Scientists discover dangerous neurotoxin is responsible for killing bald eagles

HALLE, Germany — A devastating neurological disease called vacuolar myelopathy (VM) has been plaguing bald eagles in the United States since the 1990s. The disease quickly spread across the nation, afflicting scores of amphibians and avian species, and outpacing wildlife experts’ ability to contain it. The disease seemed unstoppable until recently. In a major breakthrough, scientists from Martin Luther University Halle-Wittenberg (MLU) in Germany and the University of Georgia have confirmed that the cause of VM is a particularly nasty species of bacteria.

“The origin of the disease was a complete mystery,” says Professor Timo Niedermeyer from the Institute of Pharmacy at MLU in a university release.

For years, researchers worked to discover the cause of VM, a progressive disease leading to holes in an eagle’s brain. To solve this avian murder-mystery, they started by gathering clues. Researchers discovered that neither infectious agents nor pollution alone cause the illness. Their investigation also revealed that VM affects both eagles as well as their herbivorous prey.

Clues from prey species and Hydrilla plants

The affected prey species live in freshwater lakes that are also home to an invasive aquatic plant species called Hydrilla. However, not every lake that has Hydrilla also had cases of VM.

In 2005, Professor Susan B. Wilde from UGA’s Warnell School of Forestry and Natural Resources explained why. She identified a new species of bacteria on the leaves of Hydrilla, which appeared to cause VM. It turned out that VM only occurs in places where the cyanobacterium colonizes on the invasive plant. Wilde named the bacterium, “eagle killer that grows on Hydrilla” or Aetokthonos hydrillicola in 2014. She suspected in was a neurotoxin, a chemical that causes brain damage, but needed help to prove it.

Back in Germany, Professor Niedermeyer got word of Wilde’s discovery and initiated a collaboration.

“I stumbled across a press release issued by the university in Georgia and was fascinated by these findings, because I’ve worked with cyanobacteria for years,” Niedermeyer explains in a release.

The researcher requested samples of the bacterium in culture, then cultivated the bacteria in his laboratory and sent them back to the U.S. for further testing. To their disappointment, the tests came back negative. The bacteria from the lab did not induce the VM in birds.

“It’s not just the birds that were going crazy, we were too. We wanted to figure this out,” Niedermeyer jokes.

Identifying the bromide-containing culprit

The team headed back to the drawing board and lab with a new batch of bacteria; this time on the leaves taken from the water. A young graduate student in Niedermeyer’s lab, Steffen Breinlinger, applied a technique called mass spectrometry to investigate the composition on the surface of the plant’s leaf, molecule by molecule. Breinlinger discovered the issue. Apparently, bromine-containing molecule occurs on the leaves where the cyanobacteria grow. This molecule was not produced in the cultivated bacteria alone.

When scientists house bacteria in their labs, they follow a standard culture procedure to make media in which cyanobacteria grow. This media does not contain bromide. However, Breinlinger explained that when, “we added bromide to our lab cultures, the bacteria started producing the toxin.”

Wilde and her team then tested birds for the presence of the bromine-containing molecule. Finally, after almost a decade of research, they had proof of what causes VM.

“Finally, we did not only catch the murderer, but we also identified the weapon the bacteria use to kill those eagles,” Wilde reports.

Are more species at risk from the toxin?

Next steps for the team include understanding why the cyanobacteria is able to form the toxin on the aquatic plants in the first place. According to the study, the cyanobacterium gets the bromide it needs to make the toxin from Hydrilla, which can concentrate bromide from lake sediment in its leaves.

Bromides are relatively rare in freshwater ecosystems, but can originate from weathering of rocks, coal-fired power plant pollution, of other sources such as brominated flamed retardants fracking fluids, and road salts. Ironically, one of the herbicides used to combat the Hydrilla plant, diquat dibromide, might also contribute to the etiology of VM, as the study points out.

Saving the birds from the nefarious VM neurotoxin has already been a long fight, lasting over 25 years. However, researchers say the their work needs to continue.

“There remains a critical need for research on mammalian susceptibility to VM and on human health risks from the consumption of fish and waterbirds from VM reservoirs,” the study authors conclude.

It is important to understand how complex environmental factors interact with pollutants in general to become much broader issues that can affect human health.

The findings appear in the journal Science.