FOLLOWING infection with scab mites (psoroptes ovis), it can take a number of weeks before affected sheep start to scratch and rub, which allows time for the mites to spread to other sheep.

Scab is not the only cause of itchy sheep, so it is important to get the diagnosis confirmed to allow treatment with the right product. Pour-ons will not kill sheep scab mites and injections of 3ml wormers (eg Ivomec, Dectomax, Cydectin) will not kill lice.

Samples of skin and scab material need to be examined under the microscope in order to detect the mites, so that the appropriate action can be taken. They can be difficult to find so ask your vet to collect skin scrapings from affected sheep. Wool samples are not ideal for the diagnosis of sheep scab.

Those samples received from Scottish holdings are tested free of charge at SAC's veterinary laboratories.

However, we now have a new means of identfying sheep scab. Scientists at the Moredun Research Institute have developed a new blood test that detects antibodies to sheep scab mites. This is capable of diagnosing scab before infected sheep start to scratch.

They have also shown that by the time a small number of sheep show signs of scab, more than half the group can test positive with the blood test. The test has a number of possible uses:

l Confirming infection in itchy sheep when mites are hard to find.

Blood samples submitted from Scottish holdings at the same time as skin scrapings from the same animal will be tested free of charge.

l Detecting infection in sheep that are not showing any signs of scab.

The test could be used as a quarantine screen for added animals. The timing of blood sampling will be important. Sheep could become infected with mites the day before they arrive and it takes 2-4 weeks for antibodies to be produced.

l Proving a response to treatment.

If two samples are collected a few weeks apart the antibody levels will fall.

l Reduction in the use of OP dips and 3ml injectable wormers.

It is not always possible or desirable to treat groups of sheep either during quarantine, or annually, as a precaution against scab. This could be due to withdrawal times, organic regulations, poor handling facilities or a wish to reduce the use of medicines and the associated time and costs.

l As part of local control programmes.

Testing could be carried out to estimate how many flocks in a local area may be infected. If multiple flocks are treated at the same time those farms at the edge of the area could be monitored for re-introduction of mites.

l Proving freedom from infection.

This could apply to a country, area, farm, group of sheep or even an individual animal. The Shetland Isles are considered to be free of sheep scab. Testing carried out by Moredun suggests that this may also be the case for Mull, which was based on testing 12 sheep per management group.