The Costs of Maedi-Visna for Sheep Farms
The Plague of the Sheep Part 2 - Economics and Options
This week I will visit the economic costs and options for risk reduction available to sheep producers when it comes to Maedi-Visna. Part 1 focused on the epidemiology of Maedi-Visna and Small Ruminant Lentiviruses. Maedi-Visna (MV) is common enough that it should be of great concern to all sheep producers (and goats, CAE is the same). If you missed part 1, please read it here. It takes only one poorly planned purchase of a ram or some replacement ewe lambs to bring MV into your flock. Research has shown that mixing infected sheep with healthy sheep will result in 50% of the negative ewes seroconverting within 9 months. By the time you notice, it will be a problem and the costs are huge.
How It Happens
The latent period of MV and the length of time it can take until a sheep shows clinical signs means that MV can spread undetected. I’ll illustrate the best that I can wherever possible with a fictional established 250-ewe flock. This fictional flock got infected with MV when they bought in extra replacements because they wanted to grow to 250 ewes. This flock is a typical eastern Canadian flock which is lambed and housed indoors for at least the winter, they manage about 1.3 lambings per ewe per year. They are not closed and buy replacement rams and occasionally ewe lambs from a couple of breeders. They like to keep back approximately 50 ewe lambs each year, and usually have 1.5 lambs per ewe ready for market per lambing (538 total lambs per year, 1.66 per ewe overall). It took a couple of years to notice that they’re having more mastitis and sometimes ewes don’t respond to medication for pneumonia. They don’t have as many older ewes but were attributing this up to newer better genetics. This is the scenario flock that will appear throughout the article.
Economic Costs
The economic cost of Maedi-Visna (MV) is huge and hidden due to its prevalence. One UK case study of a flock with 1,500 ewes estimated that productivity decreased by 20-40% and was costing £30,000 to £50,000 per year ($49,000 to $82,000 CAD). Another study found that MV cost flocks €50 per ewe per year ($74 CAD). The main costs are premature culling of ewes and poor growth rates of lambs due to decreased milk production. Depending on the variant present, the increased mortality of both ewes and lambs would be contributing to the losses. Anecdonatly, MV comes in waves, everything will appear more or less fine for months or years until one day everything seems to go wrong. Infected flocks have higher rates of secondary infection.
Fewer Lambs Per Ewe Over her Lifetime
MV greatly impacts the longevity of sheep as they typically die or are culled before 5 years old and have less than 3 lambings. Greater longevity in sheep leads to an increased overall average age of the flock, more lambs available for sale, and higher reproductive performance1. Premature culling means that ewes are having fewer lambs over their lifetime. Healthy ewes in a properly managed flock are definitely productive until they are 9-10 years old.
As Australia and New Zealand don’t have MV, looking at their production records and practices is a good place to start. Their predominant culling criteria is the age of the sheep after production factors. They expect ewes to be productive to at least age 7 with a minimum of 5 lambings annually. Based on the production limitations discussed in part 1, we can estimate that MV reduces production by at least 2 lambings for an annual system. If you have an accelerated 3-in-2 system where ewes would generally be productive until 7 years old, the optimal number of lambings would have been 10 lambings. In that system, the loss would be 5 to 7 lambings. Seropositive ewes have a less than 5% rate of even achieving 5 lambings in their lifetime, most of them only lamb once or twice.
There are two costs to this issue, the lost revenue from lambs and the extra replacements. A ewe in an annual system would be expected to bring in $1,312.50 in revenue in her lifetime (5 lambings, 1.5 lambs, $175 per lamb). In an accelerated system, that would be $2,362.50 (9 lambings, 1.5 lambs, $175 per lamb). Within an MV-infected flock, assuming you do get the 3 lambings, that’s a loss of $525-$1,575 in revenue per ewe. That’s a drop of 66% in potential revenue for the investment in a ewe. It would be even higher for a more prolific breed.
If we expect an average of 5 lambings per ewe and once infected with MV, we’re at best achieving 3 lambings, the minimum number of replacement ewes required will be up 40% from expected. This means the farm either has to buy those replacement ewe lambs or keep them back. In the 250-ewe scenario flock, this would be an additional cost of $6,000 provided they can get the additional 20 ewe lambs for $300 each. This cost will likely be higher as the cull rate increases.
