I haven’t written about Ovine Progressive Pneumonia Virus (OPPV) in a while, so it’s time to catch up. I have continued to test my flock for it since 2011. I had been considering it before then, but was deterred by the cost. A buyer had requested testing a group of ewes at their expense, so I figured while the vet was out, I might as well test my ewes too. At the same time, I learned how to do the blood draws myself, and how to send the samples to the WSU lab, for the future. My ewes tested clear, so I figured, hey, I might as well roll with this, continue to test, and use it as an advertising strength. Though I believed in the seriousness of OPPV, the concept was still a little abstract to me, I didn’t have much personal knowledge of it.

That fall, I traded rams with a friend, bringing home a big yearling ram for breeding. Shortly after I brought the ram home, he got this weird ballooned-up knee joint. I had the vet test it, it had gram-positive bacteria in it. So we treated him with Penicillin, it got better. It was puzzling what might have caused it, with no apparent wound; but I chalked it up to an injury sustained while we pulled him out of a drainage ditch that he had fallen into (a newbie mistake every sheep seems to make here).

The next fall, my friend let me know that she had detected OPPV in her flock. For her, it was really bad. It was the culmination of several years of worsening struggles with weak and dead lambs, tons of orphan-rears, difficulties keeping the orphans alive, ewes with milk and udder problems, and more found-dead ewes in the pasture than one would ever think normal. But, she said for a while, she did think it was normal; that is, until she didn’t. This is because we don’t often talk about losses with others in detail, so it’s hard to gauge whether one’s personal loss rate is normal, or not. And a phenomenon like OPPV creeps up slowly, so you adjust to the abnormalities; until one day you just realize, wait a minute, this cannot be what everyone else is experiencing, this is crazy.

She sought vet help; but as you may know, we don’t have cadres of vets here with decades of sheep experience. They tinkered with some theories and treatments. They thought it might be bacterial infection brought in from flood waters. That’s another aspect of OPPV, is it mimics so many other diseases, you can chase several red herrings before you detect it. But finally, they were led to test for it, and found significant infection. She never told me a number, other than, it was a lot. It’s probably hard to say the number out loud. She reported that looking back through her records, all of a sudden it was acutely obvious what had been mystifying before: all the bottle lambs, troubles and losses followed the lines of those infected ewes. She did a massive cull. And, she let some people know.

The letting people know part is sticky. This is a big flock. My friend has bought sheep from all sorts of local and distant breeders, and sold sheep to lots of people. She has a lot of farm visitors. She doesn’t know how and when OPPV came into her flock. By the rate of infection, which is slow to spread, we can guess that she’d  had it for a long while, maybe many years. We can probably conclude that it came from a local peer, and also that it went to lots of other local peers. How far back into history do you notify buyers that their purchases may have had potential exposure?

It’s really not practical, or even sensible, to track down everyone from the past, and alarm them. If they weren’t already concerned about OPPV and testing for it, especially testing incoming animals; why bother with it now? They may have been exposed from her flock, but also any other non-testing flock from which they purchased. A tiny part of me wanted to be mad at my friend for exposing my flock; but I realized how fruitless is this line of emotional thinking. Because you really have to get mad at the person who unknowingly exposed her, and the person before that, etc.; and who even knows who these people are? Nobody passed it on purpose, it is what it is. We really just need to be mad at the disease and work to eradicate it. Those of us who care about keeping this  disease out need to test, test, test; especially incoming sheep. Even incoming sheep from reportedly tested-clear herds (which I now do). So, it really comes down to personal responsibility; not passing the blame upstream.

This is when the concepts went from abstract to personal to me: realizing that this virus is probably common at many local farms; and many of those people just don’t yet know they have it. And that it is a big, fat mess once it has spread to large portion of your ewes. (I should point out that there are those in the industry who still maintain that OPPV is a don’t-care; but that opinion is now pretty clearly in the minority.)

