HILL sheep may be less susceptible to sheep scab than lowland breeds.

Recent studies suggest the Swaledale breed has some natural resistance, with an extremely long sub-clinical period and fewer scab mites and lesions.

Mr Peter Bates, head of entomology at the MAFF veterinary laboratory agency at Weybridge, said the thickness of the fleece appeared to play a key role.

"One of the main differences between hill and lowland breeds is that the lowland has more wool fibres per square centimetre, which gives the mite its ideal living conditions," he told a meeting at Skipton auction mart last week, organised by the NFU.

The scab mite likes the heat and humidity of a dense fleece.

"Swaledales have fewer wool fibres so the micro-climate is not as good for the mite," said Mr Bates. "They also have horns, which they use on the lesions caused by the mite. If you take away the micro-climate, you reduce the number of mites. We have found there are some sheep naturally resistant to scab; we have not been able to give it to them."

Studies involved researchers infecting Swaledales, Polled Dorsets and Mules and resulted in the Mules having the highest number of mites.

Mr Bates said OP dips had been the most effective weapon against scab. He believed they would return, but not for this season, and warned that they might not be back in use for long.

It was vital, he said, for producers to be extremely careful in choosing alternative methods of control. While some might cure scab, they would not prevent it returning.

The situation had got far more complicated since OP dips were withdrawn. There were many different products and treatments which meant farmers had to read the labels carefully to decide whether they were suitable for their needs.

Mr Bates also recommended taking advice from vets or other professionals to decide exactly which parasite was affecting the sheep. A wrong diagnosis meant not only the wrong treatment but could also heighten the risk of insecticide resistance.

Each animal in the flock should be treated at the same time. "Missed sheep will be a reservoir of infestation," he said. "All treated sheep should be moved to clean accommodation immediately after treatment."

Mr Bates explained that scab mites spent their entire life on the sheep. They had piercing and sucking mouth parts but it was still not certain what they lived on.

"They do not take in blood, but we think they graze around the edge of the lesions," he said. The scab lesion was a form of allergic dermatitis to the mite faeces. "The resultant inflammation causes the heat and humidity and the mites actively feed off the moist periphery of the lesion so the lesion gets bigger."

Sub-clinical scab symptoms include restlessness, rubbing against fence posts, soiled and stained areas of wool, head tossing and rags of pulled fleece. They can be confused with other ecto-parasites.

In clinical cases, excess rubbing, lesions, violent head tossing, extensive wool loss, loss of condition, excessive soiled and stained wool and epileptic fits can be seen.

"Probably not all these happen at the same time," said Mr Bates. "If the animal is susceptible you may get all, but if it is not particularly susceptible you may just get related forms."

Lambs up to one month old do not show typical signs of scab. Studies have shown the symptoms manifest themselves in hoggs and shearlings more than in ewes.

Although scab had always been thought of as a winter disease, with the peak between September and April, research had shown the real peak was between June and February.

"You can have a considerable number of cases in summer," said Mr Bates, "As long as the animal has a fleece it can have scab."

It can be passed on by bits of wool caught on fence wire, thorns and fencing.

Mr Bates again stressed the need to use the most effective treatment. "If you have highly susceptible sheep, one mite can cause disease to the whole flock," he said.

Vaccines had been researched for some time but he believed it would be ten to 15 years before they were likely to be in use