CSIRO research has helped to breed sheep resistant to internal parasites

Nemesis: breeding for worm resistance in Merino sheep

The Nemesis worm-control project used selective breeding of Merino sheep as an effective and sustainable means to control internal parasites.

• 27 March 2006 | Updated 14 October 2011

• The problem
• The solution
• Breeding for resistance
• From lab to farm

CSIRO scientists have shown that Merino sheep can be bred to resist infection by parasitic worms with little or no chemical treatment.

Genetic tests for worm resistance are now becoming an integral part of Merino breeding in worm-prone regions.

The problem

One third of sheep production in Australia is dependent on anthelmintics (anti-worm chemicals) to control the devastating effects of internal parasites on production.

The growing problem of anthelmintic resistance and the assumption that there will be no new drenches due for release in the near future, presents Australia's sheep industry with a serious dilemma.

The solution

The future of worm control cannot rely on any single approach, but rather the integration of several control methods.

To generate optimal protection, several forms of worm control will all need to be considered, such as:

• breeding for resistance
• nutritional supplementation
• grazing management
• strategic use of anthelmintics
• novel approaches such as vaccines and predatory fungi.

The Nemesis project used genetic selection to reduce costs and improve production efficiency, through research in the area of parasite resistance.

Breeding for resistance

In the early 1970s, CSIRO realised the danger of relying entirely on chemicals. Our scientists began to determine the potential for genetic manipulation of the sheep host to minimise parasite infection.

Early indications showed that resistance to Barber's Pole worm was moderately heritable in Merinos. In 1977, work began to establish divergent selection lines: groups of sheep that were bred for worm resistance. There was no breeding between the groups, so each group could develop different traits for worm resistance.

These lines served two purposes:

• demonstrating that sheep will respond to selection for improved resistance. This has allowed the genes for resistance to be identified.
• by studying natural mechanisms of worm resistance, researchers can develop new forms of parasite control.

The resistant line has evolved to the stage where the sheep can live in medium to high rainfall areas with little or no chemical treatment for worms.

From lab to farm

In 1994, a large-scale technology transfer project was launched. Nemesis assisted breeders, with collaborative input from the University of New England and Agriculture Western Australia. Many ram breeders now test for worm resistance and this trait is becoming an integral part of Merino breeding objectives in worm-prone regions.

Nemesis is part of a large, woolgrower-supported initiative to provide sustainable worm control practices for woolgrowers.

Read more about the project in the Breeding sheep for sustainable worm control information sheet