Throughout the month of December scientists have been working to determine the cause of twenty-nine bald eagle deaths in the state of Utah. Many of the raptors were found with leg paralysis and tremors prior to death. The deaths have been reported from the northern and central parts of Utah where between 750 and 1,200 of the birds migrate from Alaska and Canada during winter months.
The bald eagle is widely known as the national animal of the United States of America. It was on the verge of extinction during the late 1900s, but was removed from the Endangered Species List in 2007 after numbers rebounded partially due to the banning of the pesticide DDT.
The recent die-off had been suspected to be due to a toxin or infectious disease such as avian cholera or erysipelas (a bacterial infection that affects wild birds). However, on December 31, officials with Utah’s Division of Wildlife Resources announced that laboratory results show the deaths are likely due to West Nile virus. Laboratory tests have also indicated that West Nile virus is likely to blame for the deaths of an estimated 20,000 water birds known as grebes since November in the area around Utah’s Great Salt Lake.
West Nile virus was first discovered in Uganda in 1937, and appeared for the first time in the United States in 1999. The disease is acquired through the bite of an infected mosquito. Scavenger birds, including bald eagles, are capable of acquiring the virus after feeding on infected sick or dead birds. It is believed that the bald eagles acquired the disease by feeding on dead grebes infected with West Nile virus. The sick birds do not pose a threat to humans, however officials advise against handling any dead animals bare-handed.
It is unusual for such high numbers of deaths due to West Nile virus to be seen during winter, when mosquitoes are not usually active. The number of bald eagle deaths in Utah since December 1 is unprecedented, and investigations continue to look into contributing factors that may have led to the die-off. According to Utah’s Wildlife Disease Coordinator, Leslie McFarlane, "Eagles have been known to feed on birds infected with West Nile virus but the transmission hasn't happened on this large of a scale. And the total number of birds we're talking about is on a grand scale that may not have been seen before."