SeaWorld's 35-year-old killer whale, Tilikum, is suffering from an incurable bacterial infection in his lung. Over the last few weeks, the theme park's veterinarian staff noticed that the whale's health has been deteriorating, with is behavior growing very lethargic.

Although Tilikum has undergone treatment, the respiratory condition he is believed to have is very resistant to medicine and a cure for it has not yet been found. It appears that as the whale ages, his disease is taking a greater toll on his health.

"He does have what we believe to be a respiratory condition that is extremely difficult to treat," SeaWorld veterinarian Scott Gearhart said. "He has a disease, which is chronic and progressive, and which, at some point, might cause his death."

At 12,000 pounds, Tilikum is the largest marine mammal living at SeaWorld in Orlando, Fla. He is also the killer whale responsible for the death of three people, including a SeaWorld trainer, and is the star of the documentary "Blackfish."

Tilikum has lived at the marine theme park for 23 years, after being relocated from another park, Sealand of the Pacific, when it closed in 1992. He was originally captured off the coast of Iceland in 1983 at age two. While in captivity, he has sired 21 calves and is thought to be one of SeaWorld's most prolific breeders. 

Had Tilikum remained in the wild, Gearhart said he would have died a long time ago. Even so, SeaWorld officials noted that 35 years old is the upper end of the life expectancy scale for male orcas in the wild. When, Tilikum dies, SeaWorld does not plan on replacing him with another orca.

In the meantime, SeaWorld veterinarians plan to manage Tilikum's illness and keep him comfortable in any way they can. However, like many older animals facing significant health issues, his condition may continue to fluctuate. 

Last year, the California Coastal Commission approved SeaWorld's $100-million tank expansion project, known as "Blue World," in San Diego. However, the commission's approval meant the park could no longer breed killer whales in captivity. As a result, SeaWorld announced it would start phasing out its killer whale show beginning in 2016. Instead, the park plans to create a more "natural" orca experience, set to debut in 2017.