Parasitism: a perspective from a retired veterinarian

In 1962 the first truly broad spectrum anthelmintic was released onto the market. The Benzimidazoles (Thibenzole, Telmin, Panacur etc.) or white drenches were followed by the clear drenches, Levamisole (Nilverm) and the avermectins (Ivermectin , Abamectin) and Milbemycin (Moxidectin or Cydectin). Finally, and relatively recently Monepantel (Zolvix) and Derquantel.

Over the last 60 years worms have developed significant resistance particularly to the white, clear and mectin drenches and now even to Zolvix. Triple drench resistance is not uncommon.

What is now clear is that the use of drugs alone will not solve the parasitism problem in NZ pasture-based farming systems. Even if a new class of drugs is discovered then if history is anything to go by, overuse will result in rapid development of resistance by worms.

Climate change is likely in some cases to enhance larval survival and development increasing the challenge to sheep.

For instance, should the worm challenge be such that a drench is warranted, the interval between drenches would be longer in resistant sheep and the proportion of the flock requiring a drench is smaller. These practices reduce the development of drench resistance in worms.

Farmers are often very reluctant to leave sheep un-drenched in a mob but having worm resistant sheep can give them confidence.

To maintain our current farming system there is no silver bullet, and it is obvious that the problem must be attacked from many directions, i.e., an integrated approach.

Breeding sheep that are resistant to internal parasites is an essential tool in this battle. It has positive benefits to many of the recommended procedures advocated in Wormwise.

Ewe drenching is not a recommended practice if you want to avoid drench resistance but personal knowledge of two flocks in this area with FEC’s of 2500 show that parasitism is still a major problem. One of these flocks had been drenched a month before the samples were taken indicating possible worm resistance to that drench. Both flocks were in very poor condition with abysmal scanning results. Resistant ewes are far less likely to develop such a high worm burden.

Safe pasture in practice is a relative term and resistant sheep are able to graze areas that are more contaminated with infective larvae than susceptible sheep without production loss.

Finally breeding sheep that are resistant to worms is a sustainable practice looking to the future in these uncertain times.