Learning of my friend’s troubles, my mind immediately went to that ram, and his formerly swollen knee (joint problems are one of the many clues of OPPV). I tested him, and sure enough, he was positive. I had missed it before because I was only testing my ewes. At the time, less info was available on OPPV, and I had falsely believed it is uncommonly, or never, spread by rams. At the time, general thinking seemed to be that it was mostly transmitted “vertically”= from ewe to lamb; not “horizontally” = between peers. Now, we know better; that it does spread horizontally, it just spreads very slowly. It doesn’t survive in the environment long, so it mostly relies on things like nose-to-nose contact at a feeder in order to propagate.

What to do, what to do…  I really needed to breed that ram, there wasn’t time to find another. Still operating on the theory that it’s unlikely to spread from rams to ewes, I decided to risk it. I kept him ‘til spring, then re-tested all my adults, including the rams this time. I found one more infected ram, and two positive ewes. So, in a year and a half sharing the same pasture, it appeared he’d managed to spread it to three other sheep (out of, I dunno, fifty-some adults that came and went in that time?). That’s not terrible, and shows how OPPV is an inefficient spreader, especially when the sheep are not in close quarters.

I did one re-test before I culled those sheep, which I had separated from the flock, and curiously, one of the ewes re-tested negative. Huh. I spared her. Also curiously, a lamb of one of the infected ewes tested negative, despite still nursing on her dam (normally, we’d expect to see the lamb carrying the passed-through-milk maternal antibodies from the dam). I DNA tested that lamb, and she did show she has the “less susceptible” gene. That lamb’s twin had been a weak lamb that died, so I’ll never know whether it was OPPV that caused it, but it’s certainly suspicious.

So, with that culling, I assumed this had been a near-miss for me, and I was grateful. I re-tested that fall, guessing that I’d be totally clear; but preparing myself for the possibility that maybe one or two had slipped through. Because, when you test, if an animal had just picked up the virus in the last few weeks, they may not yet test positive. So, eradicating the disease can take a few rounds of test-and-cull, before you catch any stragglers. But this didn’t seem likely for me, since my incidence was already so small.

Imagine my shock when EIGHTEEN of my mature ewes came back with positive tests that fall- eighteen out of forty-one sheep! Nearly half my herd! I just about had a coronary. Many of them were my best ewes, in the prime of health. No symptoms. What could possibly be going on? Slow spreading for a year and a half, I get rid of the spreaders, and now all of a sudden half my sheep have it? I called the lab. The doc on the phone was vague, but suggested I re-test. So, I did, a month later, and I was down to thirteen positive ewes: five “converted” to negative. Ok, hold on. Something was clearly not right here.

According to the test result summary sent to me in 2012, the test that the WSU lab uses (the cELISA test) claims a sensitivity of 95%.That means, about 5% of an infected population tested will receive false negative results, meaning the test isn’t sensitive enough to always detect the virus when it is present. (And this totally makes sense, because we know that if a sheep has just been exposed, she may have the virus, but not yet be presenting enough immune reaction to come up positive in a test result.) The test has a specificity of 98.4%. This means that only 1.6% of an uninfected population tested will receive false positive results, where the test was not able to specify whether or not the disease is truly present.

So, we can see right away, all this doesn’t add up mathematically. Those five ewes were 12% of the total population tested. They were 27% of the previously tested-positive population; and 22% of the previously tested-negative population. How could all five be either false-negative or false-positive, given the sensitivity and specificity claims of the test? That implies a sensitivity and specificity more like in the 70-percent range; which isn’t very good. And, again, how could so many of my ewes have suddenly picked up this disease when it spread so slowly before, and is known to spread slowly? Thirteen ewes still positive also seemed unlikely. I could make no sense of it.

I called the lab again to ask about this. I still didn’t get a very clear explanation. There was even a subtle insinuation that maybe all I cared about was getting a negative test result, not actually knowing whether or not my sheep had the virus. To which I took offense, because the whole reason I was testing was to know; but my trust in the test was eroding fast, based on the math. And it’s heartbreaking to think one might be making culling decisions based on a test with poor accuracy. It was mentioned that maybe I should try the test used by the University of Minnesota lab, with no clear reason about why. So, that was mentally logged.

