One of the last four remaining northern white rhinoceros -- a subspecies of the white rhinoceros -- died Sunday. Nola, a 41-year-old female, had been a popular attraction at the San Diego Zoo Safari Park since 1989, and was the only white rhino in the United States.

According to the zoo authorities, Nola underwent a surgery to drain a pelvic abscess on Nov. 13. However, despite the surgery being successful, the rhino began showing signs of a reduced appetite and lower activity levels.

“Nola, who lived here since 1989, was under veterinary care for a bacterial infection, as well as age-related health issues,” the zoo said, in a statement released Sunday, adding that the decision to euthanize her was made when her condition continued to worsen.

“Nola was an iconic animal, not only at the San Diego Zoo Safari Park, but worldwide,” the zoo said, in the statement. “Through the years, millions of people learned about Nola and the plight of rhinos in the wild through visits to the Safari Park, numerous media stories and social media posts. Nola leaves a legacy that her keepers and animal care staff hope will continue to help rhino conservation for years to come.”

As recently as 1960, there were more than 2,000 northern white rhinos -- most of them in South Africa, Namibia, Zimbabwe and Kenya. However, in 2008, following years of indiscriminate poaching by hunters seeking their prized horns -- used in traditional Asian medicines -- the subspecies was declared extinct in the wild.

Now, only three northern white rhinos -- including the last surviving male rhino Sudan -- remain. All of them are kept closely guarded at the Ol Pejeta Conservancy in Kenya.

“It is sad what human greed has done and now we must keep watch every minute because it would be unimaginable if the poachers succeeded in killing these last few animals,” Mohamed Doyo, a ranger at the Ol Pejeta Conservancy, told the Guardian in April.

The San Diego zoo recently took possession of six female southern white rhinos -- which currently number at just over 20,000 -- from South Africa. The zoo authorities plan to save the northern white rhino from extinction by implanting its embryos into surrogates from the southern white rhino subspecies.