Maedi Visna – the hidden disease

The Sheep Farmers of Ontario (OSF) is encouraging sheep producers to consider taking steps to mitigate a silent, hidden illness that may be having a greater impact on their bottom line than they realize.

Maedi Visna (MV), also known as ovine progressive pneumonia, is a chronic viral illness that is 100 per cent fatal. The most common symptoms are pneumonia and hard udders (the virus affects primarily lung and mammary tissue), reduced milk production and weight loss. Less commonly seen is arthritis or neurological signs.

Why it matters: Maedi visna is difficult to detect in sheep until they are older so biosecurity plays an important part in mitigating the disease when new animals are brought into a flock.

MV is difficult to detect because symptoms don’t usually appear for one to three years. That’s why OSF refers to it as an “iceberg” disease; while only a few sheep in the flock may appear affected at any one time, many more are infected with the virus and will eventually become ill themselves. The effects of the virus in tissues accumulates over time, and when symptoms are obvious the organ damage is already severe.

Successful control includes a combination of monitoring and biosecurity.

Kim Schneider, who has a commercial flock of 100 ewes of mostly purebred Dorset with some F1 Rideau cross, said she “dragged her feet” to enrol in a Maedi visna monitoring program, but it was her veterinarian that convinced her that it would be beneficial. She raises lambs for meat and for breeding stock on her farm near Sunderland.

Working with her veterinarian, she has made a few modifications to her sheep housing to eliminate nose-to-nose contact and provide areas for quarantining animals that are new to the flock.

According to the OSF, 30 to 40 per cent of sheep can be infected (seropositive) in a typical flock with uncontrolled MV. The virus is quite contagious and is spread via nose to nose contact, saliva, and through respiratory secretions. When sheep are housed indoors, the OSF estimates the prevalence of infection can reach more than 50 per cent.

The OSF has a voluntary program to help sheep producers determine the status of MV infection in their flocks, called the Ontario Maedi-Visna Flock Status Program (OMVFSP).

OSF Executive Director Jennifer MacTavish says while the organization has seen an increase in enrollment in the program by its dairy producers, there is less uptake from producers raising meat animals.

“Dairy producers realize the benefit in terms of milk production,” she said, “but meat producers may not see the benefits right away.”

“MV can be like a slow burn under the radar, and some producers may just choose to live with it.”

But the OSF hopes that producers realize that the illness results in economic losses, even if they’re not obvious.

An OSF research report on MV says a seropositive ewe not only produces less milk, but she’s more likely to have fertility issues and that two-thirds of MV seropositive ewes that appear healthy had udder lesions, and their lambs had lower than average weaning weights.

Infected ewes are often culled due to poor productivity when they should be the most productive (between three and five years old) as the illness starts to manifest.

Dr. Rex Crawford, owner of Dufferin Veterinary Services, has worked with the OSF on its MV program. He says the challenge of MV is that it’s a hidden disease and that the decrease in milk production is “probably the most important economically. The more prolific your flock, the more important that milk production is.”

“There’s a big risk to producers that are trying to leave triplets on mothers,” he says, as ewes with MV will not produce enough milk to feed her lambs and producers will have to use milk replacer, which can be costly.

“The production side of things is very real, but hard to see sometimes.”

The surveillance project was initiated several years ago in conjunction with the Animal Health Laboratory at the University of Guelph. Producers who enrol are required to follow the rules of the program, which requires serological testing of blood samples from each animal in the flock, removal of positive animals from the flock, and following a specific set of biosecurity requirements.

The program was developed to help sheep producers manage the risk of bringing MV into their flock when purchasing new stock. The OSF encourages producers to bring in new animals from flocks that have superior genetics and health status, “but the onus is really on the buyer,” says MacTavish.

Two flock-monitoring options

The OMVFSP offers a whole flock program, and a monitored flock program. For the whole flock program, the program tests all animals in a flock 180 days and older. If all sheep test negative, the flock status is ‘enrolled-negative’, and to qualify for a ‘B’ status flock animals are tested 180-395 days after the initial test. If this second test is negative, the ‘B’ status is given. To qualify for an ‘A’ status, a third qualifying test of a random sample of sheep 365 days or older must also be negative. This third test can be completed 180-395 days after achieving ‘B’ status.

If the initial flock tests positive, an ‘enrolled’ status is given and all positive sheep must be removed, and lambs less than 180 days old must be isolated until they reach market weight. A follow-up test is required in 90 to 180 days.

The monitored program does not require the entire flock to be tested, but a random sample of animals one year and older. The test is repeated yearly.

Producers can enrol in either program, depending upon what their goals are – to provide greater assurance to buyers of breeding stock, or to monitor MV if they are experiencing productivity issues (or both).

The barrier for many producers wanting to join the program is the cost of the blood test, says MacTavish, which is a little more than $11 an animal. There has also been some concern amongst producers over the accuracy of some of the serological tests that can be used.

“There is no gold standard,” says Crawford, saying some of the tests are not as sensitive as they would like them to be.

He was part of a research study with the University of Guelph and the AHL on test sensitivities, which showed that the ELISA tests are most sensitive, particularly the Elitest ELISA. But he emphasizes that a negative test result doesn’t necessarily mean an animal is free of MV.

“Being negative doesn’t mean for sure that the virus isn’t there, but positive means it’s there for sure.”

The virus is tricky, he says, and the reasoning for the protocol set up in the OSFMVP is that it can take a long time for the virus to produce enough antibodies to be detected.

Biosecurity plays a big role

Also required in the program are biosecurity protocols for producers to help ensure they minimize the risk of bringing MV into their flock. The importance of biosecurity is underrated, says MacTavish.

Modifications don’t have to be complicated. Schneider got an old calf hutch from a dairy farming neighbour and used some gates she already had to create an outdoor pen to quarantine lambs. To house animals new to the flock, which should be kept where sheep haven’t been kept before, she uses an empty horse stall at another neighbour’s property.

“It can be a little bit of a puzzle, but if you’re committed it’s possible,” she says.

She encourages other producers to be cautious of where they buy from, and to quarantine new animals until the second test is passed. “Quarantine is essential,” she says.

She also only uses needles once, and has a pair of booties that stay in her truck that she can wear to the sales yard, so she doesn’t bring anything home on her boots. As a committed 4H leader, she also houses sheep for showing separately.

“A lot of it is just common sense.”

Crawford’s takeaway for producers is that MV is a disease of concern and a hard one to see.

“It’s far better to start from free stock than it is to figure out what to do once you’ve got 40, 50 or 80 per cent of the flock infected.”