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It is understood two more birds are ill. The dead birds' remains were found on private land in the Killyleagh area.
A spokesman for the Department of Agriculture and Rural Development (Dard) said the swans had been removed so that post-mortem exams could be carried out.
"This is the second case that has been brought to our attention," he said.
"The first case was investigated by Dard and the post mortem revealed lesions suggestive of bacterial infection."
"We would ask the public to refer to the Dard website or helpline for information on the types of bird deaths that we would like to be brought to our attention, so that Dard can monitor the situation and respond quickly."
There are more mysterious deaths occuring in the already hard hit UK seal population.
"The sickness, which has baffled experts, leaves the animals weak, starving and barely able to breathe. Most of the casualties are pups, spotted struggling on Norfolk’s sandy beaches by passers by.
Marine experts can find no signs of viral disease or bacteria as they nurse the animals back to health. While many have been infected with lungworm parasite, this is likely to be an underlying problem, not the cause.
But the season has extended and we have been getting emaciated and dehydrated older pups throughout the autumn and wint...er. Many have dry eyes and are coughing up blood and struggling to breathe.
Dr Dave Thompson, marine scientist at the Sea Mammal Research Unit at St Andrews University, added: ‘If it was disease it would have been obvious by now.
‘It is possible there is something else happening at sea that we don’t know about."
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