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Hundreds of elephants are dying in the Okavango Delta in Botswana but, as yet, no-one can explain why.

There's now concern that whatever is killing them could spread further, and that poachers could take advantage of the situation by taking the animals' tusks.

Kate Evans is founder and director of the charity Elephants for Africa, which has a research camp in northern Botswana.

"It's a completely open border... it's not like elephants can be told to stay at home."

(Photo: Dead elephants are seen in the Okavango Delta. Credit: Reuters)