Sheep scab - on the rise and costing more

Sheep scab costs the UK sheep industry £78m to £202m every year¹ and the mite causing it is showing signs of resistance to injectable treatments.

The Psoroptes ovis mite, which can be transmitted between animals or via contact with surfaces or clothing, can survive for up to 17 days in the environment.

Clinical signs

The mites’ faeces can cause a severe allergic reaction, causing skin lesions and intense itching. While most farmers are familiar with the classic signs of scab, fewer are familiar with the signs of chronic sheep scab disease and may be unaware long-term infections can cause no obvious clinical signs at all.

Biosecurity

Animals repeatedly exposed to scab are likely to have developed an immune response. This will not eradicate disease and will not prevent transmission but will dampen or even prevent obvious clinical signs. All new incoming stock should always be considered a risk.

Diagnosis

Skin scraping to detect mites can be carried out by your vet. An ELISA test detects the presence of antibodies following sheep exposure where mites are not detected on scrapes, or for surveillance and subclinical cases.

Treatment and management

If sheep scab is diagnosed, engage with your vet or Registered Animal Medicines Advisor (RAMA) to discuss options.

There are currently two treatments available: plunge dipping with organophosphate dip or injecting with Macrocyclic Lactones. (Group 3 clear wormers)

Diazinon has been used in showers or water jetters, however, the product is not licenced for this use and there are numerous concerns, including:

Mites can survive in ear canals facilitating reinfection
Fleece dip concentration following showering has been demonstrated as significantly lower in trial work vs plunge dipping². This could result in treatment failure and reduced duration of protection
Exposure of mites to sub-lethal concentrations may lead to resistance
Harmful aerosol generated may increase risk of inhalation
Medicines must only be used in accordance with their licenced use

Mobile dipping contractors

For those unable to dip themselves or who would prefer to use a contractor, a cross industry ‘Code of Practice’ is now available on the SCOPS website. Responsible dipping contractors must adhere to this code for the welfare of sheep and to ensure waste disposal is being carried out responsibly. If you are considering using a contractor, read the code and ensure they comply.

Resistance

Scab mites resistant to injectable treatments have been demonstrated as susceptible to plunge dipping with diazinon allowing elimination of resistant mites from the flock.

Injectable ML treatments are also used to target gastrointestinal roundworms. Using injectables to target scab, when there is a low worm burden which does not require treatment, may also speed up the development of roundworm resistance. If the roundworm burden is low and does not require treatment, consider using plunge dipping which only targets external parasites.

Summary

Numerous control schemes to tackle scab are being rolled out across the UK. For up-to-date information on management of sheep scab disease, consult the Sustainable Control of Worms in Sheep (SCOPS) website: www.scops.org.uk/external-parasites/scab/

References:

The high cost of sheep scab. Emily J Nixon, Richard Wall, Hannah Rose Vineer, Lesley Stubbings. Vet Record, 2020.

M. H. Clear , P. R. Kettle & T. J. Hynes (1982) Retention of diazinon in wool on Romney and Drysdale sheep and in hair on goats, New Zealand Journal of Experimental Agriculture, 10:1, 19-21, DOI: 10.1080/03015521.1982.10427836