American bald eagles are dying, and scientists may finally know why

The bald eagle has been through a lot in the past few decades: Once a species on the brink of extinction, it’s bounced back thanks to conservation efforts.

The living symbols of the United States made it off the endangered species list, but it isn’t totally in the clear yet. Researchers have discovered a new virus in the bird, and it’s infecting nearly one-third of the bald eagles in North America, says a study published Friday in the journal Scientific Reports.

Bald eagle hepacivirus (BeHV for short) is a distant relative of hepatitis C in humans. The researchers found it when they were studying Wisconsin River Eagle Syndrome (WRES), a disease that has plagued parts of Wisconsin since the 1990s.

WRES first turned up near its namesake, the Wisconsin River, around 1994. Bald eagles started showing symptoms of weakness, incoordination, tremors, vomiting, and seizures.

“It was horrible,” Tony Goldberg, Ph.D., lead author of the study and a professor of epidemiology at the University of Wisconsin-Madison, tells Inverse.

“We’d get calls from the public or local veterinarians that eagles were stumbling around, vomiting, or having seizures. They’d be raced into veterinary hospitals but they’d never make it.”

Researchers at the National Wildlife Health Center in Madison, Wisconsin, began the race to figure out what was happening. Twenty years earlier, the federal government had banned the insecticide DDT, which had devastated populations by thinning eagle eggs. Studying potential toxins, however, left researchers empty-handed.

“It just kind of sat on the shelf for years,” Goldberg says.

New gene sequencing technology helped Goldberg and his colleagues uncover new clues, including the new virus, BeHV. Rather than traditional tests to look for specific conditions, the researchers used “shotgun sequencing,” or looking at all the DNA and RNA in a sample.

The new technology has helped researchers discover all kinds of new viruses. Goldberg says that when he’s looking in a new place, he usually expects to find between 3 and 10 new viruses.

“We’re kind of in a global age of discovery,” Goldberg says. “Virus hunting is all the rage.”

But discoveries in the Hepacivirus genus, like BeHV, don’t happen every day. And in this case, the new discovery has potential links to the deadly, mysterious disease near Wisconsin River.

Researchers found BeHV cases all over the United States, from Florida to Washington state. In Wisconsin, the only state where WRES is a concern, eagles are about 10 times more likely to be infected with BeHV. For eagles in counties where WRES has been diagnosed, the virus is 14 times more likely.

Those patterns raise questions about what role the virus plays, if any.

“If the virus is causing this, why isn’t it causing it everywhere?” Goldberg wonders.

He has a few theories: Conditions for bald eagles are particularly friendly in Wisconsin, so it’s possible that birds survive until the last stages of the infection, meaning more of them are discovered. It’s also possible that the virus weakens birds — many of them die from traumas like electrocution or being hit by cars — putting them at risk, particularly in areas outside of Wisconsin.

BeHV may also be completely unrelated to to the puzzling Wisconsin disease.

“Even if it’s not the virus in the end, it gives us an entry point,” Goldberg says. “We can use it as a way to understand eagle health in general.”

In the past, concerns around toxins have dominated conversations about bald eagles, and for good reason. “The banning of DDT and the recovery of the bald eagle is one of the great conservation success stories of our nation,” Goldberg says.

With the resurgence of populations, a notable success, new research opportunities can take on a wider scope: things like infectious disease, genetics, climate change, and breeding habitat.

“We don’t have to be so laser-focused on toxins,” Goldberg says.

Researchers now have the basis — the freedom, even — to better understand the various factors important to keeping America’s national bird around.

Abstract: The bald eagle (Haliaeetus leucocephalus) once experienced near-extinction but has since rebounded. For decades, bald eagles near the Wisconsin River, USA, have experienced a lethal syndrome with characteristic clinical and pathological features but unknown etiology. Here, we describe a novel hepacivirus-like virus (Flaviviridae: Hepacivirus) identified during an investigation of Wisconsin River eagle syndrome (WRES). Bald eagle hepacivirus (BeHV) belongs to a divergent clade of avian viruses that share features with members of the genera Hepacivirus and Pegivirus. BeHV infected 31.9% of eagles spanning 4,254 km of the coterminous USA, with negative strand viral RNA demonstrating active replication in liver tissues. Eagles from Wisconsin were approximately 10-fold more likely to be infected than eagles from elsewhere. Eagle mitochondrial DNA sequences were homogeneous and geographically unstructured, likely reflecting a recent population bottleneck, whereas BeHV envelope gene sequences showed strong population genetic substructure and isolation by distance, suggesting localized transmission. Cophylogenetic analyses showed no congruity between eagles and their viruses, supporting horizontal rather than vertical transmission. These results expand our knowledge of the Flaviviridae, reveal a striking pattern of decoupled host/virus coevolution on a continental scale, and highlight knowledge gaps about health and conservation in even the most iconic of wildlife species.