Maedi-visna (MV) the chronic infectious disease affecting sheep and goats could be significantly reduced in the UK if more breeders aim to breed sheep containing a double copy of the TMEM154 K allele, or the K gene.
That was the good news story from a new investigative study at Nottingham University and the Moredun Institute. The project builds on work demonstrating that some breeds of sheep are more resistant to the disease than others.
In turn, more MV resistant types of sheep could be sold at a premium and bolster the confidence of flock masters who have already lost breeding females to this long-standing illness.
Worryingly, the number of flocks affected by MV has increased sharply in the last 30 years from 1.4% in 1995 to 9.4% in 2019, with at least 1.6m animals out of the UK’s 32m strong sheep flock thought to be affected.
Furthermore, according to industry specialists, farms often do not realise their animals are affected with MV until more than 50% of the flock is infected, with many animals thin and dying.
The Moredun Research Institute and the University of Nottingham have been awarded a joint £1.1m grant to tackle the devastating impact of MV by developing genetic resistance to this debilitating disease, thereby potentially transforming sheep farming and enhancing animal welfare worldwide.
K gene linked to resistance
The three-year award will revolve around extensive research into multiple sheep breeds and production systems, which have shown the variation in the K gene to be strongly and reproducibly associated with genetic resistance to MV in sheep.
This opens the possibility of using genetic selection for resistant forms of the gene to help reduce the impact of MV on UK sheep farms.
David Griffiths, Moredun Research Institute and project co-lead, said: “Maedi-visna is a serious issue for sheep farmers. This new funding will allow us to understand how best to implement genetic selection to control the disease. The result will be healthier animals and more sustainable farming.
“The project has already stimulated a lot of interest as to what can be done with genotyping of sheep. We are talking to a number of sheep breeds about how to integrate this into their breeding programmes and would be really keen to talk to any interested breed societies about participating.”
Rachael Tarlinton, University of Nottingham, project co-lead said: “A major part of the grant is to establish the breeds that are most susceptible to MV and those that are most resistant. For some breeds (those that have been participating in the AHDBs signet programme or the Welsh hill sheep genome project) we know what the level of the genetic resistance allele (specific mutations in the K gene) is.
“For many breeds we don’t know. A major part of the project is to generate genetic sequence data for breeds that don’t have it and we are really keen to talk to any interested breed societies.
If breeders are currently genotyping sheep it is likely that the TMEM sequence data or K gene can be retrieved from the existing SNP chip testing data and we are working with Neogene and Wetherbys to make sure the relevant alleles are on the genotyping chips and they can produce reports on this.”
It should also be noted that Neogene is hoping to provide a genotyping service for the 2024 ram sales, but producers will have to have to be organised to get the test done and sent away next month, to ensure the information is back in time for the back-end sales.
Increased incidence of MV
She added that there is no real reason why the level of incidence of MV has gone up so much in the last 20 years, but it may be due to changes in sheep husbandry (more indoor lambing), changes in the sheep breeds in commercial operations, changes in the strain of the virus or a combination of all of these.
There are two main ways that MV spreads – the first is by respiratory secretions, particularly when animals are housed and in close contact with each other. This is probably the main way it is spread in the UK.
The second way is via colostrum from infected ewes to their lambs (this route is really important in dairy flocks or where pooled colostrum is fed to lambs).
It is also possible to spread it via semen and embyros (though not as common a route) and if you are using artificial insemination programmes, semen and embryos should really be coming from known disease free flocks and animals
MV is identified as sheep with chronic ill thrift (thin, poor doing sheep) and can be easily confused with other common diseases. Sheep can also have have multiple infections.
The disease primarily affects the lungs and udders in sheep so will affect their breathing and result in a ‘hard bag’ (indurative mastitis or hard lumps in the udder). It decreases milk production in ewes.
Some sheep can also develop neurological problems (but that is rare in the UK). Sheep in earlier stages of the disease may not have any visible signs that they have it. Postmortem examination of the lungs can however pick up changes in the lungs (enlarged lungs that don’t collapse). Often flocks don’t realise they have a problem until they have a lot of very ill and dying sheep and losses can be very large in this situation.
The virus is diagnosed by serology (blood) tests for antibodies to the virus. As the virus has a long latent period (the period between when an animal is infected and when they test positive or develop clinical signs) it is necessary to test an animal at least twice over a six-12-month period. There are no reliable PCR tests for the virus as it is so variable.
Voluntary ‘accredited-free’ scheme
Currently, there is a voluntary scheme for flocks looking to become ‘accredited free’ which is run by the SRUC premium sheep and goat health scheme. However, there is no vaccine for MV, with control limited to test and cull.
If farmers or their vets think they might have a problem with MV a screening test of 12 thin sheep is the first step, if the flock is infected then the next step usually involves screening all the animals.
Control strategies include culling all infected animals (and retesting all animals until the flock is completely free – this can take several years) or splitting the flock by age group and testing status (older animals are more likely to test positive) and gradually culling out the infected flock or in extreme cases it may be more viable to cull the entire flock.
Dr Tarlinton added that USDA, in America, had done a lot of the work over the years on breeding for animals that are genetically resistant to MV. They’ve been working with individual producers and breed societies to introduce screening for the resistant alleles of the K gene (animals have to be homozygous – ie have two copies of the same gene variant to be resistant) as part of disease control programmes.
While the genetic selection does not mean they are 100% protected from infection, it does mean they are less likely to get infected and less likely for the disease to progress or cause problems if they do get infected.
In infected flocks, which animals test positive and which ones test negative is very strongly correlated with their genetic resistance status.
