You may remember in May when there was a mass die-off of around 85,000 endangered saiga antelopes. Now, the same thing has happened again, and researchers are just as helpless to stop it.

Researchers recently arrived in Kazakhstan to monitor a herd of these antelopes as there had been reports of multiple deaths.

"But since there happened to be die-offs of limited extent during the last years, at first we were not really alarmed," Steffen Zuther, a geoecologist, told Live Science.

Now there has been a second bout of deaths, this time 60,000 antelopes have fallen foul to this unknown perpetrator.

"The extent of this die-off, and the speed it had, by spreading throughout the whole calving herd and killing all the animals, this has not been observed for any other species," Zuther said. "It's really unheard of."

There are a few herds of the critically endangered antelope across Kazakhstan, Russia and Mongolia. They congregate during the winter and migrate to Kazakhstan during the fall and spring. The herds then split up to give birth, and it is during this calving season that the die-off began.

To try to figure out the cause of these widespread deaths, researchers have been taking samples from everything the antelopes touched over the summer. The soil, the water they drank from and the vegetation they ate.

A field of dead saigas in May 2015. Albert Salemgareyev.

In addition, they observed the behavior of the animals before they died and took samples from the carcasses. They observed that the females in the calving herds were struck first. Next were their calves, who were not yet old enough to eat vegetation. Whatever was causing these die-offs, it's possible that it was being transferred through the mother's milk.

The results of the necropsies showed that the antelopes' gut tissue contained toxins that are known to be produced by two types of bacteria: Pasteurella and Clostridia. These may have caused bleeding in the animals' organs. However, researchers aren't 100% convinced that they the toxins are responsible.

Pasteurella doesn't typically inhabit the saiga antelope and would only have a harmful effect if the animals' immune systems were compromised.

Another possibility is a slightly altered environment. The previous winter was unusually cold and spring was especially wet, leading to an abundance of vegetation and standing water: perfect for colonies of bacteria to thrive. Zuther points out, though, that this one wet spring doesn't seem that unusual.

There is still the possibility that the cause of the deaths is something more unusual. An example of an unlikely culprit happened in a strange case of vulture deaths in India. The vulture population in India was nearly wiped out, and it turned out that their main food, cows, were receiving a drug that poisoned the vultures. While the cause of death for the saiga antelope might be not be as convoluted as this, finding the cause may still take some creative thinking.

In the meantime, Zuther and his colleagues are continuing their quest to find the elusive killer that's wiping out whole herds of saiga antelopes.

[H/T: LiveScience]