Vets say they believe a parasite is responsible for the deaths of two swans found dead on Brayford Pool.
Both of the birds were found to be carrying the Nematode worm. There are now concerns it could spread to the rest of the 200-strong flock.
The parasite kills the swans by taking all the nourishment out of the food which the birds eat - starving them.
Wildlife experts are monitoring the situation as they say conditions are right for the parasite to spread.
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