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Disease that kills bald eagles linked to invasive plant

Bald eagles have been dying from a neurodegenerative disease in the southern United States, but the cause of the disease has eluded scientists for more than 25 years. In a new study led by the Martin Luther University Halle-Wittenberg (MLU) in Germany, experts have now identified the source of the mysterious ailment.

The researchers have linked the unexplained deaths of bald eagles to a toxin produced by cyanobacteria that grow on invasive aquatic plants. The experts report that the problem is potentially worsened by the herbicides used to control the plants. 

The brain disease was first detected in 1994, when more than two dozen bald eagles died in Arkansas. The animals last control over their bodies and developed holes in their brains. The neurodegenerative disease was named vacuolar myelinopathy (VM), but its origin was unknown.

Researchers later found that the herbivorous prey of bald eagles were also affected by the disease. The invasive aquatic plant, Hydrilla verticillata, grows in freshwater lakes. However, there were some lakes that contained the aquatic plant where the disease was not detected. 

In 2005, Professor Susan B. Wilde of the University of Georgia identified a previously unknown cyanobacterium on the leaves of Hydrilla verticillata, which appeared to be the cause of the disease. This led to the discovery that vacuolar myelinopathy only occurs in places where the cyanobacteria had colonized the invasive plants. Professor Wilde called the bacterium Aetokthonos hydrillicola, or “eagle killer that grows on Hydrilla.”

“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,” said MLU Professor Timo Niedermeyer. He obtained samples of colonized plant leaves, cultivated the bacteria in the lab, and then sent them back to the U.S. for further testing. However, the tests came back negative, which meant the bacteria from the lab did not induce the disease. 

“It’s not just the birds that were going crazy, we were too. We wanted to figure this out.” Professor Niedermeyer once again colonized the leaves that were sent to him. Steffen Breinlinger, a doctoral student in his research group, used a new imaging mass spectrometer to investigate the molecular composition on the surface of the plant’s leaf. 

The analysis revealed a new substance that only occurs on the plant leaves where the cyanobacteria grow, but is not produced in the cultivated bacteria.

Breinlinger’s investigation into the chemical structure of the isolated molecule showed that it contains five bromine atoms. The properties are unusual for a molecule formed by bacteria.

The chemical structure of the isolated molecule also explains who the toxin did not form under laboratory conditions – standard culture media in which cyanobacteria grow do not contain bromide. 

“We then added bromide to our lab cultures, and – the bacteria started producing the toxin,” said Breinlinger. 

Further research is needed to determine why the cyanobacteria form the toxin on the aquatic plants in the first place. One of the herbicides used to combat the invasive aquatic plant contains bromide and may stimulate the production of the toxin that is killing bald eagles and other wildlife.

The study is published in the journal Science.

By Chrissy Sexton, Staff Writer

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