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LONDON, Jan 16 (Reuters) - A fourth swan in southern England has tested positive for the deadly H5N1 strain of bird flu, Britain's farm ministry said on Wednesday.

"There is currently no evidence to suggest widespread disease in the wild bird population but enhanced surveillance is taking place and poultry keepers in the area are reminded to remain vigilant," a spokeswoman said.

Last week, the ministry confirmed that three wild swans at a bird sanctuary in Dorset had died from the virus. In the latest case, the dead swan was collected on Jan. 11 as part of a wild bird surveillance programme in the same area.

The spokeswoman said there was no evidence of the disease in domestic birds.

Britain's first case of the strain was in a wild swan found dead in Cellardyke in Scotland in 2006 and there have subsequently been outbreaks at poultry farms in eastern England, most recently in November 2007.

The virulent H5N1 strain has killed more than 210 people worldwide since 2003 and millions of birds had either died from it or been killed to prevent its spread.

(Reporting by Veronica Brown and Nigel Hunt; editing by Chris Johnson)

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