Worm control options

By Tom Ward

In Livestock7 Minutes

The first option is the preventative-blanket approach where the lamb receives four to five drenches from different families at four-weekly intervals.

Secondly, preventative – based on a monitoring approach. Lambs are drenched based on faecal egg counts (FEC), body condition scoring (BCS), liveweight gain (LWG), etc. If the FEC, which should be done at four to five weeks, shows less than 200 eggs/gm, do another test in two weeks.


This approach has been shown to reduce the frequency of drenching, and therefore reduce drench resistance, ho wever it does not always maintain satisfactory production levels.

Ewes should not have to be drenched, although there are times when a dench may be beneficial.

Quarantine drenches are used to ensure sheep coming on to a farm do not bring worms. Drench, hold off pasture for 24 hours, then graze contaminated pasture, then FEC 10 days later. There are two options: Combine no less than four unrelated drench families with at least one being the drench active chemicals monepantel or derquantel, and drench with a combination containing monepantel or derquantel. Watch withholding periods. Every farmer needs a written quarantine policy.

CARLA is an acronym promoting selection of animals with high levels of genetic immunity to worms. The test measures antibodies against worms in sheep saliva.

Drench resistance and refugia

Drench-resistant worms which survive breed resistant ones. Over time this increases the proportion of resistant worms. A 2006 survey showed 36% farms showed no resistance to drench. Resistance to Ivermectin was shown to a half dose on 36% of farms and to a full dose on 25% of farms. Resistance to Levamisole occurred on 24% of farms, and to Albendazole on 41% of farms. Resistance to Levamisole and Albendazole combination occurred on 8% of farms.

To fight drench resistance we can use a strategy called refugia, which is described as: a part of an animal population left unexposed to drench, allowing the worms in those animals to develop and reproduce.

In practice, refugia ensures there are some drench-susceptible worms available to reproduce, the aim is to increase the proportion of susceptible animals, thereby reducing the pool of resistant worms, and leading to an improvement in productivity.

In practice, the challenge is to find ways to maintain low levels of worms but retain a useful pool of susceptible worms.

A maximum of 10% left undrenched has been shown to be sufficient. Not drenching the best lambs when feed is plentiful and of good quality is desirable. Even if you have low drench resistance, refugia could still help as some worms will be resistant.

It is about finding a balance. Refugia will increase worms on pasture, but long-term drench resistance could be a greater problem than short-term lowering of production.

The risk of developing resistance in ewes is high where a long-acting pre-lamb drench is used and moderate where a weaning one is used. Consider whether any drenching of ewes is necessary, or drench only part of a ewe mob. In some districts haemonchus (barbers pole) is a problem and ewes will need drenching over summer.

Reducing contamination

Preventative drenching of lambs, five or six times after weaning, is the main approach to reducing the amount of pasture contaminated by worms. Consider drenching every 28 days, thereby allowing susceptible worms to re-infect the gut and pass to the pasture, ensuring a mix of susceptible and drench-resistant lambs to be on the pasture.

Buying in lambs is a high-risk strategy. Using ineffective drench is high risk, as is using a single active ingredient one.

Drenches can be divided into the different ‘active” family groups: the so-called “white drenches”, the “clear” drenches and the “avomectins”. They can also be classified around how many worm types they kill (broad or narrow), or whether long or short acting.

Broad spectrum drenches can be divided into five action groups: Benzimidazoles (BZs), levamisole/morantel and macrocyclic lactones (MLs), amino-acetonitrile derivatives (AADs) and spiroindole (Si).

BZs are sometimes called “white drenches”, although not all are white and they prevent the worm from digesting nutrients. They are effective against most worms in sheep although may not be as effective against lungworm and inhibited larvae as the MLs. Some are effective against fluke.

Levamisole and morantel are sometimes called “clear drenches” although again are not all clear. They work on the worm’s nervous system and are effective against most worms in sheep.

MLs include avermectins and milbemycins and work against both internal and external parasites (endectocides), their action paralysing the worm.

AADs act on receptors that occur only in nematodes, blocking those receptors, paralysing the worm.

SIs are the latest type of anthelmintic to be developed. One of the group, derquantel, blocks the use of acetylcholine, causing flacid paralysis. It has excellent efficacy, apart from only 95% efficacy against Ostertagia. It works better in combination with an anthelmintic from a different class.

Narrow-spectrum drenches include clorsulon, closantel and praziquantel which are used for killing specific worms such as liver fluke , tapeworms or barbers pole.

Acknowledgement to Wormwise for assistance with this article.



  • When best to not drench stock
  • What proportion to not drench
  • Implications for production
  • Drench resistance status of the worms
  • Sheep/cattle ratio and stocking rate
  • Other farm enterprises
  • Effect of climate
  • Key risk periods for animals
  • Ewe drenching policy
  • Feed quantity and quality

Tom Ward is a South Canterbury farm consultant.