Waterford Township, MI — State officials have eliminated poisoning as the cause of death of some mute swans at a small suburban Detroit lake and put the blame on a disease caused by a parasite. The Michigan Department of Natural Resources says necropsies on three of eight of the birds conservation recovered since January from Maceday Lake in Waterford Township determined they were killed by a disease caused by parasitic flatworm that's found in certain freshwater snails. A DNR wildlife pathologist says it's not certain what killed a dozen swans this past winter at the same lake northwest of Detroit, but it's highly likely that they also died from the disease.