3 November 1995

How causal mites thrive and spread

THE mite which causes sheep scab (psoroptes ovis), is 1mm long, pearly white, breathes through its skin, and needs a moist environment to survive.

These were some of the facts revealed by Peter Bates, senior scientific officer with the Central Veterinary Laboratory.

He said the mites eggs were laid on the edge of lesions and took up to three days to hatch. Average life-span was 40 days, with females mating once only .

Mr Bates is currently undertaking a PhD to study the mites behaviour further. He told the conference that sheep experience allergic dermatitis as a reaction to the mites faeces or the parcel of gut which was excreted along with the faeces. Antibodies were produced by the sheep, and Mr Bates is hoping to examine this reaction.

When moulting the mite did not eat. This factor was important when systemic injections were used, as it was likely the mite could not be affected during this period.

Mr Bates said sheep could recover from scab if the lesions had spread so far that there was nowhere for the mites to go. But in such cases, the fleece would be affected for three years due to follicle damage. When a sheep did die, it would not usually be the direct result of scab, he said.

Secondary ailment

More often a secondary ailment, such as hypothermia or dehydration, would be the cause.

Mites could survive dipping, for they had been found in sheeps ears, he said. This was one of the areas, along with the eyes and in under the back legs, where they spent the summer.

Cattle could also be infested with the mites, which was another way in which they can re-infest sheep.

The CVLs Peter Bates warns that scab mites can survive dipping.