By Steve Gorman

LOS ANGELES (Reuters) - A rare northern white rhinoceros, one of just five left on Earth, is undergoing treatment at the San Diego Zoo Safari Park for what veterinarians believe is an abscess under its skin, a park spokeswoman said on Tuesday.

The 41-year-old female rhino, named Nola, was found last week to have swelling on her right hip that grew larger until veterinarians on Saturday lanced the site to drain and flush it with saline solution, spokeswoman Darla Davis said.

Samples of tissue and pus taken from the site are being tested, with results expected in a matter of days, though vets do not believe a tumor exists, Davis said. In the meantime, Nola has been put on a course of antibiotics as a precaution.

Davis said the entire procedure was performed in the field where Nola was grazing, inside the 65-acre exhibit she inhabits inside the sprawling park, without the use of anesthesia.

Vets were able to approach the animal, lance the swollen area and flush it as they walked along beside her.

"She has such a great relationship with the keepers and vet staff that she would allow them to do things that other rhinos wouldn't," Davis said.

Though considered elderly by rhino standards, with a touch of arthritis, Nola has been fairly healthy otherwise, and has exhibited no signs of illness since recovering from a sinus infection in January, according to Davis.

Another, older male northern white rhino at the park died in December, leaving Nola as the only living member of her kind in the Western Hemisphere.

There are three in a reserve in Kenya and a fourth in a zoo in the Czech Republic, though like Nola all are past breeding age or have reproductive issues.

DNA from northern whites has been stored in hopes that new reproductive technologies might allow scientists to bring them back from the brink of extinction. Studies are under way to determine if southern white rhinos, of which some 20,000 remain in the wild, are close enough genetically to serve as maternal surrogates for implanted white rhino sperm.

Scientists remain unsure whether northern and southern whites are two distinct species or subspecies of each other.

Southern white rhinos, like their northern cousins, have been decimated by poaching for their horns. They are now being killed off in South Africa at the rate of about one every eight hours, Davis said.

(By Steve Gorman; Editing by Doina Chiacu)