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Brain disease killing bald eagles traced to unknown cyanobacterium

Researchers have identified a cyanobacterium as causing deaths of bald eagles in the southern U.S., but more work needs to be done to figure out how to eradicate it from invasive aquatic plants. Photo by skeeze/Pixabay
Researchers have identified a cyanobacterium as causing deaths of bald eagles in the southern U.S., but more work needs to be done to figure out how to eradicate it from invasive aquatic plants. Photo by skeeze/Pixabay

March 25 (UPI) -- North America's bald eagles, once severely threatened by DDT exposure, have rebounded dramatically over the last 50 years.

In pockets of the American South, however, scientists have found local eagle populations suffering from outbreaks of a mysterious neurodegenerative disease, called avian vacuolar myelinopathy

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Now, nearly 30 years after an outbreak of AVM killed hundreds of eagles in Arkansas, scientists have traced the origins of the deadly disease to a previously unknown cyanobacterium that grows exclusively on an invasive aquatic plant.

The revelation -- detailed Thursday in the journal Science -- was made possible by a series of vital breakthroughs, and a fruitful collaboration between scientists in Georgia and Germany.

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Several years ago, a group of researchers led by Susan Wilde, an associate professor of aquatic science at the University of George, determined that AVM could spread throughout local food-chains, from waterfowl to birds of prey.

The outbreaks, she found, seemed to be concentrated around lakes that had been colonized by the invasive aquatic plant Hydrilla verticillata.

Upon closer examination, Wilde realized that the map of AVM outbreaks didn't perfectly match the map of Hydrilla's southern invasion.

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"Susan observed that not at all hydrilla-infested lakes had an AVM problem -- only in those where the leaves of Hydrilla were covered by a cyanobacterium, blue-green algae, which grew on the plants," study co-author Timo Niedermeyer told UPI in an email.

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"This was the first breakthrough: suspicion that a cyanotoxin might cause AVM," said Niedermeyer, an expert on cyanobacteria and a professor at Martin-Luther-University Halle-Wittenberg.

After reading a press release about Wilde's work, Niedermeyer reached out to see if he could help figure out how exactly the algae was damaging the brains of bald eagles.

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By the time Wilde and Niedermeyer began collaborating, researchers had already observed birds develop AVM after being fed cyanobacteria-tainted hydrilla leaves.

"She then sent samples to us, and we cultivated the bacterium," Niedermeyer said. "This took two years, because the bacterium grows so slowly. We sent back the cultivated bacterium to Susan, who tested it on birds -- but it was not active! This was of course very frustrating."

Wilde sent Niedermeyer more samples, and this time, Niedermeyer and his colleagues in Germany used mass spectronomy to study the cyanobacteria's chemical makeup.

"This is like taking a digital picture of the surface, but instead of light, one detects molecules," he said. "So we could identify a molecule that was co-localized with the cyano colonies on the leaves."

"This was the second break-through, because we saw that this molecule contained five bromine atoms -- and we suddenly knew why our lab cultures were not toxic -- there was no bromide in the cultivation medium," Niedermeyer said. "We added some, and the lab strain also started producing this putative toxin."

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Once scientists realized the toxin contained bromine atoms, they were able to test for it. They quickly identified the toxin in the lakes and animals where AVM outbreaks had been recorded -- the toxin was missing in healthy animals.

Because the toxin bioaccumulates, it can quickly spread throughout an ecosystem and its food web. The toxin is quite potent against fish and nematodes, and researchers estimate that it doesn't take much to kill a bird.

Because eagles prefer to target injured fish and fowl, they're especially susceptible to AVM. When the disease impairs the escape capabilities of coots, a type of waterfowl, they become an obvious target for hungry eagles.

Now that scientists know what's causing AVM, they're beginning to work on ways to stop the disease.

Because hydrilla is very hardy and can regrow from just tiny fragments, eradicating the invasive species is next to impossible.

Instead, researchers are looking for ways to stem the supply of bromide. To do that, they need to figure out where it's coming from.

"Bromide concentrations are high in marine environments, but much lower in freshwater. So where does the bromide come from?" Niedermeyer said. "There are, of course, natural sources from minerals of rocks, but we suspect that some or most of it comes from human activities."

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"One example: Hydrilla is fought using herbicides like diquat dibromide, which contain bromide," he said. "Other sources may be wastes from flame retardant industry, gasoline additives, road salts. But another significant source of bromides can come from coal-fired power plants, where bromides are used to detoxify wastes and flue gasses. These bromides can reach the waterbodies in these areas."

In addition to tracking down sources of bromide, researchers also plan to investigate the toxin's impacts on humans and mammals and to study precisely how the toxic molecules disrupts the nervous system.

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