SAN DIEGO (CNS) - Veterinarians at the San Diego Zoo Safari Park Thursday performed a surgical procedure on an abscess on the right hip of an aging northern white rhinoceros in an attempt to get to the cause of the recurring ailment.

Nola, 41, is one of just four northern white rhinos remaining in the world. She's been on antibiotics since near the beginning of the month for the abscess, which animal keepers flush daily.

According to the zoo, the veterinarians couldn't immediately determine the cause of Nola's condition, but they took tissue and blood samples for further investigation. Results of the tests are due in a week of two.

Going deep into the abscess also helped it drain better and relieve pressure that can cause pain, park officials said.

"We are treating Nola for a bacterial infection on her right hip," said Dr. Jim Oosterhuis, a veterinarian at the park.

"The inside of the abscess is very hard, and we want to determine what may be causing this," Oosterhuis said. "At this point, we simply don't know. Our main priority now is to keep Nola comfortable -- we will continue to monitor her and keep her on her daily treatment plan until we know more or until the abscess properly heals."

The procedure on the 4,500-pound rhino was performed in the field where she lives in order to keep her comfortable. She was given a mild sedative, and her handlers rubbed her back, head and ears while the veterinarians did their work, park officials said.

While the abscessed area is sore to the touch, Nola appears to be feeling well, and is eating and walking normally, according to the Safari Park.

The three other northern white rhinos are under human care in the Ol Pejeta Conservancy in Kenya. The species is at the brink of extinction because of poaching for their horn.