After many thousands of years roaming much of central Asia, the saiga antelope has had a terrible couple of decades—and a devastating few weeks. A mysterious illness that causes severe diarrhea and breathing difficulties has wiped out what could be up to half of the remaining population of the species, New Scientist reports. Officials in Kazakhstan say the disease has killed 85,000 of an estimated world population of 257,000 since about May 10, but experts say they've heard unofficial estimates closer to 120,000. The species—known for its distinctive tubular nose, the AP reports—numbered more than a million in the 1990s but has been in decline since, thanks in large part to poachers, and was already considered critically endangered before the latest die-off.
"It's very dramatic and traumatic, with 100% mortality," the leader of an international team investigating the outbreak tells New Scientist. "I know of no example in history with this level of mortality, killing all the animals and all the calves." The team, which has been taking soil and vegetation samples, believes the mass deaths may have been caused by a bacterial infection or by a virus carried by mosquitoes, Smithsonian reports. Whatever the culprit, it has been able to cause a staggering death toll because the population's females all have their calves in the same week, allowing disease to spread quickly, New Scientist notes. (In better news for an endangered species, the US government says most humpback whale populations have rebounded.)