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A mystery illness has swept through mute swans at a Dorset wetland reserve - killing half the resident herd.
Sixteen swans at Radipole Lake Nature Reserve in Weymouth have died since September.
Two sick swans are currently being treated at the RSPCA's West Hatch Animal Centre in Somerset.
It is not yet clear what caused the deaths, though the swans have tested negative for bird flu.
Tony Whitehead, speaking for the RSPB in the south west said: "We started to find poorly swans at our Radipole Lake Nature Reserve six weeks ago.
"They were acting lethargically, and clearly struggling. The RSPCA took the birds to West Hatch and to date 16 birds have been affected - which is half the resident herd on site.
"We've spoken to neighbouring nature reserves to let them know - including the swannery at Abbotsbury - but we have had no reports from elsewhere of the same thing happening.
"And we haven't had any birds affected over the past couple of weeks - so it appears quite contained. And it's a real mystery. But one, naturally we're keen to solve.
"The birds have tested negative for bird flu. But we await a fuller post mortem from RSPCA to see if a cause can be identified.''
The illness is being investigated by the Animal Health and Veterinary Laboratories Agency.
A spokeswoman for the RSPCA, said two surviving swans were ``doing well at present''.
She added: "The others were sent to the AHVLA for a post mortem but the results came back as negative for influenza.
"The AHVLA said they didn't know what the cause of death was. It would really be for them to do further investigations on this.
"Obviously, it is always sad when animals do not make it, especially for our staff who work tirelessly to save the lives of all the animals which come into their care.''