Science Tackles A Different Pandemic in Our Valley

I’m driving over the washboard road toward the Whiskey Mountain Trailhead, when my hiking partner gasps. I brake.

“Look over there!” she says.

It’s a rare and very special sight: A group of bighorn sheep, up close. We count 26.

“Are those lambs?” she asks, spotting a few small ones. Maybe last year’s, I respond. The new lambs won’t be born for a few weeks yet.

I resist the urge to add what I learned recently, at the Annual Meeting of the National Bighorn Sheep Center in Dubois.

So far, from what Rachel Smiley and Brittney Wagler said at the meeting, I assume that none of these sheep that gaze at us in the spring sun are infected. With luck, they never will be.

Smiley and Wagner, both graduate students at the University of Wyoming, are somewhere high in the mountains right now, working to capture female bighorns which will be tested and then released. After a long, slow process, they have amassed enough data points to draw some conclusions about what should be done.

As most locals know, these sheep are just a remnant of the core native herd of bighorns in the greater Yellowstone region. Before 1990, the Whiskey Mountain herd was so large that almost 2000 wild sheep were transferred out of here to populate other parts of the Mountain West. That stopped abruptly after the herd suffered a die-off in the 1990s. It has never recovered.

Scientists and interested local volunteers have spent decades trying to understand the problem. Only in the last few years, with the same DNA technology used to test for COVID, has research identified the root cause: the bacterium Mycoplasma ovipneumoniae. (Researchers shorthand this to “Movi”.)

 It  affects sheep worldwide, both domestic and wild, causing a form of pneumonia. Movi is easily transmitted and (especially in this area, for some reason) often fatal in bighorns.

The parallel to COVID isn’t lost on researchers, who observe that sheep don’t wear masks, have a habit of sniffing each other’s noses, and behave with the opposite of social distancing. There is no vaccine against Movi.

There’s another poignant parallel for the Sheep Center. Its Director (who is also, like Wagler and Smiley, young and very fit) nearly died last year from pneumonia due to a form of plague that she caught from her sick cat.

Like so many other pathogens, Movi came to America with the Europeans – specifically, with their sheep. It jumped easily to wild sheep. There aren’t any domestic sheep near here these days, but Movi has persisted. It isn’t the only respiratory pathogen now endemic among local bighorns, which may harbor any or all of 5 different varieties. It’s just the most deadly.

Since 2015, a multi-agency project begun in coordination with the Sheep Center in Dubois has been pursuing a coordinated effort to come up with solutions. As part of it, Smiley and Wagler capture ewes in December and March and test them for Movi as well as for their physical condition, having fitted each one with a radio collar and a vaginal implant transmitter (VIT) that signals when they give birth.

Since 2020, they have also been capturing and collaring newborn lambs. Much of the time, off-schedule, they go back in when sheep (or sometimes just their collars) have died. Every reading and result becomes a point in a larger image.

“Every one of those data points has a ton of effort that has gone into it,” said project advisor Kevin Monteith PhD at the Sheep Center Annual Meeting. But he didn’t expand on that. I was able to learn more from a long interview with Smiley and Wagler posted on the Artemis Podcast in 2020.

This was just after they had begun to radio-collar lambs, which in their rugged high-mountain habitat is a matter of tactics, stealth, endurance, and extreme physical agility. Smiley, a recreational rock climber, often relies on those skills.

“Sometimes it seems crazy that they choose to do what they do,” she said. “A cliff edge is their favorite place in the world.”

When a VIT signals a new birth, they have to race to wherever the GPS reads, because both sheep may easily move away. Some ewes favor giving birth near Down’s Lake, at 12,000 feet in the back and beyond of the Wind River range. “We go up on the old Glacier trail,” said Wagler, “steep, steep, steep switchbacks and then across Goat Flat, which is like 4 miles of rock-hopping. By the end of the summer, we’re all pretty just drained of doing this hike over and over again.”

Other ewes seem to want to give birth just after a snowstorm rolls through, so Smiley always waits for VIT signals after a spring blizzard. Occasionally, she has gone out on skis to find a newborn lamb.

Getting there is only part of the fun. Usually a spotter joins two people going out for the capture, all in contact by radio. They need to creep up as close as possible to the ewe and her newborn, then race in for the capture the instant the sheep spot the humans.

Lambs only a few hours old are quite mobile, and can easily dash out of sight in seconds or leap a 10-foot cliff, climbing and scrambling over rocks and steep snowbanks that no sensible human would attempt.

The researchers also have to trek out to retrieve carcasses, hoping to learn the cause of death. Sometimes the remains are too far gone to accomplish this. And sometimes they wonder what’s actually watching them.

Once, Smiley went out alone to pick up a collar only a few miles from the highway up a ravine. She found herself staring up into the steady gaze of the mountain lion that must have just swallowed the collar, along with the unlucky lamb.  (She backed away, very slowly.)

