6 May 2022
Dr David Cutress: IBERS, Aberystwyth University.
- Sheep scab eradication is vital as the disease impacts the UK sheep sector via huge economic losses and as a significant welfare concern
- Control would benefit from combined focus from regional groups of farmers alongside veterinary experts
- New tests and vaccine options, as well as government-funded projects, should provide good opportunities to work towards again achieving national eradication
Sheep scab, caused by the mite Psoroptes ovis, is the most important ectoparasitic (external parasite) species in the UK. It is endemic and causes detrimental disease across the whole sheep sector. In the past, sheep scab has been tackled effectively via legislation and control leading to its eradication in England, Scotland and Wales. Despite this, it has re-emerged and since has undergone shifts between being considered a notifiable and non-notifiable disease at different times. Deregulation of controls, based on its status, is thought to have led to its increased prevalence following initial eradication. Reports place the economic burden of sheep scab in the UK sheep at around £8 million a year, though a recent scientific opinion piece from experts in VetRecord suggests these previous figures, based on outdated 2005 data, may now realistically be more like £78 - £202 million per year. Even at the lowest end, the economic impact is almost 1% of the £1.3 billion industry, but if underestimations prove true this could range as high as 15% losses of the total sheep production value for the UK. Alongside economic losses, sheep scab has a significant impact on sheep welfare and comfort and as such it is a clear problem to the sector and UK agriculture as a whole. Recent letters to ‘The Veterinary Record’ suggest that the cases of sheep scab may be higher in the UK than currently realised. Worryingly, P. ovis mites can also cross infect cattle species, with particular evidence of susceptibility in Belgian Blue beef breeds suggesting increased complications in some regions where these are farmed alongside or close to sheep. Much of the veterinary impacts, diagnosis and treatment of these ectoparasites have remained relatively unchanged over time, as such our previous article on the topic still provides a useful background for those less familiar.
Any disease based on a vector, be that bacterial, fungal, parasitic or viral, due to their natures, have the risk of resistances developing over time. As with many diseases, sheep scab has had a history of anecdotal evidence of treatment options having little to no effect. But it wasn’t until 2018 that the first scientific confirmation was made of scab populations in West Wales, the Wales/England border and Leominster showing significant signs of resistance to the macrocyclic lactone (ML) moxidectin, one of the two primary current treatments for scab prevention licensed in the UK. Following this initial confirmation, a report released the following year, demonstrated detrimental resistances in outbreak populations in Wales and south-west England to ML moxidectin but also ivermectin and doramectin. This suggests that resistances to licensed treatments for this disease and other diseases (ivermectin being used as an anthelmintic wormer) are a significant issue of future concern. Resistance to treatment is an issue for the UK as a whole, but further to this it is a problem when considering disease prevalence and spread in organic farming systems. Organic farms face limitations on treatment options due to their farming practices and accreditation requirements with the soil association having strict rules regarding the use of organophosphorus (OP) chemicals and sheep dips. As such when organic farmers do need to treat for scab this is far more complex and can lead to the inability to sell treated sheep and achieve the organic premium, or may require farmers to adhere to significant meat withdrawal periods. Furthermore, organic sheep are at even higher risk to the impacts of any resistances due to the limited pool of treatment options available to them.
Sheep scab is a large-scale endemic issue with spread influenced by the transportation of animals to market places and replenishing stock from off-farm with insufficient quarantine facilities. However, scab has also been shown to occur in national hotspots. This demonstrates that local transfer of infection between nearby farms plays a key role in this disease. For this reason, scab disease control cannot just be considered on a farm by farm basis but there must be a collaboration between neighbouring farmers to attempt to remove any reservoirs of re-infection from an area.
As agriculture as a whole looks towards improved environmental impacts and sustainability then optimal animal welfare should increasingly be a concern. Where animals are diseased and ill they perform sub-optimally, as such they often require more inputs (feed, bedding, water etc) over a longer time to produce the same product outputs, or outputs are lost entirely if animals have to be culled. This reduced efficiency can be considered directly as higher carbon dioxide equivalents emissions per kg of meat/milk produced and based on high impacts across the UK sector could be considerable (though no life cycle assessment studies of scab in the UK could be found). Furthermore, the production of chemical scab treatments themselves are associated with energy and resources which have an impact on global emissions, whilst the disposal of sheep dip chemicals is a well-known area of concern for polluting waterways and toxicity to humans. As such control and eradication of P. ovis mites have considerable environmental impacts as well.
The practicalities of on-farm considerations regarding scab control are well discussed and still entirely relevant from our previous article. These note the mite’s ability to live off-animal in the environment for 16-17 days making biosecurity, animal handling and hygiene considerations a high priority. It is also vital that correct diagnosis is performed as scab and lice can present with very similar symptoms but require entirely different treatment regimes. As such, veterinary practitioner flock sampling, usually 12 animals per flock (for ELISAs or microscope identification), and discussion with them, of the results, are key before treatment. Also noted previously, the Animal Health and Welfare Framework (AHWF) commissioned a survey on Welsh scab prevalence in 2016 through Bristol University researchers. Whilst the survey was completed by only 14% of the possible respondents, due to its voluntary nature, it did reinforce some core concerns. Outbreaks were noticed seasonally by farmers and clustered to Brecon and Bangor regions, with clear evidence that common grazing led to higher scab risk for sheep due to higher potential farm to farm animal interaction.
