Aketi and Aru were spotted foaming at the mouth around five days ago, confirmed zoo curator Heiner Klös. Since then they have been suffering from colic and are finding it difficult to breath. They have also stopped eating.

“We think that someone threw something in [to the enclosure],” Klös told local newspaper Der Tagesspiegel.

To check what exactly is wrong with the pair would require a blood sample. This would only be possible under general anaesthetic – which the pair are too ill to undergo. They are being looked after carefully though, said Klös.

The zoo has declined a full police investigation, the paper reported.

The zoo has ruled out that the food given to the lions by the keepers could have been poisoned – the meat is shared between all 16 big cats and only the brother and sister pair are ill.

This is the first time that a potential poisoning has happened in the past 25 years, Der Tagesspiegel said.

Berlin zoo has come under fire from critics since Aketi and Aru arrived from Wuppertal in 2009. The pair have had two cubs and animal rights activists have protested against the zoo allowing incest.

The Local/jcw