For the last two decades, bald eagles living in and traveling through the Lower Wisconsin River Valley have become infected with and perished from a mysterious disease that causes seizures.

While searching for the origins of the Wisconsin River Eagle Syndrome, scientists discovered a new type of virus that could partially explain the strange disease. Researchers described the newly named bald eagle hepacivirus, or BeHV, this week in the journal Scientific Reports.

BeHV is a relative of the human hepatitis C virus. In humans, the virus causes liver damage, and researchers found similar effects in eagles. But scientists also found healthy eagles carrying the virus, making it difficult to directly connect BeHV and Wisconsin River Eagle Syndrome.

In the 1950s and 1960s, bald eagle numbers plummeted as hundreds perished from exposure to toxic pesticides like DDT. But improved environmental regulations in the 1970s, including the banning of DDT, helped the species rebound. Today, bald eagles are flourishing across much of North America.

In the 1990s, when biologists began documenting bald eagles vomiting, staggering and dying along the banks of the Wisconsin River, they suspected environmental toxins were to blame. Over the years, tests have turned up nothing.

Approximately 50 birds have been documented with the disease during the last 20 years. Many of the birds showed signs of liver damage, while some also suffered brain damage.

For the new study, scientists at the University of Wisconsin deployed thorough genetic sequencing tests designed to identify new viruses. The team of researchers, led by Tony Goldberg, analyzed liver tissue from nine birds diagnosed with WRES. Their tests revealed the presence of a virus family not previously observed in birds.

When researchers looked for the virus in other eagles from across the country, they found it, but in smaller numbers. Eagles in Wisconsin were more likely to be found with the virus, and eagles near the Lower Wisconsin River were even more likely to be BeHV-positive.

However, none of the bald eagles from other states with the virus showed signs of WRES.

Scientists aren't sure whether there's a connection between the virus and disease. It's possible that most eagles infected with the virus perish before the telltale signs of WRES appear, they said.

Eagles gather along the Wisconsin River because its open waters remain mostly unfrozen during the winter, offering prime hunting conditions. It's possible the region's environmental advantages allow eagles in the area to survive for longer, increasing the odds that the disease progresses and allowing scientists can make a WRES diagnosis.

"It is curious that the liver pathology of the eagles resembles the damage to human livers caused by hepaciviruses," said study co-author LeAnn White, a scientist with the USGS National Wildlife Health Center White. "But since the lesions are nonspecific there's more that needs to be explored if we want to understand the virus itself or really get at what is the cause of WRES."

The study's authors say the disease does not threaten the continued resurgence of the iconic bird.

"We don't think this virus is having a serious impact on the bald eagle population, but the fact that WRES is an unknown condition keeps our interest," said co-author Sean Strom, an environmental toxicologist with the Wisconsin Department of Natural Resources. "This study is another piece of the puzzle. Hopefully we can find more pieces and figure out what is happening."

"This study has opened our eyes to glaring knowledge gaps about infection in a species of great national importance," says Goldberg. "It's a more complicated story than we thought it might be at first, but that makes it more interesting."

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