Making the most of testing for sheep scab
Preparing for Sustainable Farming – making the most of testing for sheep scab
As part of Scottish Governments ‘Preparing for Sustainable Farming’ initiative, farmers can claim £250 per intervention for undertaking up to two animal health interventions per year in 2023 and 2024. The standard payment is £250 per intervention, regardless of the size of the flock or the number of tests done. You do not have to provide details of their expenditure in order to claim. With the first claim, an additional £250 will be received to fund further on-farm development activities appropriate to the flock. Interventions carried out January to December 2023 need to be claimed by the end of February 2024. One of the interventions included in the scheme is to investigate sheep status in a flock by screening 12 animals per management group using the sheep scab ELISA test. Further information on the scheme can be found here.
Sheep scab is caused by the mite Psoroptes ovis and has been a notifiable disease in Scotland since 2010. The number of notifications to APHA has remained relatively static at around 100 per year, with higher numbers of notifications usually seen from October to March.
Following infection there can be a lag phase of weeks to months prior to development of clinical signs in the flock. The scab ELISA offers a method for screening sheep for subclinical scab infection. Animals seroconvert around two weeks after infection and taking 12 serum samples from each management group has a 95% chance of detecting a prevalence of 20%. Antibody can be detected for three to six months after successful treatment, therefore positive results do not always indicate active infection and results should be interpreted in the context of the flock history. Sampling of the same individuals after three weeks to look for a rising/falling titre may be useful in some cases. At SRUC the ELISA costs £8.95 each if screening 10 to 39 animals or £7.00 each if over 40 animals are tested.
As the infection progresses, higher mite numbers result in restlessness, rubbing, staining of the fleece especially over the shoulder with yellow crust at the skin surface, progressing to wool loss, weight loss and occasionally seizures. Diagnosis in clinical cases is usually by microscopy. Plucked wool, crust material and skin scrapes should be taken from the edge of lesions from several animals, and examined under the microscope for the presence of mites. SRUC will examine skin scrapes from suspect cases in Scotland free of charge (please do not send sharps in the post).
Following the diagnosis of sheep scab, a report should be made to APHA via email or by phoning your local APHA field office. Following diagnosis the source of the infection should be identified if possible. Neighbours with sheep in adjacent fields should be contacted with the farmers permission and where possible co-ordinated treatment in an area facilitated.
Good biosecurity is essential to precent sheep scab from entering a flock. The scab ELISA can be used to screen purchased animals, as described above, prior to them being added to the flock. Incursion of sheep from neighbouring flocks is another risk of infection and double fencing may be required to distance sheep from high risk neighbours in some cases.
There are two treatment options for sheep scab:
- Macrocyclic lactone injection
- Plunge dipping with organophosphate (by licenced operator)
Macrocyclic lactone (ML) treatment paralyses the mites, and sheep may remain infective for up to 12 days following injection. Mites can survive for 17 days off the sheep, therefore fields and handling facilities remain a source of infection for this period. When using Ivermectin or Doramectin sheep should not return to contaminated fields or which share a fence line with contaminated fields. With more persistent (moxidectin) products sheep can be returned to contaminated fields but it is still preferable not to do so. Whole group/flock treatment with macrocyclic lactones will select heavily for avermectin resistance, especially in winter where the refugia population is low. Additionally, ensuring every sheep is correctly injected (all animals with the correct dose by the correct route) can be challenging, especially in larger groups of animals. Resistance of scab mites to macrocyclic lactones has been reported, and the VMD should be informed when treatment failure is suspected. Further information on ML’s used for scab control can be found at scops.org.uk
Plunge dipping will kill mites within 24 hours and provides up to eight weeks of residual protection depending on the product used. Sheep should remain within the dipper for 60 seconds and be submerged at least once. The dip should be regularly replenished as per the product instructions and may need to be emptied and refilled after dipping larger numbers of sheep. Where possible, plunge dipping with organophosphate products is considered the treatment of choice. For further information on the use of mobile dippers see the mobile sheep dipping code of practice 2023.
Posted by SRUC Veterinary Services on 22/09/2023