All the data points gathered in this challenging effort have documented what the Sheep Center already knew: Nowhere near enough lambs are surviving to guarantee the Whiskey herd a future. Around half to three-fourths of lambs born to the Gros Ventre herd near Jackson survive to adulthood. Last year was a banner year for Dubois: Two lambs actually made it through to winter.

The first few weeks of life are perilous for any bighorn lamb. Although predators do kill some lambs, including some that are healthy, most lambs born to the Whiskey herd die of pneumonia, the team has found.

The problem is most grave in the Red Creek sub-herd, the bighorns that can be seen sometimes grazing dangerously near the highway beside the huge red-rock formations at the eastern edge of the Wind River Reservation.

One member of the herd, Sheep 108, was caught for the first time in March 2019 and twice since then. Each time, she has tested positive for Movi. She has never recovered.

(However, escape is possible: Sheep #1, also a member of the Red Creek sub-herd, has been tested 9 times and never tested positive. Many sheep in herds elsewhere catch the bacterium and do recover. What makes the difference? The team would like to know, and hope to learn.)

Sheep #6 was first caught in March 2015. She tested positive for Movi a year later, and then began to harbor the other 4 pathogens. Although she never appeared sick, she tested positive ever after, and died last December. At necropsy, her lungs were full of tissue killed by pneumonia. She also had a sinus tumor. These tumors, invisible to human observers in the live animal, are known to harbor bacteria.

Among the 10 lambs born to ewes that chronically tested positive for Movi among the Red Creek herd, only one survived to the next winter–and it had been born to a ewe that first tested positive only afterwards. The chronically infected ewes are not contributing to the herd. Instead, they are endangering it.

Obviously, it is not only the lambs born to infected mothers that are at risk of catching Movi. “Essentially any lamb is susceptible to dying of pneumonia if there is Movi in that subgroup,” said Daryl Lutz, wildlife management coordinator for Wyoming Game & Fish, at the Sheep Center meeting.

Over the Zoom screen, Lutz and Monteith looked troubled at times. Beginning his remarks, Monteith signaled the unhappy conclusion that was ahead. “When we engage in working to address difficult questions like this, we often hope to find a silver bullet, and oftentimes things are complex enough that maybe there’s not a silver bullet. I think this is one of those instances.”

After Smiley and Wagner painstakingly laid out the results of their research, Lutz had the more difficult task of explaining the consequences.

The collaborative team had already sought and won approval for its strategy from the Intertribal Council, the elected governing body on the Wind River Indian Reservation, which is part of the herd’s habitat. The jargon they use for the strategy is “test and remove.” It has shown some success in infected herds in Idaho, South Dakota, and British Columbia. (The Sheep Center is hosting a webcast about test and remove on April 9.)

“Remove” means to kill the chronically infected “super-spreaders” – in this case, three who had never produced a viable lamb since becoming chronically infected with Movi — because there’s no good option for actually relocating a very sick bighorn. (Nature has already removed the fourth, Sheep #6.) The team intends to examine the ewes that have been culled, anticipating they will find more sinus tumors.

I think a more appropriate euphemism would be the one we used back when I was doing cancer research on laboratory mice:  Sacrifice. That research provided no advantage for the rest of the mice, but in this case the ewes are indeed being killed for the potential benefit of the rest of the herd.

The decision entailed “a lot of thought, a lot of discourse, and also angst on my part and I’m sure others felt it too,” Lutz said, “because what we’re talking about doing is a pretty aggressive management tool.”

Twice he stressed that the decision was not “cavalier,” adding that he hates killing any animal unnecessarily. “But I do think we’re at a point where this is the best thing to do.”

Someone asked whether it might be possible to treat the sick animals and isolate them instead. But there are no effective antibiotics or good place to keep bighorns in captivity, Monteith said. There is simply no time left to take any chances with this herd, Lutz added.

There’s no guarantee that the strategy will rescue the sub-herd at Red Creek, in part because the herd there is already so small. But it may help to protect others nearby, like those we saw last week.

Many questions remain.

Although Movi doesn’t survive outside a living body the way Coronavirus can, the outdoor environment may still play a role. Oddly, bighorns in the Absaroka range just across our valley test positive for the same number and kind of bacteria as the ones on the Whiskey Mountain side. But that herd is thriving, numbering around 1000, while the Dubois herd has been dying off for decades.

One reason might be nutrition. But then why did the Whiskey Mountain herd thrive before the 1990s, and not after?

Smiley and Wagler have found that ewes in the Whiskey herd don’t gain as much fat during the summer as bighorn ewes on the Gros Ventre side near Jackson. They’ve been systematically gathering samples of forage in both locations.

Their initial findings won’t surprise hikers in the Winds and the Tetons: Plant life is much less abundant in the mountains around Dubois than in the greener terrain over the Pass.

It could be that the lambs here are more susceptible simply because they (and the pregnant ewes) aren’t fed well enough. What to do about that, if it’s true, is yet another question to address.

Often, Wagner said during the podcast, people told them that what Montieth had assigned them to do was impossible.

“He just said we’re going to get it done. And yeah, we’re getting it done. It is possible.”

© Lois Wingerson, 2022

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