SOUTH BEND, Ind. (AP) — Zoo officials say a rare leopard was euthanized at South Bend's zoo after the 19-year-old animal's health suddenly worsened.

The Potawatomi (pot-ah-WAH'-toh-mee) Zoo announced Saturday that the Amur leopard, named Sergei, was euthanized after his health rapidly declined due to old age and chronic health issues.

Zoo veterinarian Dr. Ronan Eustace says zoo employees are in mourning but comforted knowing that they helped Sergei live "a long and comfortable life that ended with grace and dignity."

Sergei had fathered five rare cubs with his zoo mate, Pearl.

Amur leopards are believed to be the Earth's rarest big cat with only about 40 remaining in the wild. They typically live to be 10 to 15 years old in the wild in their native range throughout northern China and Russia's far east.