It was December when the first reports started coming in: All across the frozen Mongolian steppe, saiga were dying.

The antelope species, with its tawny coat, ringed horns and incongruous oversize snout, has roamed the world's chilly northern grasslands since the Pleistocene. But the International Union for Conservation of Nature now deems it critically endangered. And in the past two months, the rare Mongolian subspecies Saiga tartica mongolica has been decimated by a deadly virus.

You can literally look out across a plain and see sick individuals and carcasses,” said Amanda Fine, a veterinarian with the Wildlife Conservation Society in Asia. 

The organization said this week that a quarter of the Mongolian saiga population has been lost to the small ruminant plague, or PPR. The virus evolved in Africa and normally affects livestock like goats and sheep. But it has spread in recent years, first to the Middle East, then to Asia. Fine and her colleagues think it probably was introduced to Mongolia via livestock imported from China, then passed to the saiga in the grazing areas the wild animals share with domestic herds.

This is the first time PPR has been identified in saiga, and the impact is pitiful to see. The virus initially infects the animals' nasal cavities, producing crusty secretions around their noses and eyes. Then it migrates into their lungs, causing pneumonia. The animals struggle to breathe, stretching out their necks and gaping their mouths in an attempt to get more air. They stop eating and become dehydrated.

When PPR infects domestic goats, it kills anywhere between 30 and 90 percent of the herd. Carcass counts of the Mongolian saiga suggest that 2,500 animals — a quarter of the population — have already died, Fine said. The rate of infection seems to have slowed, but it has not run its course, and the virus could flare up again when the animals congregate in the spring.

The saiga have been in decline for decades, a result of habitat loss, hunting and poaching for their horns. By the start of the millennium, there were just 50,000 left — a 95 percent decline from their numbers in the 1970s. The creatures were added to the IUCN Red List of endangered creatures, and conservation work started in earnest.

By 2015, the efforts were starting to pay off. The global population of saiga had grown to 300,000. In Mongolia, numbers jumped from fewer than 1,000 to nearly 10,000.

Then a mysterious disease struck the larger saiga population that live on the grasslands of Kazakhstan. Over the course of a few weeks in May 2015, when the saiga gather to give birth to their young, 200,000 saiga died. A BBC camera crew, there to film the documentary series “Planet Earth II,” watched the tragedy unfold, along with three British veterinarians who wrote about the experience in the Conversation:

“As they gathered to give birth, an increasing number of females became weak and uncoordinated, dying in a matter of hours. Soon a vast area stretching over hundreds of kilometers was littered with corpses.”

More than a year of research revealed that the proximate cause of the die-off was Pasteurella multocida, a usually harmless bacteria that resides in the animals' respiratory tract. But the overarching cause — the factors that made P. multocida suddenly so deadly — was more complex. Stress from short-term weather changes, compounded by the physiological strain of giving birth, probably made the saiga vulnerable to infection, setting off a “cascading effect of virulence,” the vets wrote.

Fine sees a link between these two die-offs. Not a direct one — the Kazakhstan outbreak was caused by a bacterium, while the current Mongolian plague is viral. Plus, the two populations are separated by thousands of miles and the Altai Mountains.

“But I think we probably are looking at similar situations in the sense that these populations are highly vulnerable to the introduction of new diseases and the stress that goes along with that and factors that can trigger it,” she said.

Because they recently were so close to extinction, because their ecosystem is undergoing change, because they increasingly come into contact with humans and their domestic herds, and because of some less-than-ideal traits of their own (like the habit of gathering by the tens of thousands to give birth), saiga are especially susceptible to being devastated by a fast-acting infectious disease.

There was no way scientists could have predicted or stopped the Kazkhastan die-off. But Fine and her colleagues say a global PPR vaccination program in domestic herds could protect saiga and other wild ungulates from the disease. (It would also save an estimated $74 billion for the livestock industry, according to a recent study in the journal PLOS One.)

“That is the silver lining here,” Fine said. “We don't have a mystery disease. There is something we can do.”

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