The ongoing challenge of worm resistance

It started with the single active products which when they first arrived on farms were miraculous in killing internal sheep parasites.

The trend of developing resistance caused manufacturers to formulate products that contain double products and then triples and now we are moving to the so-called novels (Zolvix Plus and Startect when they are available) which combine “new” actives with traditional ones.

It is fair to say that come each autumn we receive faecal samples taken 10 days after drenching and it is a rarity to find a farm where a triple drench is fully effective.

While there is great store in using the novels Startect and Zolvix Plus these are also becoming ineffective when used as before. Some within the industry are crying out for new products and while you can never say this won’t occur, there is nothing in the immediate pipeline.

In July, I spoke at two seminars that we held in our Feilding clinic; the audiences were highly engaged sheep farmers who are rightly concerned. In addition to giving some historical perspective on anthelmintic resistance my pitch was on making profit in the face of anthelmintic resistance.

Resistance has not just happened, and it is not absolute; for the last five years we have been working with farms where resistance occurs, and we are now producing heavier lambs more efficiently than before.

Whether it is the combination of warmer and wetter late summers/early autumn periods and continuing to graze young stock on areas contaminated with internal parasite larvae from either other lambs or from lambing ewes, we are seeing a parasite larval challenge like never before.

These higher challenges combined with the lowered effectiveness of drenches is why we are seeing what we are.

If a product is working at 75% and the larval challenge has resulted in an internal worm burden of 10,000 worms, the effect of leaving 2,500 worms behind after drenching is significantly more damaging than if these lambs had been carrying 2,000 worms prior to being drenched. It is simple maths.

Farm management changes to ensure we protect lambs from massive larval challenges take planning and time to implement and if you would like a hand, we are here to assist.

I’m happy to take a call (027 446 3616) and either myself or one of our team will help you navigate this wicked problem.