26 May 1995

How resistance to sheep wormers is growing in the UK

UK SHEEP producers are urged to improve their anthelmintic control strategies or suffer the resistance to sheep wormers that is now experienced in the southern hemisphere.

The warning comes from Dr Gerald Coles, Bristol University, after attending an an anthelmintic conference in New South Wales, Australia.

"Total reliance on anthelmintics for worm control simply cannot last," he says. "But reducing reliance on them will help to preserve their efficacy."

International scientists discussed alternative methods of helminth (roundworm, tapeworm and fluke) control. These included worm vaccines, breeding sheep resistant to nematodes, and use of diet to decrease the spread of nematodes.

"But none of these options offer either a quick fix or a complete alternative," says Dr Coles.

The biggest problem, particularly in warm, moist summers, is the stomach worm, haemonchus contortus, he says. Once ewes and lambs have picked the larvae off the pasture, the worm ingests blood in the stomach. Sheep become anaemic, weak and lose weight. Mortality rates are high.

With one worm producing thousands of eggs, turnover is high, which makes it easier for the worm to become resistant to wormers. In Britain it is resistant to benzimidazoles, but not to levamisole or ivermectin.

"The clear message from the conference was that nematodes are showing they are not beaten, and we either improve our battle plan through research and education or suffer defeat," says Dr Coles.

&#8226 Now is the time to dose ewes and lambs grazing old pasture for the nematodirus worm.

The intestinal worm, whose eggs are hatching after they were laid last year, causes sudden scouring and death in sheep.

Two worm doses are required, three weeks apart, advises Dr Karl Linklater, director of veterinary services at the Scottish Agricultural College.

"Most modern wormers carry an indication for Nematodirus," says Dr Linklater. "The older the pasture the greater the risk of picking up the worm."