Three lionesses have died at the Tokyo Zoo, suspected of suffering from heat-related illness, amid Japan's worst summer heat wave.

Three lionesses died within a week at the Tama Zoo in western Tokyo. According to the zoo, autopsies revealed that the animals, aged between 3 and 15 years, suffered from dehydration and multiple organ failure, suggesting heat as a possible cause of death.

"This is the first time something like this has happened to us. Last year was also hot, but none of our lionesses became so ill," said zoo spokesperson Miwa Kosaka.

Summer in Japan is becoming increasingly hot and dangerous, with some cities experiencing several days with temperatures above 40°C – so-called "sweltering hot days."

According to the zoo, a three-year-old lioness named Mugi lost her appetite on July 19, and the following day she was unable to stand. Zoo staff cooled her with ventilation and water, and later administered subcutaneous fluids, liver medications, and vitamins. However, she died on July 28.

Eleven-year-old lioness Ichigo died a few days later, on July 31. Fifteen-year-old lioness Luena died on Sunday.

According to autopsy results, all three lionesses suffered systemic dehydration and multiple organ failure, suggesting heat stress as a possible cause. The exact cause of death will be determined after test results are available, the zoo noted.

Three more lions—two males and one female—remain in critical condition as of Monday and are unable to stand, the zoo said in a statement. Staff are administering fluids, medications, and vitamins.

"Some lions are showing signs of improvement," Kosaka said.

According to the zoo, four more lions are showing signs of recovery, and six animals have remained healthy from the start. They will be housed in a separate enclosure equipped with portable air conditioners and industrial fans.

Tama Zoo houses approximately 260 species of animals. Lions have long been one of the park's main attractions—they have been bred and exhibited to visitors since 1964. According to the zoo's website, a safari-style bus is available for exploring the lion enclosure.