(KRON) — The Oakland Zoo mourned the loss of one of its elderly African elephants, Lisa, at age 46, after her battle with progressive arthritis. Lisa was humanely euthanized on Sunday.

“We are heartbroken,” said Dr. Alex Herman, VP of Veterinary Services at Oakland Zoo. “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 came to the Oakland Zoo in 1979 and was one of only sixteen female African elephants 46 years and older in Association of Zoos and Aquariums-accredited zoos, officials said. She recently celebrated her 46th birthday where zookeepers presented her with a large spread of treats, enrichment, and pinatas filled with her favorite fruits.

“Lisa has been a deeply beloved part of our Oakland Zoo family since 1979. She was truly special to me, and to all of us here. Her loss is felt by so many, especially her closely bonded elephant keepers. We’ll miss her greatly,” said Colleen Kinzley, VP of Animal Care, Conservation and Research at Oakland Zoo.

Lisa’s health issues emerged in 2013 with arthritis and have since progressed to a point where she began avoiding bending her body to lie down and rest, even to sleep, in her heated elephant barn, zoo officials said. She has undergone extensive medical treatment for her condition but ultimately, animal care experts decided euthanasia was the right course.

“This was the best thing for Lisa, but the most painful and heart wrenching for all her Zoo family and the Bay Area community that has loved her for decades,” Herman said.

‍Lisa was transported Sunday evening to the UC Davis School of Veterinary Medicine for an autopsy.