Unna, an 18-year-old orca at SeaWorld San Antonio, has died after a months-long battle with an infection.

SeaWorld said that the female whale had been undergoing medical treatment after it was discovered that she had contracted a fungal infection in September.

'We are saddened to share the passing of Unna today,' the park's website posted on Monday, adding that it is a 'difficult time for the SeaWorld team and all of Unna’s many fans'.

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Unna, an 18-year-old orca at SeaWorld San Antonio, died Monday after battling a fungal infection for months. Above, Unna is treated by SeaWorld staff

SeaWorld said that traditional treatment was not working, but that a new approach had seen positive effects before the whale's death. Above, Unna lies on her side as she is treated

In September Unna reportedly began being monitored round-the-clock by medical staff at the Texas SeaWorld because she had an inflamed bladder from the fungus Candida.

The park said that traditional medicine was not working, but that a 'novel approach' was having a positive effect on the 17-foot long, 4,600-pound animal before her death.

On December 8, the team said it was 'cautiously optimistic that she is on the road to recovery'.

Candida is a fungus that also occurs in whales and dolphins both in the wild and in captivity, though some say that stress of captivity could be a contributing factor in cases such as Unna's.

A necropsy be done to find out the exact cause of her death, which caused the cancellation of shows on Monday at the Texas park.

Unna is the third whale to die at the Texas Sea World in six months, according to My San Antonio.

In November a two-year-old beluga named Stella died because of gastrointestinal problems and in July a baby beluga died after being born prematurely.

A two-year-old beluga whale named Stella (pictured) died this November from gastrointestinal problems. Unna was the third whale to die at San Antonio's SeaWorld in the last six months

A baby beluga whale died this July after being born prematurely at the Texas park. Above, the newborn beluga calf

Some observers have said that killer whales should not be kept in captivity, though SeaWorld has claimed that the orcas it keeps in captivity live just as long as those who live in the wild.

Two former SeaWorld trainers John Jett and Jeffrey Ventre wrote a paper earlier this year about whale survival rates in different facilities and said that their evidence diminishes the parks claims.

Dr Ventre told The Dodo that 'In regard to the captivity debate, specifically, the survival to age milestones data does undermine recent claims made by SeaWorld that its whales live just as long as killer whales in the wild'.

The study showed that the median survival time for whales in captivity was 12 years for US facilities, higher than the 4.4 years for those held in other countries.

Whales born in captivity, such as Unna, had a median survival time of 14.1 years, according to the study.

However, the study said, 'Survival of captive killer whale cohorts has generally improved through time, although survival to age milestones are poor when compared to wild killer whales'.

SeaWorld has faced scrutiny over its treatment of orca whales since the movie Blackfish in 2013, including concerns about the whale's longevity in captivity. Above, a trainer speaks to a whale on the side of a pool

SeaWorld's new CEO has said that 2017 will see whale shows that are in a more 'natural' setting than the current conditions. Above, spectators watch a show at SeaWorld San Antonio

The park has accused animal rights activists of using different studies to compare the average lifespan of a whale in captivity with the high maximum lifespan of whales in the wild, where females can live into their 50s but are not certain to do so.

Different SeaWorld parks currently have one orca in its 40s and several in their 30s.

It also said that it is not fair to include deaths of calves in captivity that are not observable in the wild, and that the annual survival rate measure in captivity, or which percentage of an orca population is likely to survive a year, is comparable to the wild.

SeaWorld suffered from criticism of its treatment of orcas, as well as lower attendance, after the 2013 documentary Blackfish about the whales and the park.