I took a pause to think about what to do. I re-evaluated the DNA testing, wondering if maybe it was a better choice to try to breed my way out of this box, rather than cull. But there is no ideal promise there either, since the OPPV gene testing is more about “less and more susceptible” –not immune, resistant, or invulnerable. I contacted Judy Lewman, the president of the OPP Concerned Breeders Society to ask for advice. Judy is extremely helpful, knowledgeable, comforting, calm, and level-headed. We talked about a lot of avenues, the whole history of my experience and testing, this and that, what to do, and the confusing test results. At this point, Judy asked, did you happen to vaccinate for Chlamydia sometime recently? I looked in my records and realized, why yes, eight days before that blood test.

Bingo. It turns out, there had been some suspicion for some time that the Chlamydia vaccine cross-reacts with the cELISA test. Creating false positives. Judy encouraged me to send samples to the U of Mn lab, where they use a different test (the Elitest), one that is believed to have better specificity. So, more blood draws. By this time, I had 35 ewes still here, some had been sold or butchered during this short period of time where I was figuring all this out. Only one came back positive: a little ewelamb that hadn’t previously been tested. I’ll be damned. For curiosity’s (and science’s) sake, the lab re-ran those same samples using the cELISA test. They re-tested 11 samples: focusing on the population of 13 positives I’d had from the previous cELISA test, minus one I’d sold, and minus the one that tested positive with the Elitest. All negative! Same blood tubes, different tests.

Going back the earlier-mentioned ewe that escaped culling from going negative->positive->negative. At the time she tested positive, she was an immune disaster: recovering from fighting a molar abscess, pneumonia, and mastitis. So, this may hint that the cELISA test can cross-react with other immune system events as well, not just the Chlamydia vaccine. That ewe tested negative on multiple subsequent tests, and was spared to be bred another year.

I tracked down three ewes I’d sold during this period of confusion, went and drew blood from them, and tested those with the Elitest. They were negative. I re-tested the one positive ewelamb again, and she was still positive. She was a tiny little thing, but healthy. And pregnant. Smile with tongue out I felt weird about butchering a pregnant ewe, so I kept her in the barn with some butcher wethers while I thought about it. Meanwhile someone contacted me looking for some pet sheep, with no plans to breed. Pet buyers often enjoy the chance to save sheep from the butcher channel, so I threw it out there as an option. I wound up selling a pet ewe and the OPPV ewe’s castrated lamb to her for pet/butcher prices, and threw in the OPPV ewe for free, with full disclosure. My hunch is that OPPV is not as much of a concern for pet sheep which experience no stress; and of course, hard bag isn’t a concern for a ewe that will never be bred. Potentially that ewe could have her life shortened by arthritis or pneumonia; but there is also a good likelihood she’ll live a completely normal, long life. The other mystery of OPPV is some sheep that have it never become symptomatic. And this is probably especially true for those that aren’t under the stress of production.

So, in theory, the last of the infected sheep left my farm spring of 2014, and I tested all fifty ewes here in May and they were all negative. I tested all purchased sheep coming in last summer. So I believe I’m in the clear again, though I will continue to test to confirm that. What a freaky circus it’s been figuring all this out. I can’t help but reflect on my friend, and also another local acquaintance I’ve spoken with, who did big culls based on the cELISA test; and hoping that it didn’t mislead them on their rate of infection, and cause them to cull some ewes which were healthy.

Subsequently, researchers associated with the U of Mn published a poster on this topic. And guess whose data is a little bit famous now? Of course, they treated my data anonymously, but I’m OK with it being open: Flock “C” is me! You can see in the table how I had sixteen ewes test negative, then positive right after being vaccinated. Then, there is a “falloff” of subsequent positive cELISA test results as they slowly return to negative, as their immune systems presumably ramp-down on the response to the vaccine. And, meanwhile, the two Elitest samples convey that all those ewes are actually negative all along.

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So, some pretty cool science, and I’m glad to have had the accidental chance to contribute some data to furthering this learning and theory. Not to mention, greatly relieved that I didn’t experience some of the more catastrophic impact of this disease that some of my peers have!

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