Sheep Scab

• Sheep scab, or psoroptic mange, is caused by infestation of the skin surface with the scab mite, Psoroptes ovis.
• The Sheep Scab (Scotland) Order 2010 is in place in Scotland regarding the reporting and control of sheep scab. In England and Wales, the Sheep Scab Order 1997 addresses the movement and treatment of sheep affected by sheep scab.
• The disease is characterised by crusty yellow scabs on the skin surface and is accompanied by restlessness, scratching, loss of wool, bleeding wounds and loss of condition.
• It is one of the most contagious endemic parasite diseases affecting sheep, causing annual loses of £78-200M to the UK sheep industry.
• Scab is common throughout the year, but majority of outbreaks occur between September and March.
• Psoroptes mites can survive off-host for 16-19 days and sources of infestation include anything the infested sheep has contacted, e.g. fence posts, trees, equipment, trailers and anyone handling the sheep.
• Treatment options are limited to organophosphate (OP) plunge dips and macrocyclic lactone (ML) injectables. Resistance to the ML injectables has recently been confirmed in the UK and is spreading.
• Accurate diagnosis is crucial to ensure that the correct treatment is given at the right time. For example, other skin infections or infestation by multiple ectoparasites can affect treatment selection.
• Diagnosis is classically performed by a skin scrape but sensitivity can be low and often fails to detect sub-clinical disease. A diagnostic blood test (ELISA) for sheep scab, developed at Moredun, is available and can diagnose the disease before clinical signs appear.
• Good biosecurity, such as well maintained double fencing, can prevent the introduction of sheep scab into a flock by preventing direct contact with neighbouring flocks.
• Test all incoming stock to see if treatment is required, quarantine for at least two weeks and observe for signs of infestation such as nibbling, rubbing, scratching, deranged wool and areas of wool loss.

Research at Moredun seeks to understand the mechanisms of immunity to the sheep scab mite, Psoroptes ovis, with a view to developing effective vaccines and diagnostic tests based on sound scientific rationale.

The development of resistance to current chemical classes of acaricidal compounds presents a real threat to the long-term viability of the animal health industry. Alternative control strategies including vaccines and novel chemical control agents are being actively investigated at Moredun and elsewhere, but are unlikely to be widely available in the near future and even then they will need to be integrated with the existing chemicals used to treat and prevent infestation with scab mites.

The Moredun sheep scab blood test is now available as a commercial testing kit and is available via BioBest. Future improvements to the test will include the capacity to discriminate between currently infested and recently treated individuals by including selected biomarkers into the existing assay. One further development that may be required once a vaccine is available would be the inclusion of an additional antigen to allow the test to differentiate between infested and vaccinated sheep.

Moredun is currently developing a novel pen-side test for scab, which is based on the existing blood test. This test aims to offer a more rapid, pen-side diagnosis of disease with an obvious use in livestock markets.

Control of scab by vaccination is supported by demonstration of protective immunity in sheep previously infested with P. ovis. Moredun has been working towards a sheep scab vaccine for a number of years and we recently tested a prototype vaccine in repeated vaccination trials. Vaccination resulted in highly significant reductions in both lesion size (up to 63%) and mite numbers (up to 56%) following challenge. This vaccine represents the greatest reduction in lesion size to date with a sheep scab vaccine, providing encouragement for future production of a commercially-viable means of sheep scab control. We are now seeking a commercial partner to further develop the vaccine; however it is not likely to be commercially available for a number of years.

Infestation of sheepskin by P. ovis results in a rapid cutaneous immune response, leading to the crusted skin lesions characteristic of sheep scab. Little is known regarding the mechanisms by which such a profound inflammatory response is instigated and to identify novel vaccine and drug targets a better understanding of the host-parasite relationship is essential. One aspect of our research at Moredun has involved a combined network and pathway analysis of the in vivo skin response to infestation with P. ovis to gain a clearer understanding of the mechanisms and signalling pathways involved.

Research at Moredun is supported by a number of funding bodies, including:

• The Scottish Government under the Strategic Research Programme (2016-2021) and the Centre of Expertise on Animal Disease Outbreaks (EPIC)
• The European Union Horizon 2020 project PARAGONE
• The Genomia Fund
• Agriculture and Horticulture Development Board (AHDB)
• The Perry Foundation

Diagnostic blood test

Infestation with the sheep scab mite P. ovis can often be asymptomatic, and in these cases the detection of mites in skin scrapings (the traditional diagnostic test) is unlikely to be successful. The inability to diagnose infestation before disease has spread through the flock is an obstacle to effective control and disease prevention.

