looks like he is all but dead.... (CNN) Tilikum the killer whale may be "beginning to deteriorate" as the estimated 35-year-old animal battles a yet incurable bacterial infection in his lungs, SeaWorld said Tuesday. "Despite the best care available, like all aging animals, he battles chronic health issues that are taking a greater toll as he ages," SeaWorld said on its website. "However, the suspected bacteria is very resistant to treatment and a cure for his illness has not been found," SeaWorld said. The bull orca "has become increasingly lethargic" the past few weeks, according to the marine mammal park in Orlando, Florida. SeaWorld says that at 12,000 pounds, Tilikum is "the largest marine mammal living in a zoological park." Tilikum was at the center of the 2013 CNN documentary "Blackfish," which gave a disturbing portrayal of the captivity of the killer whales in SeaWorld, which in response called the film false, misleading and "emotionally manipulative" propaganda. Tilikum was captured off the coast of Iceland and has sired 21 calves in captivity. He has been involved in the deaths of three people, the most recent being trainer Dawn Brancheau in 2010.