A 46-year-old African elephant has been euthanized at the Oakland Zoo after suffering arthritis and other age-related problems, the zoo announced Monday.

Lisa was euthanized on Sunday after years of chronic and degenerative health issues that had irreversibly worsened, a zoo statement said.

"We are heartbroken," Alex Herman, vice president of veterinary services, said in the statement. "Her disease progressed to a point where we didn’t want her last day to be her worst day, and it was time to do the most humane thing and to let her go."

Lisa had been in the zoo since 1979.

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The zoo said her ailments included eye ulcers, progressive arthritis that was diagnosed in 2013, foot and nail lesions and recently ventral edema, or swelling of her body caused by fluid retention.

"Her medical care plan included physical therapy, pain medication, laser treatments, cryotherapy, and daily foot soaks in Epsom salt, and two cutting-edge stem cell procedures in October 2022 and January of this year, in partnership with researchers and veterinarians from Michigan State University," the zoo said.

Those involved infusing stem cells from a healthy elephant into Lisa's blood in an effort to ease her arthritic inflammation, the zoo said.