More than 350 elephant carcasses have been spotted in South Africa's Botswana's Okavango Delta, in the past two months and no one knows the reason.

A cluster of elephant deaths was first reported in early May, with 169 individuals dead by the end of the month.

Botswana, home to a third of Africa's declining elephant population has said it has started its investigation regarding the unexplained deaths. Acting Director of the Department of Wildlife and National Parks Lucas Taolo, said local communities are being advised not to tamper with the dead elephant’s tusks.

The officials from South Africa, Zimbabwe and Canada have collected samples, while anthrax has been ruled out as the cause. Anthrax can be found naturally in soil and commonly affects domestic and wild animals around the world.

As the carcasses were found intact, poaching too has been ruled out, international media reported. Dr Niall McCann, of the UK-based charity National Park Rescue, said, "The way the animals appear to be dying - many dropping on their faces - and sightings of other elephants walking in circles points to something potentially attacking their neurological systems."