Maedi Visna: the overlooked virus killing sheep in the UK

04 February 2020

The prevalence of a virus that affects sheep has doubled in the UK over the past 10 years, researchers say. Dr Rachael Tarlinton, Associate Professor at the University of Nottingham, has been investigating ways to control the spread of Maedi Visna (MV), a virus that infects an estimated 100,000 sheep in the UK.

Originally introduced to the UK from imported sheep, Maedi Visna (MV) is a virus that causes lung disease and chronic wasting, causing the sheep to become emaciated.

MV is difficult to diagnose until symptoms appear, which can take several years after infection. To make matters worse, awareness of the virus is low in both farmers and vets. According to Dr Tarlinton, there is no formal surveillance of the virus in the UK, meaning the virus can spread untreated. She said, “We don’t monitor systematically for the virus and the disease is often not obvious until very late in the infection, meaning affected flocks are often not picked up.”

In an effort to develop an effective diagnostic test to help control the disease, Dr Tarlinton has been investigating ways to use nasal swabs to identify the virus. This may be a more effective way of monitoring the virus at a lower cost. She said, “One of the big costs for a farm in getting testing done is having to get a vet out to take blood samples, tests that the farmer can collect the samples themselves and send in make it much easier and cheaper for them to do (and hopefully more likely that they will take it up.)

“The cost of testing and disease control versus the extra output a farmer would get from eliminating the disease means that for sheep, the test has to be very cheap for commercial farmers to be able to use it and the current tests are only just cost effective.”

Named after the Icelandic for pneumonia and wasting, MV can be fatal in as many as 20% of adult sheep infected. In lambs, mortality is even higher, and infection can be transmitted from mother to lamb through milk. “The virus can spread quite quickly, particularly in dairy flocks if infected colostrum is fed to kids or lambs. The onset of disease is often very delayed as the virus can stay dormant for a long time and animals may not test positive on PCR or serology tests for three to six months post-infection, meaning a lot are missed on screening programmes” said Dr Tarlinton.

The effect of an MV outbreak in a flock can be devastating, and when not fatal, the virus can stunt growth in lambs, leading to further financial losses for the farmer. At present, there is no vaccine for MV, and as the currently available treatment options are not cost-effective; infected sheep are often culled instead. Dr Tarlinton said, “Control relies on testing whole flocks and removing infected animals, however as so many are missed on initial tests, repeated rounds of six-monthly testing are needed (sometimes for years) until the entire flock is free of disease. This is really expensive and can be very hard for farmers to have to cull animals they have cared for. The extreme variability of the virus has meant that developing a vaccine hasn’t been feasible so testing and removing animals is the only real control method.”

MV is caused by Visna-Maedi virus, a virus in the lentivirus family which spreads when sheep are in close contact through respiratory secretions. In December 2018, there were an estimated 22.5 million sheep in the UK, where MV outbreaks have been seen in dairy, meat and wool flocks. Dr Tarlinton and her colleagues have also been assessing other ways the virus could be transmitted: “We’ve been looking at whether sexual transmission (from rams to ewes) plays a role in spreading and maintaining the disease in a flock. This has involved an artificial insemination trial of semen from naturally infected rams to ewes and so far, shows that the risk of sexual transmission is likely very low.”

In the future, Dr Tarlinton hopes to understand more about the strains of the virus circulating in the UK and assess how successful control measures are at eliminating the virus from an infected flock. She also hopes to find out whether some breeds of sheep are more susceptible or resistant to the virus due to their genetics.

Dr Tarlinton will present her research as part of the Starve the livestock pathogen: feed the world session at the Microbiology Society’s Annual Conference in Edinburgh this year. Her poster, titled ‘Failing to control Maedi-Visna’ will be available to view on Thursday 2 April and Friday 3 April, with presentations taking place on Thursday evening between 18:30–20:00.

Image: Rachael Tarlinton.