Here's What's Killing Bald Eagles in the US

More than 130 bald eagles have died from a mysterious disease; now researchers have found the cause.

Hood Canal Bald Eagle
More than 130 bald eagles have been found dead since the disease was first observed. David Reddish / 500px / Getty Images

Bald eagles first began dying around an Arkansas lake in the mid-1990s.

Their deaths were attributed to a mysterious neurodegenerative disease that caused holes to develop in the white matter of their brains as the animals lost control over their bodies. Other animals, including waterfowl, fish, reptiles, and amphibians, were soon found with the same illness.

Now, after nearly three decades, an international team of researchers discovered that the deaths were caused by a toxin produced by cyanobacteria or blue-green algae. The bacteria grows on invasive aquatic plants. It affects the animals that eat the plants as well as the predators like eagles that prey on those animals.

The results of the findings were published in the journal Science.

More than 130 bald eagles have been found dead since the disease was first observed.

“Most likely, many more have died but nobody noticed,” study co-author Timo Niedermeyer, a professor from the Institute of Pharmacy at Martin Luther University Halle-Wittenberg (MLU) in Germany, tells Treehugger.

“But it is not only eagles and other birds of prey that are affected, but also waterfowl, fish, amphibians, reptiles, crustaceans, nematodes.”

It started over the winter of 1994 and 1995 at DeGray Lake in Arkansas when 29 bald eagles were found dead. It was the largest undiagnosed mass mortality of bald eagles in the country. More than 70 dead eagles were found over the next two years.

By 1998, the disease was named avian vacuolar myelinopathy (AVM) and had been confirmed at 10 locations in six states. In addition to bald eagles, AVM has been recorded across the southeastern U.S. in various birds of prey and many waterbirds including American coots, ringnecked ducks, mallards, and Canada geese.

Lab vs. Real Life

In 2005, Susan Wilde, an associate professor of aquatic science at the University of Georgia, first identified the previously unknown cyanobacterium on the leaves of an aquatic plant called Hydrilla verticillata. Researchers dubbed it Aetokthonos hydrillicola, which is Greek for “eagle killer that grows on Hydrilla.”

Next up was identifying the specific toxin that the bacteria produced. And Niedermeyer found his way to join the team.

“Of course, it is kind of shocking in the U.S. if their iconic bald eagle dies from an unknown cause. I came to the project by chance,” he says.

“In 2010, I was still quite new to cyanobacterial natural products and wanted to learn more about their toxins. But working in industry, I didn’t have access to proper scientific literature databases. So I used Google to get a first overview.”

He came across a blog post discussing that a mysterious disease affecting the bald eagle might be caused by a cyanotoxin.

“I had loved bald eagles since I was a child and I was intrigued by the story. The cyanobacterium grows on an invasive water plant which is consumed by waterfowl, which is in turn preyed upon by bald eagles – a transmission of the putative toxin through the food chain,” he says.

Niedermeyer contacted Wilde and offered his help. He cultivated the bacteria in his lab and sent it to the U.S. for more testing. But the lab-created bacteria didn’t induce the disease.

“We then took a step back and analysed the bacteria as they grow in nature, on hydrilla plants collected from affected lakes,” he says.

They examined the surface of the plant’s leaf and discovered a new substance, a metabolite, that was only on the leaves located where the cyanobacteria grow but wasn’t found in the bacteria grown in the lab.

“This opened our eyes, as this metabolite contained an element (bromine) that was not present in our lab cultivation medium – and when we added this to the growth medium, also our lab strain started producing this compound.”

The researchers call their discovery aetokthonotoxin, which means “poison that kills the eagle.”

“Finally, we did not only catch the murderer, but we also identified the weapon the cyanobacteria used to kill those eagles,” said Wilde in a statement.

Fixing the Problem

bald eagle with drooped wings
A bald eagle's drooped wings show signs of brain infection caused by the bacteria Aetokthonos hydrillicola.

University of Georgia

Researchers don’t know yet why the cyanobacteria form on the invasive aquatic plants. The problem might be made worse by herbicides that are used to treat those plants.

“One way to fight the invasive plant hydrilla is to use a pesticide, diquat dibromide. This contains bromide, which could stimulate the cyanobacterium to produce the compound,” Niedermeyer says.

“So in a way, humans might add to the problem with the good intention to solve another problem (hydrilla overgrowth). To be honest, I do not think it is a good idea to treat whole lakes with herbicides in the first place.”

Other sources of bromide can include some flame retardants, road salt, or fracking fluids.

“However, most important in my eyes, also from the quantities of bromide released into the environment, might be coal-fired power plants, where bromides are used to treat the wastes,” Niedermeyer says. “Maybe this sounds a bit too strong, but maybe stopping burning coal might help stopping the eagles to die.”

He says it can be difficult to prevent more animal deaths.

“One important factor is studying where the bromide comes from, and then stopping this. So monitoring of waterbodies for the cyanobacterium, the toxin, and also bromide is important in the future. Also, removing hydrilla from the lakes (e.g. using grass carps) might be a good strategy to remove the host plant of the cyanobacterium.”

However, both hydrilla and the cyanobacteria are hard to kill, Niedermeyer says, and can likely be spread by boats and maybe also by migrating birds.

View Article Sources
  1. https://science.sciencemag.org/content/371/6536/eaax9050

  2. co-author Timo Niedermeyer, a professor from the Institute of Pharmacy at Martin Luther University Halle-Wittenberg (MLU) in Germany

  3. https://pubmed.ncbi.nlm.nih.gov/9823589/

  4. https://news.uga.edu/international-investigation-discovers-bald-eagles-killer/