At Moredun we have developed an alternative method to diagnose infestation with sheep scab mites, which involves the detection of antibodies in blood. This diagnostic blood test can reliably detect the presence of P. ovis on sheep even before clinical signs of disease are evident. Therefore, the test will be of particular use as an aid to control the highly infectious mite and is available commercially via BioBest Laboratories.

This ELISA test is based on the detection of antibody (IgG) with specificity to a single recombinant protein from P. ovis, Pso o 2. The test is highly sensitive (98.2%) and specific for sheep scab (96.5%) with no known cross-reactivity with other common parasite infections of sheep.

Potential applications of the test

• Testing individual animals e.g. when sheep scab infestation is suspected
• Testing at the whole flock level to detect the presence or confirm the absence of sheep scab
• Use the test as a biosecurity tool prior to the introduction of new stock onto farms
• Can also be can be used for local/regional eradication programs to confirm the effectiveness of treatment or eradication of disease.
• The optimum use of the test is to apply it at the whole flock or management group level in which case we recommend that a minimum of 12 sheep from the group (up to a group size of 2,000) are tested.

Mull eradication work

Based on the numbers of notifications since the introduction of the Sheep Scab Order (2010) Scotland, we were able to identify areas where sheep scab was either absent or present at very low levels. The data collected suggested that sheep scab was either absent from Mull or present at very low levels. If it was absent, then we sought to maintain that status and if it was present at low levels, then this would provide an opportunity to eradicate it and keep it out.

To achieve this, Moredun Scientists, in conjunction with NFU Scotland and Mull Vets used the Moredun sheep scab ELISA on an island-wide basis (Isles of Mull & Iona) to determine whether or not flocks had sheep-scab. The study was spread out over two years and was completed in 2015. The first year involved applying the whole flock testing regime to as many flocks on the islands as possible and resulted in >700 samples being tested from >70 premises. No confirmed positives were detected and at this point the islands were assumed to be either free of sheep scab, or that scab was present at very low levels.

The second year of testing was designed to maintain this status and involved the use of tups as sentinels of disease, by serologically testing them pre- and post-tupping. This resulted in a very different pattern of responses (more border line and positive tests) and resulted in the detection of an outbreak of sheep scab in a defined geographical area.. The outbreak was isolated and controlled and animals on neighbouring properties were also tested with the ELISA; as well as any animals that were recently moved from/or to the affected area. No further outbreaks or cases were detected but this clearly highlights the need to be vigilant even in an area of relative low risk.

This study has demonstrated the use of the sheep scab diagnostic test as a means of assessing disease status as part of a local eradication campaign.

• We have had a series of comic-style informative posters commissioned about testing for and treating sheep scab.
• Our scientists regularly give talks and Moredun events and roadshows. Please see our events page for any forthcoming activities in this area.
• If you would like to arrange a speaker for an event, please contact us.

The first cartoon titled ‘a test is best’, describes exactly how the blood test works through identifying antibodies produced in response to a protein in the sheep scab mite faeces.

The second cartoon called ‘invest in the test’, describes the financial gain in testing sheep as opposed to treating an entire flock, giving a comparison of £160 for testing 12 individuals including vet fees as opposed to £1100 for treating a flock of 500 sheep.

The third design labelled ‘who let the tups out’, delivers a key biosecurity message about bringing animals on to the farm, which can be a particular issue when new tups are bought in.

It advised farmers to isolate new tups for at least two weeks, carry out the test and when the results are clear they can be turned out with the ewes.

The fourth cartoon, called ‘a race to the finish’, details the issue of store lambs coming in to be finished and the importance of testing for sheep scab, so if treatment is needed there is plenty of time for the withdrawal period to pass.

The final design ‘it is good to talk about scab’, looks at the need for coordinated treatments and the importance of talking with neighbours and sharing an effective biosecurity plan.

• A test is best
• Invest in the test
• Who let the tups out?
• A race to the finish
• It’s good to talk about scab
• Take control of sheep scab poster