The health of Tilikum the killer whale at SeaWorld in Orlando, Florida, is failing and the infection he has could kill him, say officials at the marine performance theme park.

Tilikum, who is estimated to be 35 years old, has become increasingly lethargic in recent weeks, CNN reports with a bacterial infection in his lungs resistant to treatment.

"Despite the best care available, like all ageing animals, he battles chronic health issues that are taking a greater toll as he ages," SeaWorld said on its website. "The suspected bacteria is very resistant to treatment, and a cure for his illness has not been found.".

At 12,000 pounds (5440kg), Tilikum is the largest marine mammal living in a zoological park and is one of SeaWorld's most prolific breeders, having sired more than 20 calves.

The orca has lived at the theme park for 23 years, after he was moved from another park, Sealand of the Pacific, which closed in 1992. He was captured off the coast of Iceland.

In 2010 while trainer Dawn Brancheau was at the edge of a SeaWorld pool with Tilikum during a performance, the orca grabbed her arm and pulled her underwater, drowning her.

The orca has also been linked to the deaths of two other people. A Sealand trainer died in 1991 after falling into a tank holding Tilikum and other whales and was prevented from surfacing. In 1999, a man who hid at SeaWorld after the park closed was found dead in Tilikum's tand the next day.

The tragic lives and fate of Tilikum and other killer whales in captivity was underscored in the 2013 documentary Blackfish which focused on the death of the trainer and the aquarium's treatment of its killer whales.

The film sparked a public campaign against performances featuring killer whales, and led to decreased attendances at SeaWorld parks in California, Florida and Texas.

After Brancheau's death, the US Occupational Safety and Health Administration prohibited trainers from entering the water with orcas unless separated by a barrier.

SeaWorld has continued its orca shows although CEO Joel Manby has said the theme park was planning a new "orca experience" that would show whales in a more natural setting.

People for the Ethical Treatment of Animals said Tilikum has been suffering "immensely by confining him to a small concrete tank for decades, causing him to succumb to mental illness that has resulted in aggression and now to some incurable illness," PETA president Ingrid Newkirk said in a statement.

"It's time for SeaWorld to move the orcas to sanctuaries, where they could enjoy a more natural life," she added.

More about orcas and SeaWorld