If you have been involved with sheep at all, chances are you have heard of Maedi-Visna (MV). It is a common and fatal viral disease. This article is the first in a series on the plague of the sheep industry. Unless your flock is tested regularly or closed, there’s a fair chance you have it on your farm. Prevalence estimates vary by province but last time it was studied in Canada, 63% of flocks has positive ewes within their flock. Several studies have looked at the risk factors that make it more likely that your flock is positive. These include:
Purchased more than 50 sheep in the past 5 years
Having a commercial flock
You house your sheep indoors or at high densities
Flocks with more than 70 adult ewes
Frequently observe ewes with musculoskeletal/lameness issues
The disease is known by a variety of names and was first identified as Graaf-Reinet disease in South Africa in 1915. In 1923 in the USA it was called Montana Sheep disease and now usually calls it ovine progressive pneumonia (OPP). In 1939 it was named Maedi-Visna in Iceland where it did significant damage to the national flock. It is the reason no sheep can be imported to Iceland. France named it La Bouhite and the Netherlands called it Zwoegersiekte. The Icelandic name is most dominant now internationally.
The first known cases in Canada were identified in 1972. It’s been plaguing flocks in most developed countries for the better part of a century with the exception of Australia and New Zealand where MV has never been detected. MV is a reportable and monitored disease with significant restrictions in Ireland, Norway and Finland. Canada has only voluntary control programs in Ontario and Quebec at this time.
What is Maedi-Visna?
Maedi-Visna Virus is a small ruminant lentivirus (SRLV) belonging to the same family of viruses as HIV and EIA1. SRLV is known as the Maedi-Visna Virus (MVV) in sheep and Caprine Arthritis and Encephalitis (CAEV) in goats. The virus itself has many variants which all result in Maedi-Visna (MV), a highly contiguous but slowly progressing fatal disease.
Infected sheep will look completely normal and fine for long periods of time while infecting other sheep in the flock. While there is some indication of carrier sheep who do not show obvious clinical symptoms, MV is ultimately fatal. There is no cure or treatment. There are four broad categories of clinical symptoms:
Respiratory - this is the most commonly observed form and appears in adult sheep over the age of 2. Once it reaches the clinical stage, sheep will not survive more than 12 months. Sheep with the respiratory variant of MV will struggle to catch their breath with symptoms similar to pneumonia (hence the alternative name of ovine progressive pneumonia). The lungs become enlarged and heavier with reduced air exchange capacity.
Mammary - this is the most obvious production-limiting form of MV. It generally shows up in adult sheep between the ages of 3 and 5. The lesions turn fibrotic and lead to subclinical chronic mastitis which lowers or eliminates milk production. It is not painful for the ewe unlike bacterial mastitis, and will typically affect both halves of the udder. The udder will appear swollen and full at the time of lambing but the milk output will lower. Sheep with this form tend to be culled rather than die of their symptoms.
Nervous - this form attacks the mobility of sheep as it is neurodegenerative. It’s considered to be less common in sheep but several studies in Spain have found frequent outbreaks of MV with high rates of nervous system clinical symptoms. It presents in a variety of ways, one of which is encephalitis where the virus forms lesions on the brain and spinal cord. It will affect sheep of all ages, some as young as 4 to 6 months. Clinical onset to death is much faster than other forms, some sheep will appear to suddenly die (typically paralyzed and peddling their legs). Symptoms include progressive hindlimb ataxia and paralysis, it could be mistaken for listeriosis. Ataxia is when the sheep lose function in their legs, appear lame and struggle to move.
Joint - atypical in sheep, this form tends to affect goats more often. Joints become thick and enlarged with arthritis, especially the knees. They will have issues with mobility and often graze or feed on their knees.
Infected sheep can show multiple groups of symptoms, a ewe can have respiratory issues and milk issues at the same time. Sheep with MV can struggle to maintain healthy weights and consistent body condition scores, progressively becoming thinner despite healthy appetites and adequate nutrition. However, a low body condition score by itself is not an accurate indicator of MV status. It has been noted that positive ewes in an accelerated lambing system will struggle to bounce back after weaning.
Breed and geographic location can impact which SRLV strain becomes the most dominant variant in a flock. Some breeds have been studied to be more susceptible to MV than others. There is evidence of genetic resistance to MV, however, this is still being studied and cannot yet be consistently applied at a commercial scale like genotyping for Scrapie resistance. Successful studies on sheep carrying the gene were published in 2022 and research is ongoing.
Production Impact
All measurements of productivity are lower for sheep that are seropositive for MV. A study on cull ewes in Alberta found that 27% of culls were positive which can be connected to producers wanting to remove unproductive ewes. I’ll get into more details in part 2 of this series but culling alone is not enough to escape MV.
Shorter lifespan - sheep that are positive will likely die before they are 5 years old. They will have far fewer lambings in their lifetime. A German study found that 50% of positive ewes only lamb once compared to 17% of negative ewes. Only 12% of positive ewes have 3 or more lambings in their lifetime. Comparatively, 33% of negative ewes will have 4 or more lambings over their lifetime. Overall, the amount of replacements needed in a positive flock is significantly higher. Positive flocks have been found to have few productive ewes over the age of 5 if any.