Taken from Chivers et al., (2018) – scale relates to density of scab report more yellow is more reports
The results of such surveys and other experts’ advice have likely led the AHWF and other UK livestock and animal health programmes towards developing schemes for the tackling of this disease. Welsh schemes such as Hybu Cig Cymru’s (HCC) Stoc+ and the Rural Development Programme for England’s ‘For Flock’s Sake Let’s Stop Scab Together’ aim to help incentivise and inform farmers to once again eradicate scab from the UK. A trial, related to such schemes, is also part of the European Innovation Partnership (EIP) in Wales and utilises veterinary interaction and regular testing to inform treatment planning to help demonstrate effective treatment strategies and inform farmers who are taking part in the project. It focuses on farmers within a close geographical region, in Ceredigion, working together and communicating as a unit with veterinary advisors with regards to their groups' treatment and control strategies. Seasonal timing of prophylactic scab treatment via sheep dipping is often encouraged by control schemes and has shown good efficacy in many studies. Despite this, it was noted, in a 2020 thesis, that economically it lacks cost-effectiveness if there is not some kind of reduction or subsidy of the treatment costs. If the farm in question is in a known high-risk area this also makes treatment cost-effective. This suggests that organised, collaborative and targeted approaches backed by sufficient support could go a long way towards improving control and possibly eventual regional eradication of scab mites.
Alternatives and future considerations
So, if resistances have been confirmed what are the options moving forwards? Well as is common across livestock disease-control the ultimate goal should always be protection and prevention rather than cure. Control methods are vital in implementing this protection but ultimately making sheep themselves resistant to the disease is the best option. This could be achieved via vaccination strategies or potentially via breeding/genetic modification of the animals themselves.
Pso o2, the antigen that demonstrated a lot of promise for its role in accurate sheep scab diagnosis in ELISA tests, was even able to diagnose very low levels of mite infestations highly accurately. This test is also offered as part of the free blood sample diagnoses in the ‘For Flock’s Sake Let’s Stop Scab Together’ scheme and is helping to improve our understanding of the disease in key hotspots. Also, the Pso o2 antigen has shown good promise as part of a vaccine cocktail. In initial trials this vaccine cocktail causes lesion sizes associated with sheep scab, along with numbers of mites present, to both decrease by over 50%, offering a great start towards the total protection for sheep.
Whilst vaccine progress is promising with regards to the possible future options to protect sheep it does have the downside that it prevents the current commercial ELISA test from acting as a DIVA (Differentiation Infected from Vaccinated Animal) diagnostic. To combat this the Moredun research institute, which was involved in both the vaccine development and the initial ELISA development, have produced a new diagnostic using the Pso-EIP-1 antigen which should not cross-react with vaccine treated sheep, but still has high specificity and sensitivity as a future replacement diagnostic tool. Interestingly this version of the ELISA tool also shows better results identifying scab infection in non-UK sheep breeds which don’t always present positive with the Pso o2 test. This is encouraging both for when a commercial vaccine is produced and for farmers wishing to diversify their flocks with non-UK breeds with unique produce outputs including meat, milk or higher quality wool.
Alternative methods of control have been mentioned for sheep scab, often in research articles discussing ectoparasite control as a whole. Whilst interactions between P. ovis and different bacteria have been noted to play important roles in how the parasite feeds and infects successfully, little research has yet gone on to target these. If vaccines prove less efficacious than expected these bacteria/parasite interactions could offer future disease control routes, away from, or in combination with, current treatments where resistances are occurring. Also, essential oil compounds (transcinnamic acid, usnic acid, geraniol, carvacrol, eugenol, thymol and L-menthol) have shown some early success in lab studies on killing P. ovis mites Naturalistic biocontrol via Beauveria bassiana or Metarhizium anisopoliae fungi has also been considered and it was shown these can also infect and kill multiple parasitic insects.
Alternatively, producing sheep with genetic hardiness to different parasites is already an area where research has had some success. Little research has been performed on identifying animals less susceptible to scab specifically and as such, there is currently a lack of evidence of breeding research for this purpose. However, with modern technologies available for analysing host transcriptional regulation (gene control essentially) to parasite infection it is far quicker and easier than ever to highlight candidate genes that could be targeted. Selected genes could then help either strategic breeding programmes (which are slow to achieve) or more likely be targeted for gene editing, particularly as UK laws in this area continue to change. CRISPR/Cas9 genetic control for example has already been validated for editing genes to make cattle less susceptible to tuberculosis. Though if eradication schemes prove successful these longer-term genetic strategies would be unnecessary.
Sheep scab is having a huge impact on livestock welfare and the economics of the sector in the UK. Treatments are limited due to licensing and environmental considerations and evidence of resistance to treatments are apparent. Furthermore, control is complex due to animal proximity transmission as well as off-animal mite transmission due to the substantial ability for mites to survive in the environment without a host. To improve controls farmers with livestock in close proximity should work together, along with veterinary practitioners, to determine region-wide treatment and control plans. Powerful tools in the form of antigen ELISA tests for scab which are commercially available should help improve such region-wide plans. Furthermore, there is a strong prospect for a protective vaccine that could bolster eradication plans significantly. If all of these are performed optimally they should help move the UK back towards a state of nationwide scab eradication, saving the agricultural sector as a whole millions of pounds a year.
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