The National Wildlife Health Center in Madison, Wis., confirmed the theory Tuesday.
West Nile virus was suspected when sick and dead eagles were found in December, but wildlife officials were reluctant to identify the mosquito-borne virus as the cause because it has never been reported so late in the year, the Salt Lake Tribune said Wednesday.
The investigation turned to the ice-free waters of Utah's Great Salt Lake and migrating eared grebes, water birds that gather on the lake in numbers as high as 2 million. It is common for as many as 20,000 grebes to die each year of avian cholera caused by the virus, said Leslie McFarlane of the Utah Division of Wildlife Resources.
Migrating bald eagles, who are scavengers, eat the grebes and contract the illness, the newspaper said.
DaLyn Erickson, of the Wildlife Rehabilitation center of Northern Utah, has treated nine bald eagles, five of whom have died.
The four currently under her care are responding to anti-inflammatory medicines, the newspaper said.
"I wish there was a cure for West Nile but there is not," she said. "It is basically up to the bird's immune system if they can fight it off."