Three snow leopards have died at the Lincoln Children’s Zoo in Nebraska of complications from COVID-19.

The zoo made the announcement in a Facebook post Friday, describing the deaths of the three leopards — named Ranney, Everest, and Makalu— as “truly heartbreaking.”

The zoo began treating the leopards and two Sumatran tigers for the virus last month. The zoo said the tigers, Axl and Kumar, have made a recovery.