14 July 1995

Non-OP dips fail to prevent scab in separate incidents

By Rebecca Austin

TWO separate cases of sheep scab mites surviving plunge-dipping with non-organophosphorus (non-OP) dips have been reported by vets.

Both flocks – one in the far north of Scotland, the other in the south-west of England – were dipped last year with Bayticol, Bayers flumethrin-based non-OP dip.

The Scottish flock was dipped three times at the recommended dipwash strength and for the recommended time. The other flock was dipped twice for scab control and again for tick, lice and ked control under the same conditions.

But these measures failed to eradicate the mite. The apparently resistant strains were cultured in central veterinary laboratories, where again the mite survived dipping in a flumethrin wash at the recommended rate.

Isolated circumstances

"Resistance has been recognised in two isolated circumstances, but there is no evidence it is widespread," says Peter Bates, head of entomology at MAFFs central veterinary laboratory in Surrey.

"Flumethrin probably remains an important and valuable drug for scab control, but where resistance is suspected an alternative product such as an OP or injectable avermectin should be considered."

Kevin Stevens, Bayticols product manager, said Bayer was monitoring the news closely. In future there will be stronger emphasis in literature regarding correct dipping.

"There have been no other incidences of this nature, and since 1990 at least 10m sheep have been dipped with Bayticol," he said. "Parasite resistance to any product is inevitable. But farmers can help stem the speed of this happening by using products properly."

He advises farmers:

&#8226 Correctly diagnose parasites, and if unsure to seek veterinary advice.

&#8226 Choose an appropriate product to eradicate the identified parasite.

&#8226 Mix dip at the correct strength.

&#8226 Dip sheep properly for a full minute with heads submerged at least once over that period.

&#8226 Top up the dip regularly.

John Thorley, chief executive of the National Sheep Association, emphasises how important it is producers follow the instructions supplied by dip manufacturers. "We need as broad a base of armoury against scab as is possible," says Mr Thorley.

"At the same time the industry needs to develop a thorough strategy to maintain the products it has while looking for new products. The ability of parasites to adapt to the poisons used for control is quite phenomenal. What works today might not work tomorrow unless we set about a proper programme to reduce parasite resistance."