallyvets.co.nz. 

The vision of the Wormwise National Worm Management Strategy is to minimise the impact of internal parasites in livestock now and in the future.

Implementation of the Wormwise Strategy and action plan is the responsibility of the Wormwise Implementation Group (WIG). WIG comprises members from the five participating organisations: Beef + Lamb New Zealand, Sustainable Farming Fund, Animal Remedy and Plant Protection Association, Agcarm and New Zealand Veterinary Association Inc.

The Wormwise Technical Advisory Group works in tandem with the Wormwise Implementation Group to ensure all Wormwise information disseminated is technically sound.