What Is Killing Wisconsin’s Bald Eagles?

For 25 years researchers struggled to find the culprit behind a mysterious illness plaguing eagles around the Wisconsin River. Finally, a clue emerges.

When Tony Goldberg received an exuberant, enigmatic text message—“You gotta come into the lab!”—the epidemiologist turned his car around and headed straight back to his office at the University of Wisconsin, Madison. He found his postdoc, Sam Sibley, transfixed by the computer monitor. Sibley had just finished running blood serum from a long-dead Bald Eagle through a powerful machine that searches out all traces of genetic material. Comparing the results to a database of all the world’s known viruses, the computer had spit back a surprising match. 

Peering at the screen, Goldberg realized the viral code displayed before him was something completely new. Bizarrely, it appeared to be a relative of human hepatitis C, an infectious disease that causes liver damage. At the time, in 2017, a virus of this type had never been found in a bird before.

It was the first break in a case that’s perplexed wildlife investigators for 25 years. Starting in the 1990s, a strange illness called Wisconsin River eagle syndrome suddenly began afflicting Bald Eagles living around the Wisconsin River. Winter after winter, the eagle plague returned, killing every bird known to contract it and defeating every scientist who tried to find its source.“This is like an episode of ‘House’ for veterinary medicine,” Goldberg says. “It’s a mystery disease that came through the door—that people threw the kitchen sink at—and no one, despite a lot of effort, has been able to figure it out.”

Standing in the fresh glow of discovery, his fingers tingling, Goldberg remembers basking in “this grand vision of having solved the whole thing.” Months later, however, he realized that the puzzle was vastly more complicated than he originally supposed. Additional testing revealed that the strange virus occurred not just in Bald Eagles in Wisconsin, but also in ones scattered around the country, where the disease had never been recorded. Like so many before him, Goldberg had been sucked into the deepening mystery of Wisconsin River eagle syndrome.


arge Gibson knows sick eagles. In 1993 she and her late husband, Don, opened the Raptor Education Group, Inc. (REGI) in Antigo, a town three hours north of Madison. REGI was one of the first state and federally permitted rehabilitation centers for endangered native birds, including Bald Eagles, in Wisconsin, and the couple was soon inundated with patients. Bald Eagles had been making a comeback since the pesticide DDT was banned in 1972, and Wisconsin’s fish-filled waters and tall riverbank trees provide some of the best habitat for the species in the country. The raptors will eat virtually anything, though, and their affinity for roadkill, carrion, and garbage makes them particularly prone to car accidents and poisonings.

Gibson, who has been working professionally with birds for 45 years, could usually tell right away what ailed her patients. An eagle virtually choking on its own saliva, rhythmically convulsing with strong muscle contractions, and experiencing rapid eye movements had probably consumed a carcass laced with an organophosphate pesticide. A bruised bird bleeding from the mouth likely ate something poisoned with an anticoagulant. As Gibson says, “Every toxin has a thumbprint.”

In the mid-1990s, though, Gibson began receiving Bald Eagles with a combination of symptoms she’d never seen before. The birds were suffering from seizures but none had head injuries. They were throwing up but their bodies seemed to be in good condition. They were also stumbling, had virtually no muscle control, and were extremely weak. Some people told Gibson that the eagles had literally fallen out of the sky, right in front of them. Gibson was dubious at first, but she kept hearing the same story over and over again.

The Bald Eagles that REGI took in were male and female, young and old. Gibson could discern no apparent pattern, save for geography and timing. Strangely, most of the birds came in during the dead of winter. Most were also found near Sauk City, a town situated on the Wisconsin River just north of Madison, famous for the many Bald Eagles that congregate there to fish when most other water bodies are frozen.

Most distressing and confounding of all, the mystery affliction proved 100 percent fatal. “Quite honestly, we’re just not used to losing patients like that,” Gibson says. “Nothing we did made any difference other than making them comfortable in their last hours. It was gut-wrenching.”

Gibson’s husband, a retired human pathologist, agreed that something was seriously off. For several years, the cash-strapped and harried local wildlife authorities did not investigate. But the cases kept piling up, and Wisconsin’s Department of Natural Resources (DNR) eventually took note. “At that time, eagles were still listed as an endangered species, so there was a heightened concern and urgency in trying to figure out what was going on,” says Sean Strom, an environmental toxicologist at the DNR who was hired after the department had launched into action. “The mystery of it, the concentration of cases especially in this area—it was very unique.”

“A lot of people who get into this field like the murder mystery side of things.”

To expedite the hunt for answers, in 2001 the DNR formed a task force whose members included Gibson’s nonprofit organization, the U.S. Fish and Wildlife Service, and the USGS National Wildlife Health Center. Located in a squat brick building in the suburbs of Madison, the center is the country’s only federal lab dedicated to detecting, controlling, and preventing diseases in wild animals. Its experts regularly apply scientific sleuthing to determine the cause of death for carcasses shipped from all over the country to “the CDC of wildlife,” as the center is sometimes called. “A lot of people who get into this field like the murder mystery side of things,” says LeAnn White, acting deputy director at the  National Wildlife Health Center.

