A fifth swan at a sanctuary in Dorset has tested positive for the virulent H5N1 strain of bird flu, environment department Defra has said.
Workers at the swannery have been given flu jabs as a precaution
The first case of avian influenza was found at the Abbotsbury Swannery, near Chesil Beach, on 10 January.
John Houston, general manager, said: "We are expecting to have a run of positives and negatives while [the virus] works its way through the herd."
Restrictions on the movement of birds, imposed last week, remain in place.
Earlier, Defra said there was currently no evidence to suggest the disease was widespread among wild birds in the area, but officials were closely monitoring the situation.
Mr Houston said: "There are still four or five dead swans which have been picked up - and for which there are no test results as of yet."
He said Defra staff and staff from the swannery had so far taken swabs from about 60 mute swans at the swannery.
They are hoping to establish whether the birds are building up resistance to the outbreak.
The swannery will be given the all-clear if no birds test positive for the virus after 21 days.
The swannery is part of an area of wetland recognised to be of international importance.
The Health Protection Agency has monitored about 12 staff at the swannery for signs of the disease, although the risk of infection is said to be low.
Workers have been given a course of anti-viral drug Tamiflu as a precaution.