ESCONDIDO, Calif. -- A 39-year-old Asian elephant at the San Diego Zoo's Wild Animal Park was euthanized Tuesday, after veterinarians determined that a degenerative joint disease had worsened.

Carol the pachyderm, who appeared with Joan Embery on "The Tonight Show with Johnny Carson," had been under special care to treat a degenerative joint disease for several years, according to the zoo.

The elephant was brought to the San Diego Zoo in 1968 and later transferred to the Wild Animal Park, where she had lived since 1978.

Veterinary and animal care staff decided to euthanize the elephant after "determining her quality of life would not improve," according to a statement from the zoo's public relations office.

"As she approached 40, Carol has been showing signs of deteriorating health advanced by old age," according to the zoo statement.

But an animal rights group blamed the elephant's foot condition on the confined conditions at the zoo and Wild Animal Park.

"The type of severe foot disease from which Carol reportedly suffers involves painful and chronic abscesses and infection that eventually invades the bone," said Elliot M. Katz, president of In Defense of Animals.

"This excruciating condition afflicts more than 60 percent of elephants in zoos and is directly caused by the cramped and unnatural exhibits zoos force these magnificent animals to live in," he said.

According to IDA, a natural elephant's lifespan is 60 to 70 years.

Zoo officials said the Wild Animal Park had hired additional animal care staff to address Carol's condition over the last several years, providing foot treatments and exercise to increase her mobility.

The elephant was on a specialized diet and exercise plan to bring down her weight, and rubberized flooring and a central heating system were added to the Asian elephant barn, they said.

The Wild Animal Park is home to five other Asian elephants.

"We recognize that Carol will be sorely missed by the caring staff and visitors who had seen her at both the Wild Animal Park and the San Diego Zoo," the zoo's statement read.