The orca whale that made worldwide headlines when it killed its trainer during a live show in 2010 is severely ill, SeaWorld officials in the US said Tuesday.
"We are saddened to report that over the past few weeks, Tilikum's behavior has become increasingly lethargic and the SeaWorld veterinary and animal care teams are concerned that his health is beginning to deteriorate," SeaWorld said on its blog.
Tilikum - nicknamed Tilly - one of SeaWorld's biggest and best-known attractions, is believed to be aged about 35, which is advanced in years for male killer whales.
The orca, which has been in captivity for three decades, has chronic health issues and SeaWorld - which has faced fierce criticism from animal rights groups for its whale shows - is treating him for a bacterial infection in his lungs that has been resistant to treatment so far.
Trainer Dawn Brancheau, 40, died when Tilikum - which has been linked to two other human deaths since 1991 - grabbed her by her ponytail and dragged her into the water at the end of a show at SeaWorld Orlando.
Ingrid Newkirk, president of animal rights group PETA, greeted news of the killer whale's worsening condition with skepticism.
"When SeaWorld feels that it must make a statement about Tilly's ill health, PETA bets that it's anticipating someone is about to blow the whistle about his illness, or reveal that he's dying and that the people who have exploited him to line their pockets all these years know that," she said.
"Since SeaWorld can no longer capture orcas from the wild, it has used Tilikum as its chief breeding machine, churning out more performers for a miserable life that no one would ever willingly choose," she added.
PETA has been a vocal opponent of SeaWorld's treatment of marine animals at its theme parks and the critically acclaimed 2013 documentary "Blackfish" shone a damning spotlight on the impact of captivity on orcas.
SeaWorld said in November that it planned to phase out its signature killer whale show.