Worm resistance fears outlined

TRIPLE RESISTANCE to anthel-mintics in sheep worms could be more widespread than previously thought, so more than ever producers must develop a sustainable worming policy for their flock for 2005.

According to a survey by the Moredun Institute, Edinburgh, triple resistance could be present, at varying levels, in nearly one in three flocks.

Tests on samples from 16 flocks this year revealed triple resistance in more than 31% of them, said Moredun researcher Frank Jackson.

“Normally avermectins are highly effective and cut egg counts by more than 95% and will in fact reduce egg counts by more than 99%.

Reductions of less than 95% are taken as indicative of resistance. But on five of the farms surveyed efficacies ranged from 66-92%, suggesting triple resistance is present.”

Stopping the spread of resistant worms from one flock to another is the main line of defence, said Gerald Coles of Bristol Vet School.

“Quarantining animals and drenching according to the Sustainable Control of Parasites in Sheep guidelines will help keep resistant worms off farms. And these guidelines must be incorporated into annual worming programmes.”

Another useful measure is to move groups of sheep before drenching, rather than drenching sheep before moving as previously recommended, said Dr Coles.

“Treating sheep before moving means anthelmintic resistant worms are spread to new pastures.

“Treating them after moving takes non-resistant worms, too, so the build up of resistance is delayed.”

But Dr Coles said it is still unclear how long after sheep are moved that they should be treated for this method to be most beneficial.

To prevent the build up of resistance on farm, Dr Jackson advised devising a drenching programme which includes checking drug efficacy.

“Monitoring, by way of faecal egg counts establishes whether flocks need treating or not.

“And when flocks have been treated more faecal egg counts should be done to check the anthelmintic has been effective.”

When animals are treated, Dr Jackson advised treating according to the weight of the heaviest animal.

“Treating to the average weight means under-dosing some animals. This promotes resistance to anthelmintics.”

He also suggested finishing lambs quickly, as lambs have little or no resistance and are the biggest spreaders of infection.

“This may require supplementary feed, but the less time lambs are kept the better.”

Using break crops can also help, as these cut the lifecycle of the worms, reckoned Dr Coles. “But there is no proof of how long worms can survive and still cause infection.”

Dr Jackson suggested brown stomach worms can last up to 12 months and still cause infection, while for nematodes it could be  three years.

But whatever protocols are adopted, he said they will eventually fail. The aim is to prolong product life to allow time for new technologies to become available.