Toxic plants may be the cause of a mystery disease striking elephants in Zimbabwe and South Africa. The condition causes an elephant to lose control of its trunk, making it hard for the animal to eat.

Kurt Hostettmann, of Lausanne University, Switzerland, said he believes the elephants are being poisoned by the neurotoxins found in some plants.

He said his group - which normally searches for natural products to use as drugs - had now narrowed the search down to two or three possible culprits.

The disease, known as floppy trunk, was first reported in 1989 around Lake Kariba in Zimbabwe. It causes paralysis, making it almost impossible for an elephant to use its trunk to eat, drink and communicate with other members of the herd.

Imported plants

"We have absolutely no idea how to treat them. That's why we urgently have to find the cause," said Hostettmann.

"It's possible elephants have had to change their feeding habits because of overpopulation"

About 40 elephants, mainly older males, are thought to be affected in Zimbabwe and South Africa's Kruger Park. Some elephants have died, although a few have recovered.

The disease is thought to have arisen when elephants lost much of their natural habitat and were forced to eat new species of plant.

"It's possible elephants have had to change their feeding habits because of overpopulation and some of the plants they've eaten for many years have disappeared, " Hostettmann told the BBC.

"Or they may have started to eat plants that have been imported into Africa from South America, for example."