Sheep Scab

Oct 11 2021, 11:47

Sheep Scab

Sheep scab is caused by the highly contagious mite, Psoroptes ovis. The mite causes an intense allergic reaction on the skin, although shorn sheep can carry the mite with no clinical signs. Sheep scab is most often seen between September and April, as the mite likes warm, moist areas on sheep with a full fleece.

Sheep scab causes serious welfare issues and has a massive economic impact on the UK sheep sector. The annual cost of sheep scab is estimated at £8.3m, and most of this comes from control measures. Sheep scab impacts the condition of ewes, reduces growth rates of lambs, damages the wool and, in severe cases, can lead to fitting and death.

There have been attempts in the past to eradicate sheep scab but, all of these have failed. The disease was notifiable in Great Britain until 1992 and the number of cases has risen hugely since deregulation. Sheep Scab has been made notifiable again in both Scotland and Northern Ireland in recent years. Throughout the UK, it is a legal requirement to treat any flocks containing infected animals and local authorities can force appropriate action to treat sheep scab if necessary.

Spread

Sheep scab is most commonly spread directly from sheep to sheep. It can survive for up to 17 days off the sheep so, indirect spread is possible, making common grazing a high risk area. The mites can also be picked up from anything the infected sheep has rubbed against, like fence posts and hedges.

Shared equipment and handling facilities are another area where mites can be transmitted. The mite is not zoonotic so will not infect people.

Sheep scab can be introduced to a flock when bringing in new animals. A strict quarantine protocol on farms and good biosecurity will reduce this risk. The SCOPs website has great information regarding quarantine protocols and treatments that cover both internal and external parasites.

Clinical Signs

Sheep can be infected for weeks without developing clinical signs. Always assume that the whole group is infected, even if not all sheep are showing signs.

Clinical signs include:

  • Fence rubbing
  • Restlessness
  • Head tossing
  • Biting/stamping feet
  • Loose/soiled fleece
  • Crusting red lesions
  • Poor body condition
  • Dull/depressed
  • Progress to fitting/ death when severe

Other causes of itchy sheep that need to be considered:

  • Lice
  • Rain scald
  • Lumpy wool
  • Bacterial/fly dermatitis
  • Scrapie

Diagnosis

Sheep scab cannot be diagnosed from a visual examination. It is possible to get a dual infection with lice and scab, so any itchy sheep should be checked for scab, even if lice have already been diagnosed. To confirm sheep scab, a skin scrape is taken from the edge of a lesion on the affected sheep and examined under the microscope. It is possible to miss a case of scab by taking a scrape from the centre of the lesion or just looking at wool. It can also be difficult to find mites in the early stages of disease. A blood test is available to check for exposure to sheep scab. The result will be positive after two weeks of infection. This test is very useful for early diagnosis before clinical signs appear and for testing sheep post-quarantine. We can do all the testing needed to diagnose any issues within your flock.

Treatment

Treatment should only be used following a diagnosis of sheep scab to prevent unnecessary costs and reduce the development of resistant parasites.

There are two ways to treat sheep scab - organophosphate (OP) plunge dipping and injectable macrocyclic lactones (ML). It is vital to treat all in-contact sheep even if they are showing no clinical signs. If the sheep are kept on common ground, then a treatment with residual action is a good idea to prevent reinfection.

Coordinating treatment times with neighbours should also be considered.

OP Plunge Dipping

  • Only effective if the dipping is done correctly (showering or jets will not work)
  • Kills mites within 24 hour
  • Residual action that lasts for several weeks
  • Also controls blow fly, keds, lice and ticks
  • Certificate required to use OP
  • OP is ecotoxic and must be disposed of correctly
  • Health risk to operator if mishandled
  • Contract OP dippers are becoming more common

Macrocyclic Lactone Injection

  • Injection treats sheep scab and internal worms
  • Takes up to 7 days to kill mites
  • Short acting so reinfection possible
  • Does not cover for any other ectoparasites
  • Risk that the use of injectable ML for scab will increase resistance in internal worms
  • Must not use 1% cydectin in sheep that have been injected with Footvax

Treatment should be reviewed for each individual farm. MLs are also anthelmintics and there is growing resistance to these medications in gut worms. There have been reports of sheep mites developing resistance to MLs. To reduce the risk of resistance, these medications should only be used when absolutely necessary.

Control

Good biosecurity, an appropriate quarantine protocol and prompt investigation of itchy sheep are all vital for controlling sheep scab. A vaccine is being developed which in the future may be a valuable asset to sheep scab management.

Key biosecurity points to prevent sheep scab introduction to a flock:

  • Quarantine new arrivals and either treat for sheep scab, or blood test after 2 weeks to check for exposure first
  • Clean and disinfect any equipment/handling facilities the new sheep have had contact with
  • Clean and disinfect protective clothing
  • Ensure all fencing at boundaries is well maintained and double fenced

Please give us a call if you have any questions or want to discuss sheep scab management on your farm.