SALT LAKE CITY -- The number of bald eagles that appear to have died from West Nile Virus has climbed to 40, Utah wildlife officials said Monday.
Leslie McFarlane, a wildlife disease coordinator with Utah Division of Wildlife Resources, said that besides the 40 that died, five other birds were being treated in rehabilitation centers Monday.
The number of dead eagles should start to drop off as the spread of infection appears to have slowed, McFarlane told The Salt Lake Tribune.
Since Dec. 1, officials have found the birds in northern and central Utah. All were either dead or were ill and later died during treatment.
The eagles displayed similar symptoms, including head tremors, signs of seizures, weakness in legs and feet and a paralysis of the bird's wings.
Wildlife officials were not sure what was killing the animals until last week, when they received test results.
DWR said testing indicates the birds died from West Nile Virus they likely contracted after eating aquatic birds that were infected with the virus and died recently.
Eared Grebes, a duck-like aquatic bird, started arriving in Utah in October, when mosquitoes were still active.
West Nile Virus, which is spread by mosquitoes, usually infects eagles and other birds during warmer months.
Wildlife officials said the bald eagles began arriving in November and appeared to have died relatively quickly once they contracted the virus.
About 750 to 1,200 bald eagles begin migrating to Utah each November and stay until March.
DWR officials said that during those winter months, the eagles get most of their food by eating dead animals, such as grebes.