A visitor poisoned a pair of lions at the Berlin Zoo, the Daily Mail reports. 

The pair Aru and Aketi were found foaming at the mouth and became very ill last week. The lions stopped eating and drinking after the poison kicked in.

According to the reports, the Berlin zoo is the most visited zoo across Europe, with an annual three million visitors from all over the world. The zookeepers believed that some visitor must have thrown some food which is not the part of the regular diet of the big cats.

The zookeeper's claim that all the 16 big cats were given the same meal but none of the other cats fell ill. 

"We think that someone threw something into their enclosure," Curator Heiner Kloes told the Daily Mail.

The lions also developed colic, which caused them to repeatedly cry out in distress. Experts said blood samples will be needed to identify the actual cause of the lions' illness. But it will take another week because both lions need to recover fully.

The zoo staff said that they collected leftovers from the enclosure of the animals as some visitors ignore all the instructions mentioned by the zoo authority about not feeding the animals. Zoo authorities provided a guide to the Berlin Zoo with all of the instructions and necessary information. The guide also contains a map for the convenience of the visitors.

It specifically mentions the timing to feed the animals and also says that no other food is allowed to be given by the visitors.

The zoo staff said this is the first incident in the last 25 years. The staff believes that a visitor put poison in a chunk of raw meat which both animals ate.