White and her colleagues are rarely stumped by a case, but in Wisconsin River eagle syndrome, they seemed to have met their match. The scientists started their detective work, as usual, by opening up the eagles under the fluorescent glow of their level-3 biohazard lab, located in the basement. After skinning the birds and assessing their fat content for nutritional status, they removed each organ to examine it for abnormalities. In necropsy after necropsy, the only thing out of the ordinary that they consistently noticed was that some birds’ livers were a bit paler than normal, a nonspecific symptom that could indicate any number of causes, including starvation and exposure to toxins.

The seizures and stumbling seen in birds with the syndrome seemed characteristic of a toxin, so pathologists took tissue samples from the birds’ brains, livers, kidneys, and spleen. They tested them for more than 20 possible contaminants, from amphetamines and nicotine to lead and heavy metals. Each result came back negative. Undeterred, they searched the birds’ tissues for the usual—and, eventually, the rare—microbial suspects, among them avian influenza, Newcastle disease, and West Nile encephalitis. “This was years of work,” says Valerie Shearn-Bochsler, a veterinary pathologist at the center. “It was super frustrating.”

Gibson estimates that she treated a few hundred birds suffering from Wisconsin River eagle syndrome over the past 25 years, but only a fraction ever made it to the lab for testing. All told, Shearn-Bochsler and her colleagues performed necropsies on 28 eagles suspected of having Wisconsin River eagle syndrome, representing a little more than 10 percent of all Wisconsin Bald Eagles they received from 1995 to 2018. All but three of the 28 had been found within a mile or so of the Wisconsin River. Over the years the number of cases fluctuated, but to everyone’s relief, they now seem to be tapering off, to the point that none were officially recorded in 2019. Gibson, however, believes that she had five or six cases last year, although all remained unconfirmed due, in part, to budgetary constraints that limited further necropsies.

No one knows why Wisconsin River eagle syndrome seems to be fading, or if it will come surging back. Experts also cannot say whether it will ever pose a threat to wider Bald Eagle populations. Despite the syndrome’s decline, for those who struggled for years to pinpoint its cause, it continues to be a source of perplexing and nagging frustration. It likely would have remained just that—a case with no leads, a question mark among an otherwise satisfying list of answers—were it not for Goldberg’s curiosity and drive.


iruses can be hard to grow outside of their host, making it difficult to study them in the same way as other pathogens. Most often, scientists infer their presence through the clinical signs they cause, the antibodies they trigger, or, more precisely, their genomes. As Goldberg says, “In the end, classifying viruses is genetics.”

Until recently, virus hunters looking for novel species had to settle on time-consuming methods that often failed to yield results. But technological advances have made it possible to analyze vast amounts of genetic data incredibly quickly. In a matter of days or hours, a method called deep sequencing can produce tens of millions of reads of the DNA or RNA contained in a single sample. Algorithms then compare the code to databases of known viruses to look for similarities. Just a 20 percent overlap will trigger an alert.

“It’s like pulling a shotgun rather than a precision rifle,” Goldberg says. “It’s turning the problem on its head by blindly sequencing as much DNA as possible and then letting the computer tell us if there’s something there.”

Goldberg had recently splurged on a $90,000 next-generation sequencer and assembled his own database of 40,000 virus genomes for analyzing results—and he was eager to try his new toys out in the field. “I was looking for a slam dunk, to be honest,” he says. Cracking one of the National Wildlife Health Center’s unsolved cases seemed like a good place to start, and White welcomed the offer. In an initial meeting, he flipped through a thick gray binder whose table of contents read like wildlife-themed Nancy Drew plots, from the baffling “pond of death” that killed a flock of swans and gulls to the peculiar case of the Alaska polar bears that lost their hair.

Goldberg—whose freezers are typically packed with samples from far-flung destinations like Uganda, Malaysia, and Chile—latched on to Wisconsin River eagle syndrome. He liked the idea of working on something much closer to home, and he felt compelled to take a stab at this most vexing of cases. “This isn’t just any bird: It’s the national bird of the United States,” he says. “It was screaming for an answer.”

Immediately, though, the team hit a hitch: They struggled to simply find enough samples to run the study. Wildlife agencies are notoriously skint, and the National Wildlife Health Center is no exception. Space is limited, and over the years, most samples from the 28 afflicted eagles had been used up in testing for various diseases and toxins, including liver samples, as that organ consistently showed damage. “It would have been nice to have paired serum and tissue from multiple organs from every eagle we ever tested, but we had to make do with what was in the freezers,” says White.

