Famously, the United States of America once nearly allowed its own national bird to go extinct. In 1963, fewer than 500 breeding pairs of bald eagles (Haliaeetus leucocephalus) survived, following years of poisoning with DDT insecticide.
Since agricultural DDT was banned, they have bounced back to more than 16,000 pairs. Understandably, potential threats to these avian icons get taken seriously.
One such threat is a mysterious disease that was first documented in the 1990s along the Wisconsin River, where eagles gather year-round because of the rich fishing opportunities. Wisconsin River Eagle Syndrome (WRES) causes weakness, stumbling, tremors, vomiting, seizures, brain damage and, rapidly, death, although it is not considered an immediate threat to bald eagle populations.
Its cause has remained elusive. But scientists led by Tony Goldberg of the University of Wisconsin–Madison have now identified a virus whose distribution largely coincides with the reported cases of the syndrome.
Bald eagle hepacivirus (BeHV) is closely related to the human hepatitis C virus. Eagles in Wisconsin are nine times more likely to carry BeHV than birds from other states, and within Wisconsin, birds in regions where WRES has been reported are 14 times more likely to harbour the virus.
But it is still not certain that BeHV is the cause. The scientists have also found the virus in healthy birds without symptoms of the syndrome and in areas where the disease has not been recorded. Proving it would require monitoring healthy birds to see if infection leads to symptoms, says Goldberg.
“We are focusing on places where eagles appear very healthy and other places where populations have not been doing so well, and we’ll compare infection rates between those locations over time. I’ve been contacted by many people across North America who are studying eagles, so my colleagues here in Wisconsin and I are trying to assemble an ‘eagle health strike force’ to investigate the new virus and others that may be circulating.”
It may be that WRES is not even such a new threat, and that we are only seeing the disease now because of the eagles’ success in recent decades. “In a sense, the recovery of eagles from declines due to DDT has opened a window into other causes of eagle mortality.”
Main image: Bald eagle in Alaska. © Keren Su/Getty