DENVER -- The number of mysterious duck deaths is higher and more widespread than first thought and the problem is not just limited to the Metro wastewater treatment plant, 7NEWS first reported.

A document from the Colorado Department of Health shows that not only have dead ducks turned up at the metro Denver plant, but also along the South Platte River. Wastewater treatment plants in Thornton, South Adams County, Westminster, Northglenn, and Littleton-Englewood have also reported a higher than normal number of duck deaths.

Dead ducks have also been found at the Sunfish Lake near The Breakers apartments in Denver, officials said.

But at this point, nobody knows why.

"No one remembers ducks dying in these kinds of numbers," said Steve Frank with Metro Wastewater District.

The problem first showed up at the Metro wastewater treatment plant. Employees found more than 400 ducks dying from hypothermia but don't know what caused it. Plant operators have scrambled to figure out why and have come up empty.

"Right now, we haven't found anything different in the water now as opposed to years past," Frank said.

Crews have taken the surviving ducks to rehabilitation facilities around the state. Since no one knows why the ducks are getting sick, treatment is limited.

"We just give them things to help flush out what is new in their systems because wildlife are involved with a lot of environmental toxins," said Annette Archambeau, with Archway Wildlife Rehabilitation.

Scientists have ruled out avian flu and avian cholera and state health officials said the deaths may have nothing to with the treatments plants.

The Colorado Department of Wildlife said its veterinarians have not been able to identify what's causing the Northern Shoveler to die.

"Histology from the birds shows no clinical signs of disease. There are no conclusive test results pointing to the cause. Additional tests are being conducted at United States Fish & Wildlife Service (USFWS) labs in Ashland, Ore., and the United States Geological Survey (USGS) labs in Madison, Wis. CDOW and USFWS are confident there is no significant risk to public health," the CDOW said in a release.

"We are focusing on what causes the ducks to lose their waterproofing as well as the cause of the malnutrition evident in many of the birds," said Dr. Laurie Baeten, a veterinarian with CDOW. "There are many factors we need to consider in trying to investigate a die-off and we are doing all we can to methodically apply the science available to us."

As for the survivors, they are getting better every day and may be released in April.

For those who treat the animals, it's a special moment.

"Some animals bolt off but some stop and give you a look back and a nod to say, 'Good job'" said Patrick Archambeau.

The state health department said one possible cause for the deaths is the abnormally cold winter.

The public is encouraged to report sick or dead ducks by calling the Colorado Help Line at 877-462-2911.