They focused on brain samples because afflicted eagles show neurologic symptoms, meaning whatever is causing the syndrome might be in the brain. Serum samples are useful because viruses often travel around the body through the bloodstream. 

What did remain in freezers was squirreled away, willy-nilly, and Goldberg spent the better part of a year decoding decades-old records and digging for the matching decades-old samples. “The joke was that it was much easier to find the virus in samples than to find the samples in the freezer,” he says.

Samples finally in hand, it then became a straightforward matter of grinding them up, processing them through the deep sequencer, and plugging the results into the computer. When Sibley, Goldberg’s ecstatic postdoc, found a new virus species related to hepatitis C in an infected eagle’s blood serum, everyone hoped that the case could swiftly be closed. It seemed plausible that this was the culprit, especially since liver damage of the sort caused by hepatitis C in humans was usually the only visible sign of disease uncovered in the eagle necropsies. But the mere presence of the virus wasn’t enough to definitively assign causation—not even close, given what else Goldberg found.

Goldberg and his team knew birds with Wisconsin River eagle syndrome have abnormal livers, so they collected liver samples from 47 other Bald Eagles. Eight of the birds were from Wisconsin, although not all had died from the syndrome. The remaining raptors hailed from other states and had been killed by the usual things that kill eagles: lead poisoning, cars, electrocution, starvation, and illegal hunting.

Fourteen eagles—a third of those tested—came back positive for Bald Eagle hepacivirus, as Goldberg called the new bug. Shockingly, included among the infected birds were ones that lived as far apart as Florida and Washington State. Wisconsin, however, still appeared to be the epicenter. Bald Eagles there were more than nine times more likely to have the virus, and those from counties where birds had been diagnosed with the syndrome were nearly 15 times more likely to test positive.

The curveball result presented a perplexing question: If the virus is present in Bald Eagles around the country, then why are birds in Wisconsin the only ones suffering from the syndrome? Goldberg does not have an answer, but there are a range of possibilities. Least satisfyingly, it may be that the virus is a red herring. “Birds outside of Wisconsin have it and are not sick, so it’s still possible the virus is just an interesting finding that doesn’t relate to being sick at all,” Goldberg says.

The reason the syndrome only reveals itself in Wisconsin is a matter of speculation at this point. One of Goldberg’s hunches, though, is that the virus may be weakening eagles and, in the process, making them more susceptible to getting hit by cars, illegally shot, or starving (as they’re weak and confused and not able to hunt). In Wisconsin, because the habitat is so good, it could be that Bald Eagles are able to survive with the infection until it reaches its end stage. It’s akin to humans living longer and now increasingly dying of old-age-related diseases, Goldberg says. “Everyone dies of something, so what gets you in the end is what’s left after you’ve fixed the other stuff.”

Since publishing the findings in October 2018 in Scientific Reports, multiple eagle researchers from around the United States and Canada have reached out to Goldberg, including Myles Lamont, a wildlife biologist and conservation consultant in British Columbia. Lamont regularly traps eagles to affix transmitters and take blood samples as part of the Bald Eagle Tracking Alliance, a multi-institution effort to better understand the health, ecology, and movements of the birds in British Columbia’s Lower Fraser Valley.

One limitation of Goldberg’s study was it mainly relied on samples that came from Bald Eagles in the care of rehabbers. “Given that we deal mostly in wild-caught individuals, there were some question marks I thought our population could help to address,” Lamont says, including whether the virus has reached western Bald Eagles and whether non-compromised birds are infected. Lamont also studies eagles that both congregate in groups and live as relative loners, which could help Goldberg understand if interactions among birds contribute to viral transmission. For now, COVID-19 has put a kibosh on both Lamont’s trapping efforts and Goldberg’s plans to visit British Columbia to analyze the samples. But Lamont is hopeful that the collaboration will happen in the future.

This type of follow-up study would be costly and time consuming, but Goldberg believes it’s worthwhile. Wisconsin River eagle syndrome may not pose a threat to eagle populations today, but climate change—which is projected to impact Bald Eagles through increased wildfires and spring heat waves that endanger young chicks—or other new stressors could change that.

On a February day in Prairie du Sac, onlookers braving the 30-degree Fahrenheit cold gathered with binoculars on the edge of the Wisconsin River, at a popular eagle-spotting sight. Beneath the drab sky, the water was slate gray, the hills snow-laden and still. Suddenly, someone pointed—a Bald Eagle! Perched on a tree in a small island in the middle of the river, silhouetted against the barren landscape, was our national bird. It looked like it was doing just fine. Goldberg, Gibson, and others will be holding their breaths, hoping the eagles stay this way. If the syndrome does resurge, they won’t face it with all of the answers, but at least now they will not be starting from scratch.

Neel Dhanesha wrote the Wildlife CSI cases. This story originally ran in the Winter 2020 issue as “What's Killing Wisconsin's Bald Eagles?”​ To receive our print magazine, become a member by making a donation today.