PORTLAND, Ore. -- Tusko, an Asian elephant at the Oregon Zoo, has been euthanized.

Officials said he was humanely euthanized at the zoo Tuesday, after animal-care staff determined they could no longer help his pain and mobility issues stemming from a decades-old leg injury.

“This is a very sad day, especially for all the keepers and animal-care staff who have been close to Tusko through the years,” said Oregon Zoo elephant curator Bob Lee. “The ten years Tusko spent at the Oregon Zoo was the longest period he’d ever stayed in one place. We’re thankful we were able to give him a good home with our elephant family, and on a personal level I’m thankful I got the chance to know him.”

Zoo veterinarian Tim Storms said over the past week, Tusko developed severe swelling and decreased flexibility of his right front foot and he could no longer bear weight on that limb. Keepers reported he was lying down more frequently, having great difficulty getting up, and using his trunk to help support his weight.

“It was obvious he was in a great deal of pain,” Storms said. “It was distressing to watch him trying to walk, and we felt we had run out of treatment options.”

Physical therapy techniques and anti-inflammatory medications, even at increased dosages, were no longer easing Tusko’s discomfort, Storms added. That was when management and staff made the difficult decision euthanize him.

Photos: Tusko at the Oregon Zoo

Tusko came to the Oregon Zoo in 2005. His leg injury occurred sometime earlier, while he was traveling around the continent as a circus elephant. Zoo officials said they were never informed exactly how the injury happened.

Last year, tests confirmed that Tusko had tuberculosis. Veterinarians at the time started an 18-month treatment regimen for the 44-year-old male Asian elephant. This was not connected to his leg problem.

According to zoo spokesman Hova Najarian, Tusko did well with his TB treatments right from the start, showing no signs of sickness. In June, he was deemed non-infectious by veterinarians and state and county health officials, and cleared to rejoin the herd in Forest Hall.

Tusko was the third elephant at the Portland zoo with the respiratory disease. Two other bull elephants, Packy and Rama, have been treated for TB by zoo veterinarians. Zoo officials said four female elephants and the 5-year-old male Samudra have all tested negative.

Tusko was one of the largest bull elephants in the country, weighing around 13,000 pounds. At nearly 45 years old, he was also one of the oldest males of his species.