An aging northern white rhinoceros, one of just five left in the world, appeared to be responding to treatment of an unidentified bacterial infection that has veterinarians worried for a subspecies limping toward extinction, a California zoo said on Tuesday.

Nola, age 40, was showing signs of improvement after San Diego Zoo Safari Park keepers gave her antibiotics to ward off an illness whose symptoms on Monday included a runny nose, decreased appetite and lethargy, a zoo spokeswoman said.

"She seems to be feeling better today, she has been walking around and eating," said Darla Davis, a zoo spokeswoman. "The vets think she is responding to antibiotics."

Nola, a 4,000-pound (1.8-tonne) female who came to San Diego in 1989, is considered a geriatric rhino in a subspecies whose individuals generally live 40 to 50 years in captivity, Davis said.

On Dec. 14, a 44-year-old northern white rhino named Angalifu died at the same Safari Park, where he was being treated for age-related conditions.

Angalifu's death brought the worldwide total of known remaining northern white rhinos down to five from six, with Nola in San Diego and four counterparts elsewhere, one at a zoo in Europe and three in preserves in Africa, Davis said.

"Nola had a thick drainage coming out of her nose," Davis said. "The vets swabbed the mucus, and cytology tests showed bacteria, so they began treating her with antibiotics."

The Safari Park vets also took blood for tests from the creature's tail, Davis said.

"She's such a gentle animal, she lets the keepers work on her," she added.

Northern white rhinos were driven to near extinction by poaching in Africa, where the animals' horns are prized for their supposed medicinal value. None of those remaining in captivity has reproduced.

Semen and testicular tissue from the male northern white rhino have been stored in the hope that new reproductive technologies will allow recovery of the subspecies in the future, the zoo said.

White rhinos rank as the world's heaviest land mammals after African and Indian elephants.

There are only about 30,000 rhinos of all types left in the world, and a rhinoceros is believed to be killed by poachers somewhere in the wild every eight hours, the zoo said earlier this month.

Davis said Nola did not share a habitat with Angalifu and had not bonded with him. She lives with a southern white rhino named Chuck.