RIP ... Unna, 18, passed away from an infection. Picture: SeaWorldCares.com

Unna was an 18-year-old killer whale suffering from an infection for the past few months.

SeaWorld San Antonio, in Texas, said Unna was under constant care of their veterinary team and other experts, but remained in a serious condition.

A necropsy will now be performed to determine the cause of her death.

In respect of Unna, all killer whale shows have been called off for the next week.

Unna was born on December 27, 1996 and grew to be over 17 feet long and weighed over 2090kg.

But despite the intensive medical care and the aggressive treatment by experts, such as renowned mycologist Dr Michael Rinaldi, Unna passed away today.

Some activists have claimed the illness was to be expected in wild animals being kept in captivity.

Unna’s death comes just weeks after SeaWorld announced they’d be moving away from the current killer whale enclosures into more natural environments.

SeaWorld has been under pressure since Gabriela Cowperthwaite released her shocking documentary Blackfish, exposing the consequences of keeping killer whales in captivity — which includes some of the whales injuring or killing some of their trainers.

In a statement, SeaWorld claimed Blackfish was “inaccurate, and misleading, and regrettably, exploits a tragedy that remains a source of deep pain ...”

Still, the impact was felt at SeaWorld parks with the company’s share price dropping 44 per cent in 2014 and their CEO Jim Atchison resigning.