Internal parasite management: Initial questions and answers | Beef + Lamb New Zealand
Animal Health

Internal parasite management: Initial questions and answers

When it comes to internal parasite control in sheep and cattle, there are essentially two options: the easy approach or the responsible approach.
Tuesday, 18 October 2016

Drench resistance is real. It’s already widespread on sheep farms and heading in the same direction wherever cattle are farmed.

Over coming weeks, the B+LNZ e-diary will run a series of stories around worm management, including talking with a farmer who has drench resistance, an experienced consultant’s best practice advice, the situation overseas and how to breed for resistance.

Our first story is a set of commonly-asked questions and answers. If you have other questions, please email us and we will address these at the conclusion of the series.

Q: What is drench resistance? 
A: Drench resistance is when worm populations in an animal survive after a correctly-applied dose of drench.

The resistant worms then breed, passing their resistant genes on to their off-spring. Overtime – if practices remain the same – resistant worms make up an increasing proportion of the farm’s worm population – and that is when you have a problem. As that proportion increases, so does the cost to animal health, productivity and profit.

Q: Why bother about drench resistance? 
A: Drench resistance costs money and creates an unfair legacy for future generations.

In the first instance, it makes sense to keep the older, low-cost drugs working as long as possible, because they are much cheaper:

  • Dual-acting drench = 7c-14c/lamb drench
  • Triple-acting drench = 20c-35c/lamb drench
  • Zolvix or Startect = 60c/lamb drench.

More costly, however, is the effect of continuing to use a drench that is not fully effective. You are wasting time, effort and money. The next instalment in this series is a cost-benefit analysis that includes the loss of productivity that accompanies drench resistance.

Finally – and critically – the next generation of farmers needs to be left with drench choices that work.

Q: I’ll wait until I see there’s an issue before I need to change anything.
A: Actually, you will have resistance on your farm long before you start to see a problem.  

And then, it will often be a major event that makes resistance obvious. For example, a cluster of lamb or calf deaths late autumn, or a significant problem with worms in young lambs mid to late lactation.

Q: How widespread is resistance in New Zealand? 
A: Widespread in sheep and increasingly widespread in cattle.

Beef: A 2006 farm-based survey of 62 North Island beef properties revealed:

  • 94% of farms were resistant to at least one drench family
  • Of those farms, there was 92% resistance to ivermectin (“mectin” drenches), 76% resistance to albendazole (“white” drenches) and 8% resistance to levamisole (“clear” drenches)
  • 20% of farms showed resistance in at least two worm species
  • Cooperia (intestinal worms) were commonly resistant to ivermectin and albendazole
  • Ostertagia (brown stomach worm) resistance to ivermectin and levamisole was an emerging issue.

Sheep: A 2006 farm-based survey of 112 New Zealand sheep properties revealed:

  • High levels of drench resistance to all drug families
  • 25% of farms showed resistance to ivermectin, 24% to levamisole (clear), 41% to albendazole  (white) and 8% to a combination drench of albendazole and levamisole
  • Specific problems identified were:
  • Ostertagia resistance to all drench treatment options 
  • Cooperia resistance to mectin drenches
  • Trichostrongylus (black scour worm) resistance to levamisole
  • All worm species showing a resistance to albendazole.

In the 10 years since, we don’t know much about what has happened in sheep. However, the cattle situation has got worse and we are now seeing resistance in the particularly nasty worm, Ostertagia.

Q: If you suspect you already have drench resistance, what do you do? 
A: Deal with facts. Find out what is happening on your farm.

Ideally, in the January-to-May period (when the most worm species are present in lambs), carry out a faecal egg count reduction test (FECRT), alongside a larval culture. That will tell you how effective your current drench is. 

Your vet or farm consultant will be able to help you action this.

Resources for farmers