Higher mortality - Positive ewes have nearly double the lamb mortality rate for the first 30 days after lambing (7.5% compared to 4.7%). Studies on positive ewes have found higher rates of stillborn lambs and overall lower survival rates of lambs. Ewe mortality is also higher with some farms reaching levels as high as 30%.
Lower weights - Weaning weights for lambs raised by positive ewes are on average 5kg (11lbs) lighter than weaning weights from negative ewes of comparable genetics. Most recently, a large flock in the UK with an MV outbreak found that their average daily gain (ADG) dropped by 50%2. Feeding unlimited amounts of creep feed to very young lambs is one way to offset the weaning weight difference.
No milk - Ewes that are positive produce 14% less milk per lactation with a shorter number of milking days3. Numerous studies have found that the milk components are the same, just the overall volume is lower. This is concerning for sheep dairies and prolific flocks where high milk production is necessary. Anecdotal evidence shows that positive flocks have more bottle lambs.
All of these reductions and symptoms are more pronounced as a ewe gets older. There is also some evidence that positive ewes give birth to fewer lambs per lambing and have lower conception rates. Overall positive ewes cause lost productivity and excess resource use for a flock. While genetics and nutrition can have similar impacts, if there is a progressive decline in productivity as the flock is getting older and there are not many productive ewes over the age of 5, MV status should be investigated.
How Does It Spread?
The real issue with MVV is that the virus is spreading long before a sheep shows clinical signs. A sheep could be spreading the virus within the flock for over two years with little to no indication. Most of the transmission is horizontal, from one sheep to another compared to vertical where a ewe is passing it to her lamb. It is mostly transferred through aerosol droplets. While there is some transfer through the milk and colostrum from an infected ewe, this causes less than 16% of infections per more recent studies. Nose-to-nose contact, shared water sources and aerosol droplets are the primary sources of transmission.
Once the virus has entered the sheep’s body, there is a lag in how rapidly it seroconverts. Seroconversion is when the virus triggers antibodies that can then be detected by blood testing. Once a sheep is seropositive, it is likely capable of infecting other sheep. There is a latent period between the initial infection and when a sheep become infectious to other sheep. As the disease progresses, the sheep becomes more infectious based on the most recent study information available.
Sheep that are stressed or have poor body condition scores are far more likely to get to become infected. They will seroconvert within 1.5 months compared to 8 months for healthy sheep. Accelerated lambing would be a stress-related risk factor for ewes. In a study on rams, there was limited transmission despite long-term exposure.
A very detailed epidemiology study from the University of Edinburgh found that confining sheep for as little as 9 days is enough for MV to spread. The spread of MV is reduced on pasture but activities like gathering and high-density grazing are enough to spread the disease. In one case, gathering the sheep for one hour every other week to do testing was enough for multiple sheep to become infected and seroconvert. It will spread like a wildfire in an accelerated barn system.
How Do You Detect it?
The primary method of detecting an MV infection is through a blood test. Post-mortem exams for lesions are also a diagnostic mostly used on cull ewes to determine if a flock might have positives. The test done these days is an ELISA test. There are a variety of commercial ELISA tests available. Testing is continuously evolving. Most of the tests relied upon now are within 99.3% accuracy. There is a very small risk of false positives (less than 7 per 1000 tests). The test is only a moment in time, a negative test only means the sheep was negative at the time the blood was drawn. A flock with tested negative status in Canada simply means that the risk of MV is low, not non-existent.
Now What?
Overall, Maedi-Visna is a very concerning disease that is hard to detect before it has spread to significant portions of the flock. It is by definition a plague, one that has impacted many flocks. I will detail the economic costs of MV, testing and what options for risk reduction currently exist in part 2 of this article which will be released next week. All of the above was gathered from over a dozen peer-reviewed scientific journals, newspaper articles and conversations with other producers over the last four months. Many thanks to the people who have patiently listened to me and answered my questions!
Update - check link below for Part 2!
EIA is equine infectious anemia, a fatal reportable disease in horses. If a horse tests positive even as a carrier, they are required to be euthanized. Depending on the region, all competition horses have to be tested negative (Coggins test).
“Maedi visna outbreak leads to soul-destroying 4,000-ewe cull” is the headline from January 19th, 2023 in Farmers Weekly. It spread from the indoor flock in the fall of 2019 to become a huge problem for the whole farm by the spring of 2022. The farmer ended up having to cull the entire flock.
Risk factors and milk production have been studied in detail in this 2013 study.
I am trying to get my flock of around 30 ewes tested again. I did test many years ago (all negative) for several years, so closed the ewe flock. As long as the ewe is producing twin or better lambs, she stays on the farm. Got several ewes down in condition lately and checked their teeth. Missing all the front ones but are 13 years old. Only possible sign of MV is udder issues which I put down to cold conditions and aggressive lambs. They are not confined but use shelters at night or bad weather. So problem I have run into is the vets around here NW of Edmonton, don't seem much interested in doing the testing. Of course calving is probably keeping them busy now as well. Do we have testing facilities in Alberta? Do I have to get a vet to do it?
Thanks for the article. We are fighting to eliminate MV from our flock. Emotionally and financially it is draining. But I keep hoping for good news on the next blood test.