Story highlights The female rhino was 41 and one of four of the subspecies remaining in the world

The remaining three are in the Ol Pejeta Conservancy in Kenya.

(CNN) The world's northern white rhinoceros population plunged by a quarter on Sunday with a death at the San Diego Zoo Safari Park.

Nola, a 41-year-old northern white rhino, died after being treated for a bacterial infection and age-related health issues, the zoo said.

The rhino's condition had worsened significantly in the last 24 hours, and zoo veterinarians made the decision to euthanize her.

Forty-one years is at the upper end of a white rhino's life expectancy

Photos: The great white rhino hope Photos: The great white rhino hope In December 2013, the northern white rhino Nabire walks around in her enclosure at a zoo in Dvur Kralove, Czech Republic. Nabire died of a ruptured cyst in July 2015, leaving only a few northern white rhinos left in the world. Hide Caption 1 of 6 Photos: The great white rhino hope In Kenya, where three of the remaining northern white rhinos live, armed guards prevent poaching. Hide Caption 2 of 6 Photos: The great white rhino hope Sudan was one of three northern white rhinos left worldwide. He died in March 2018. Hide Caption 3 of 6 Photos: The great white rhino hope As the only male, the fate of the subspecies depended on Sudan. Though he was too old to mate, scientists are hoping to impregnate other females using his sperm. Hide Caption 4 of 6 Photos: The great white rhino hope In addition to round-the-clock security, the conservancy has put radio transmitters on the animals and dispatches incognito rangers into neighboring communities to gather intelligence on poaching. Hide Caption 5 of 6 Photos: The great white rhino hope Rhinos are targeted by poachers, fueled by the belief in Asia that their horns cure various ailments. Hide Caption 6 of 6

Nola was one of only four northern white rhinos in the world, according to the zoo. The remaining three are in the Ol Pejeta Conservancy in Kenya.

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