For commercial flocks (particular where testing is not really economically viable) selecting breeds that already have a high level of the resistant alleles (fortunately some of our more common breeds are already in a good position here) or purchasing genotyped breeding stock (particularly rams) that are resistant is likely to increase the number of resistant animals and reduce the impact of the disease.
“As our current control strategies are not really working to limit the disease, we do need to be taking new approaches to controlling this disease. The approach we are taking here is similar to the scrapie control scheme that UK producers will be familiar with and that scheme was really successful in dropping the rate of disease in the UK flock,” said Dr Tarlinton, adding that the UK is unlikely to ever become completely MV-free.
Compulsory screening in Europe
Experience elsewhere in Europe had shown that even when screening and control programmes were compulsory it’s hard to completely eliminate the disease once it’s established.
“Most of the rest of the Europe also has a large problem with MV and there is the added complication that CAE in goats and MV are actually pretty much the same virus and can infect either species (though we don’t know if this genetic resistance also holds true in goats).
“Australia and New Zealand never had it in the first place and keeping the disease out is one of the reasons they don’t allow imports of live animals. It’s not a human health risk so its unlikely the UK Government would ever fund mandatory testing.
“We need a more viable control strategy than our present approach. We think breeding for genetic resistance needs to be a big part of that.”
MV resistance in UK-based populations
Breed % of population MV resistant
Dorset Horn 90
Scotch Blackface 64
Soay 60
Herdwick and Beaulah 58
Border Leicester 40
Easycare 30
Hardy Speckle 25
South West Mountain 17
Welsh Mountain and Texel 15
Brecknock Hill Cheviot 13
North Country Cheviot (hill) 10
Industry views on MV resistance
Derek Hall, chairman of the Stratified Sheep Group, was impressed by the possibilities of improving the overall health of the UK sheep flock and urged pedigree producers to test their rams as the first step.
“This is a huge step forward for the sheep industry. If we can build on our assurance and health schemes to have homozygous MV-resistant tups to sell in the back-end, it makes a huge difference when we are already a couple of years behind Europe when they have already been testing their sheep.
“If we can push this forward, hopefully there are gene markers that can be identified for Borders disease and OPA in sheep too which would could then look to breed for resistance from those sheep illnesses,” said Mr Hall.
Independent vet and sheep farmer, Neil Laing, was also optimistic for reducing MV in the UK flock by gene selection.
“I think any way to minimise a disease that is difficult to see, diagnose, treat and control can only be a good thing for the whole industry. As others have said, MV seems to be becoming an increasing problem and although there are diseases affecting far more flocks – mastitis and footrot will arguably be in all flocks – the ways we can mitigate and manage those diseases are far easier.
“As a breeder of commercial breeding ewes, I feel a degree of responsibility in maintaining the health of the national flock, so this seems like it could be a useful, although not inexpensive way, to eradicate this disease from the UK.
“It will take time for a market to develop for breeding sheep with this resistance to appear, as realistically it will take time to build a momentum in terms of eligible sheep, as they need to be homozygous carriers to have any effect, but also in terms of need from a buyers perspective.
“ I can see the initial market being a flock that has confirmed MV, as they can farm their way out of the disease by buying resistant sheep. It seems the resistant sheep can’t become infected and can’t become carriers, and I know that is some of the work the Nottingham team are doing over the next three years.
“Although I am going MV-accredited with all the flock, this seems like a way to stay free without all of the risks and challenges of biosecurity and continual frequent testing, with only genotype test required in a lifetime.
“Imagine what it might mean in terms of marketing genetics around the world if the UK had an entire population of sheep resistant to MV? It will take many years, probably, but it isn’t difficult in terms of management.
“It’s just breeding policy. And, the market might develop after some brave individuals take the lead and working with the Nottingham project, can prove the disease can be eradicated.”
Barrie Turner, chief executive of the Suffolk Sheep Society, added: “The Suffolk Sheep Society is in a good place to engage with this work. Since 2018 we have been DNA sampling sheep for sire verification purposes.
“The council and members have had the foresight to anticipate the potential benefits of having this data available. They quickly moved to a high density 50K SNP test at an extra cost to maximise the number of individual SNPs we have of file. We have 3700 + profiles in the bank and are now, as technology moves forward well placed and ahead of the game to screen the data and look for any benefits the science can offer.
“Our initial investigations with the Suffolk profiles we have on file indicate that we have a high presence of the MV resistant gene with it present in more than 90% of the samples we have which is a great starting point in managing the breeding programme to promote sheep who have a genetic resistance to MV.
“The work being done by Rachael, Moredun and Fiona Lovett on this is in the early stages but if the concept is proven and taken forward could prove vital in the control of MV in the UK sheep flock.”
Sandy Smith, president of the Blackface Sheep Breeders’ Association, said: “The Blackface Sheep Breeders’ Association is pleased to learn that a grant has been awarded to a project seeking to develop genetic resistance to MV.
“To eliminate any disease from the sheep population is very important. The Blackface breed is not prone to MV as over the years thousands of ewes have been tested to go to Northern Ireland and there never has been a positive case, so we look forward to the project proving the genetic resistance that we suspect the breed has.”
John Yates, chief executive of the Texel Sheep Society, went on: “The society is interested to see how this research develops when it works extensively with a variety of research organisations globally and is liaising with Moredun to consider how the Society can assist this study.
“As the only sheep breed society in the UK currently using genomic evaluations and already heavily invested in genetic testings, if the studies prove useful any future test for MV susceptibility could, in all probability, easily be added to the Society’s current genomic testing regime.
“However, as with all single trait or single marker genetic selection, care must be taken not to chase certain traits at the expense of wider, more commercially important traits. SAC’s current MV Accreditation Scheme, while not ideal, has helped protect Texel breeders for many years.”
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here