Higher Feed Resource Needs
The total feed requirements for the entire flock will be higher. Given that feed costs are the greatest expense for a sheep farm, this would have a real impact on the bottom line. Lambs from seropositive ewes need more creep or other feed to catch up. The ewes that are thin will be fed more in an attempt to maintain their body condition. Given that accelerated flocks are at high risk for MV and overall have high feed requirements, these costs can escalate quickly.
Flocks with MV always have more bottle lambs as well as a greater need for powdered colostrum. A South Dakota case study had 21% of their lambs on milk replacer compared to 2% after getting tested negative status2. A detailed study of a flock in Scotland3 found their bottle lamb rate went from 4-6% before MV to 13% with MV. These ewes were only indoors for two months of the year. The same case study also found that they needed:
33% more feed for the ewes
129% more milk replacer
12 times more creep feed (47 tonnes compared to 4 tonnes in previous years)
Seropositive ewes wean lighter lambs due to reduced milk production. Feeding lighter lambs that have lower ADG (average daily gain) or lighter weaning weights will cost more. Feed efficiency tends to average around 6kg of feed converted into 1kg of body weight gain for most breeds. Losing 5kg at weaning will take would then take 30kg of feed to recover. Even if they did make the weaning weight, they would consume extra creep feed.
For our example flock, if they had to feed an additional 10% of lambs as bottle lambs, this would cost an extra $4,300 per year (an extra 54 lambs on milk replacer when a 20kg bag is $115 and a lamb needs 14kg per the tag). If half of their flock was affected and thus weaned lighter lambs, that would be an extra 8 tonnes of feed. As this is either creep or starter feed, it’s likely costing around $800 per tonne, so that’s an additional cost of $6,400 per year. This is a total cost of $10,700 extra just for the lambs per year or $20 extra per lamb produced.
More Vet Bills
Infected flocks will have higher rates of other diseases (secondary infections) as the sheep are not as healthy as they should be. For ewes, the most common problem is pregnancy toxemia and pasteurella pneumonia. Lamb and ewe mortality is also higher than average. Since MV symptoms are similar to other common illnesses, MV is usually not the first suspect on producers' minds. A lot of additional costs can be incurred while trying to solve issues within the flock. The Scottish cost study had extra veterinary costs of £3575 which worked out to an extra £4.20 per ewe ($6.92). If you have been battling a wide range of illnesses within the flock, MV should be on your list of potential causes.
Testing
A frequent barrier to resolving MV is the cost of the testing. By the time you see MV in a flock, it’s usually spread to 30-50% of the flock if not more. In a confined flock, the prevalence rate will double in less than 2 years. The easiest is to get ahead of this. Culling alone is not enough to stay ahead of MV as you will never identify all the infectious sheep without testing. The options are autopsies on cull ewes or blood tests. The test is a moment in time so it would have to be repeated to confirm continued negative status.
Testing costs approximately $12.50 per sample plus the veterinary costs to draw the sample. Quebec is currently the only province that covers a portion of the testing costs. While the risk of false positives is cited as a reason for producers to avoid testing, the rate is really low (see part 1). There are different levels of testing, usually, the starting point is to take a statistically significant random sample4 of ewes and test them. For 250 ewes this would be 53 ewes. Once these results are available, you can consider the options.
Total Cost
I would highly recommend reading the case study in footnote 3 for a thorough case study on the cost of MV. They had a loss of £132,000 over 5 years, as they opted to depopulate and restock. The year the virus was first detected, they had 58% positive in a random sample and it cost them an additional £19,963.
The costs come in multiple ways as detailed above.
Lost revenue of $525-$1,575 per ewe over her lifetime. This would matter especially if you are starting out or investing in top-quality genetics.
33% higher feed needs for ewes. Each farm would have a different cost but the effect would be the same as corn going from $350/tonne to $465/tonne.
$20 more per lamb in feed and milk replacer. The scenario flock would have cost $43 extra per ewe per year.
$7 more per ewe in veterinary costs.
The scenario flock would easily be spending an extra $12,500 annually on feed, milk replacer and vet costs. They would also have extra costs of $6,000 for replacement ewe lambs and likely rams as well, as their life expectancy is reduced.
Running a sample test on 53 ewes to determine the flock prevalence for the 250-ewe flock would cost $662.50. That’s less than the extra milk replacer. Testing will look like cheap prevention when half of the flock is infected. The real issue to what to do with the information once you have the random sample results.
