Kansas City, Mo. ? Kansas City Zoo officials said it was likely West Nile virus that sickened the zoo’s only remaining male elephant, which had to be euthanized.

The death of Dale, a 25-year-old African elephant, comes less than six weeks after the venerable bull elephant, Casey, died.

Dale, who weighed 5 tons, was found lying on his side in his indoor enclosure early Monday by zoo staff. Efforts to medicate him and get him back on his feet were unsuccessful, and the decision was made to euthanize him early Monday afternoon.

“Any time any of our animals die it’s a sad occurrence,” Zoo Spokesman Mike Hurd said. “But Dale’s death so close to losing Casey really has been tragic. The elephants are very popular with our visitors, and they’re beloved by our keepers and staff.”

Dale had shown symptoms of illness for the last two weeks and tested positive for a virus. Zoo officials said it probably was West Nile, which is mosquito-borne.

A necropsy was being performed, and tissue samples were to be sent to an outside lab for analysis.

Steve Wylie, animal director at the zoo, said the zoo’s six female elephants have not displayed symptoms of disease.

Casey also is believed to have contracted the virus, but his death is thought to have been caused by complications of old age since he was 52 years old. Casey — who had been in Kansas City for nearly a half-century — was found dead Sept. 24.

Wylie said it was unusual for an elephant to die from West Nile, but the zoo had lost several birds to the disease.

Zoo officials said they followed animal health protocols by notifying the U.S. Department of Agriculture and the University of Missouri when Dale began showing symptoms.

Dale came to Kansas City in 1994 on loan from an animal facility in Wisconsin. He made an important contribution to the conservation of his endangered species when in 2000 he became the father of the first successful birth of an African elephant through artificial insemination. The mother was at the Indianapolis Zoo.

Samples of Dale’s semen have been sent to several animal parks around the United States, and some are in storage at the Henry Doorly Zoo in Omaha.

Wylie said it was too early to consider whether Kansas City would acquire additional bull elephants. Those decisions are made by a conservation protocol.