(CN) — A mysterious disease has been killing bald eagles across the southern United States for over 25 years, and scientists believe they’ve finally tracked down the culprit.
Known as vacuolar myelinopathy, or VM, the neurodegenerative disease behind these birds’ untimely deaths is caused by a type of bacteria that grows on certain plants, and it has been taking a toll on eagle populations in recent decades. Herbicides sprayed on these invasive plants are thought to exacerbate matters.
Researchers have been trying to track down the cause of the eagles' mysterious illness for years without much luck. Finally, teams from the Martin Luther University Halle-Wittenberg (MLU) in Germany and the University of Georgia believe they’ve made a breakthrough. They published their findings in a new study Thursday in the journal Science.
"The origin of the disease was a complete mystery," said Professor Timo Niedermeyer from the Institute of Pharmacy at MLU, in a related statement.
The rare neurodegenerative disease was first diagnosed in 1994 and causes the birds to develop holes in the white matter of their brains, which plays a key role in communication both inside the brain and between the brain and spinal cord. The illness has since been diagnosed in a range of other animals, such as waterfowl, birds of prey, amphibians, reptiles and fish. The unfortunate result is that its victims lose motor function throughout much of their body.
“Cyanobacteria are known to produce potent toxins, so we hypothesized that a neurotoxin produced by the epiphytic cyanobacterium causes VM,” explain the authors in their study.
This tainted food web works something like this: Cyanobacteria grow on the aquatic vegetation Hydrilla verticillata, which thrive in man-made bodies of water; the plants are then eaten by an animal such as a coot, which in turn is itself eaten by eagles and other susceptible species.
Over half of the watersheds studied in the southeastern U.S. were found to contain infected H. verticillata, but researchers only detected neurotoxicity in samples where the disease had been present. That’s when co-author Susan B. Wilde, a professor at the Warnell School of Forestry and Natural Resources at the University of Georgia, identified this mysterious cyanobacterium growing on the leaves of H.verticillata. She aptly named it “eagle killer that grows on Hydrilla,” or Aetokthonos hydrillicola.
"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 said in a related statement.
He had samples shipped back to Germany for further study, but, astonishingly, when he grew cultures of the same cyanobacterium in his lab it didn’t contain the dangerous toxin.
"It's not just the birds that were going crazy, we were too. We wanted to figure this out," added Niedermeyer.
Searching for clues, the team employed mass spectrometry imaging to analyze samples of H. verticillate collected at sites where the disease was found. The researchers studied infected leaves from the aquatic plant and dug into its structure on the molecular level. Finally, they discovered something new — the molecules they isolated contained five bromine atoms — unusual for a molecule formed by bacteria.
That turned out to be the breakthrough they were looking for after a decade of research. "We then added bromide to our lab cultures, and — the bacteria started producing the toxin," Steffen Breinlinger, a doctoral student in Niedermeyer’s research group, said in a statement.
"Finally, we did not only catch the murderer, but we also identified the weapon the bacteria use to kill those eagles," Wilde said.
The team hasn’t yet determined why cyanobacteria form on certain aquatic plants. They believe it may be due to herbicides used to eradicate the invasive plants, as the herbicides contain bromide, which may act as a trigger.
Fortunately, the debilitating disease affecting these majestic creatures hasn’t yet been found in Europe and has thus far only been reported in the southeastern United States.
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