Options
The MV situation in Canada leaves producers with two choices if they do not already know their flock’s MV status:
Alternative 1 - Ignore the issue and hope for the best
This option is a common choice. MV is terrifying but it also sneaks up. You could never test and hope it’s all okay. If your biosecurity protocols are strong (we’re talking hog farm level strong) and you have a flock that is permanently on pasture all year, you could likely ignore MV with minimal impact. If you bring your flock into yards or barns for even just two weeks, that’s enough for the virus to spread. It would probably take another two-three years before you noticed a serious issue at which point doing something about it is much harder.
Alternative 2 - Test
If what-ifs keep you awake at night, you can opt to do the random sample test. Quebec and Ontario do have formal programs but if that is too daunting, you can work with your vet to at least do the random sample testing so you can make informed decisions on the next steps.
Your Flock is Positive, Now What?
There are some hard decisions to make if that random sample test comes back positive. It is very important to note that seropositive sheep need to be sold for slaughter. You should never purchase sheep that are positive for MV.
Choice 1 - Continous testing and culling
There are a couple of different protocols for getting a low-risk MV flock. The main one from Ontario is outlined here.5 The entire flock over 6 months of age is tested and seropositive animals need to be removed in less than 30 days. The test-positive sheep can either be kept in a separate positive flock or sold for slaughter. They cannot be sold as breeding stock. The negative group will still have latent sheep in it, so they are retested four months later and any new positives are removed. This is repeated until there are two negative tests in a row. The exact specifics are far more detailed than this article allows but overall, this process takes a lot of time and perseverance.
Keeping the positive ewes requires very strict biosecurity protocols. A lapse in these protocols is often what goes wrong. There is the chance of recovering some additional revenue by keeping the positives for one or two more lambings but there is a risk that protocol is broken and the negative flock is re-exposed.
The costs of this program would include repeated testing, extra supplies and infrastructure for two flocks as well as lost revenue since the flock would decrease in size and more ewe lambs would need to be retained. The testing aspect would likely cost around $10,000 for the initial rounds of testing for the scenario flock.
Choice 2 - Depopulate and start over with tested negative stock
Alternatively, if the test sample rate of positives is really high, there is the option of starting over by selling the entire flock and buying tested negative ewes. The entire flock is sold for meat. The case study in footnote 3 took this route. They sold all 850 ewes and bought 450 new ewes from an accredited flock for approximately $250 each. This is much more difficult in Canada as finding large numbers of negative ewe lambs is a challenge. A typical purebred ready-to-breed ewe lamb from a negative MV status flock on Genovis will cost between $500 to $600. Most of these breeders have waitlists. If the scenario flock opted for this route, they would likely have a net restocking cost of $100,000 to bring in 250 new negative ewes considering current cull ewe prices.
In The Future
There is also a genetic component to solutions. Researchers have identified a gene, TMEM154 with a specific allele arrangement, that has shown sheep carrying it will have increased resistance to MV. Breeding for genetically resistant ewes and rams would be the way forward. The tests for this gene are commercially available but not common at this point like the genetic Scrapie tests. This would be an excellent option for pasture flocks.
Testing itself still needs further improvement and development to reduce the need for multiple repeat tests. There are PCR tests that will detect Maedi-Visna Virus but they are not cost-effective that this point. Many research studies rely on multiple tests to detect positive sheep during the latent period. This type of innovation will only happen if the entire industry makes a commitment to eradicate MV nationwide.
The increased costs of production as a result of MV are such that simply ignoring it might not be the best route to take. I will have more parts to this series at a later date covering biosecurity protocols and other issues. Thank you for taking the time to read this instalment!
Longevity in Australian sheep was the focus of this 2021 paper which investigated productivity records and culling age. Canadian statistics are harder to obtain but a seronegative closed accelerated flock owner with great data shared on average their ewes are productive until they are 7.7 years old.
They also had a bump of 45% in their lamb crop and their ewes stayed productive for twice as long. Overall, they reported significant improvements in their flock after removing MV.
This study is super detailed and worth reading. Gibson, Lynn & Dun, Kath & Baird, Andrew. (2018). The true cost of maedi visna virus (MVV) infection in a commercial flock. Livestock. 23. 238-243. 10.12968/live.2018.23.5.238.
Page 26 of the Ontario protocol has a table with all the suggested sample sizes.
Another great resource on testing and culling options is the OPP Society.