Lily, a nearly 6-year-old Asian elephant and the “darling” of the Oregon Zoo, died Thursday night after the rapid progression of a deadly virus, zoo staff reported Friday.

Lily was diagnosed with an active strain of endotheliotropic herpesvirus on Wednesday, but she didn’t immediately show any symptoms, the zoo said in a statement. By Thursday morning, the youngest member of the zoo herd became lethargic and was uninterested in food.

Vets at the zoo began treating Lily with fluids, antiviral medication and a transfusion, but she ultimately succumbed to the disease late Thursday night, one day before her sixth birthday.

“When she passed, she was with her mother Rose-Tu and surrounded by people who had cared for her since her birth,” the zoo wrote in a Facebook post.

"I can't imagine a more devastating loss for this zoo family and our community," zoo director Don Moore said in a statement. "Lily was the darling of the zoo. She was loved by everyone from her elephant family to the people who cared for her every day to her thousands of fans.”

Lily was born Nov. 30, 2012, to mother Rose-Tu and father Tusko and quickly became a favorite among zoo staff and visitors.

Lily goes for a romp with mom, Rose-Tu, 10 days after her birth. Benjamin Brink/The OregonianLC- The Oregonian

Ownership of Lily and Tusko was called into question just a few weeks later after what the zoo called “misleading” news reports indicated a breeding loan agreement gave ownership of the calf to Tusko’s owner, Have Trunk Will Travel, a private California elephant ranch. The dispute was resolved when the zoo agreed to pay $400,000 for the pair of pachyderms.

“I feel really good about it," said Kim Smith, the zoo’s director at the time. "It’s wonderful to know that we’re permanently their advocates for the rest of their lives.”

By her first birthday, Lily weighed nearly 1,200 pounds.

Experts believe nearly all Asian elephants, both captive and wild, carry some form of endotheliotropic herpesvirus, which has evolved with the species over millions of years. The virus remains latent in most animals and causes no harm, but the active strain, to which calves are particularly vulnerable, progresses rapidly and is difficult to treat.

Endotheliotropic herpesvirus is the leading cause of death in elephants under the age of 8, according to the zoo, which has been providing blood samples to researchers for 16 years in the hopes of developing a vaccine.

“It’s very difficult to study the virus in wild elephants,” Moore said. “But zoos can monitor their elephant herds closely and obtain samples of blood and other fluids. Right now, that’s our best hope of developing a vaccine.”

The zoo will be closed Friday, including the Zoo Lights event scheduled for Friday evening, so that staff members can grieve.

“Our staff did everything they could and fought to save her until the very end,” Moore said. “Everyone is in mourning here. It is just heartbreaking."

This story will be updated.

-- Kale Williams

kwilliams@oregonian.com

503-294-4048

@sfkale