In a new study conducted on Thursday, scientists say they have solved the mystery of a neurological disease that killed bald eagles and other birds in the US for over 25 years. The disease appears to be caused by a toxin produced by a species of blue-green algae that grows on an invasive plant – a toxin that can be produced in the presence of a certain pollution.
In 1994, a massive bald eagle death occurred in Arkansas. Before death, birds of prey lost their navigational skills, hitting trees or even losing their ability to fly. And when scientists examined their brains post-mortem, they found distinct lesions and holes inside, making it look like the brain had been eaten. Finally, it was established that eagles caught the disease from the waterfowl they preyed on, which often showed similar symptoms before death. The condition became known as avian vacuolar myelinopathy (AVM).
Although scientists suspected that AVM was infected in some way, the exact culprit remained unknown for years. Along the way, several outbreaks of AVM have appeared throughout the southeastern United States near lakes and other fresh water sources. In the early 2000s, a clear link had been made between the spread of an invasive aquatic plant called Hydrilla verticillata and AVM. By 2015, researchers at the University of Georgia given evidence that a specific species of cyanobacteria – photosynthesizing bacteria – that grows on this plant was responsible for AVM. The group named previously undiscovered species Aetokthonos hydrillicola, translated from Greek and Latin into “eagle killer who lives on the hydra”.
Cyanobacteria are also called blue-green algae for the color they give when grouped in massive numbers (despite the nickname, they are not true algae, a vague term given to many species of aquatic plants). They are often dangerous animals, including humans, because of the toxins they can produce. But when scientists at the University of Georgia and elsewhere tried to study A. hydrilicola In isolation, they encountered a problem: the bacteria they grew in their laboratory were harmless to birds. They only seemed to be dangerous when growing on the plant.
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In this new study, published On Thursday in the journal Science, scientists at the University of Georgia worked with researchers in Germany and the Czech Republic to unravel the final pieces of the AVM puzzle. Their work indicates that A. hydrilicola produces the toxin that causes AVM only when it is in and around bromide, the negatively charged version of the bromine element.
Once they discovered this link, the researchers finally managed to induce this toxin from their laboratory-grown samples. A. hydrilicola and found that it could kill birds just like AVM in the wild. Genetic analysis of bacteria also revealed specific bits of DNA that allow it to produce the toxin. They called their new discovery aetochthonotoxin (AETX), translated as “poison that kills the eagle.”
“We have confirmed that AETX is the causal agent of [vacuolar myelinopathy]”The researchers wrote in a summary of their findings.
While the exact method of killing behind AVM could be solved, there are still persistent questions. Namely, where exactly is the bromide that fuels the production of this toxin and why does it seem that AVM is located only in the US? Bromide exists naturally in many places, but is also observed in many synthetic chemicals that could enter the aquatic environment. In particular, it can be found in certain herbicides used to control the spread Hydrilla installed around water treatment plants and elsewhere. So, trying to get rid of a problem may have contributed to the creation of a separate environmental crisis.
More research will need to be done to confirm the role of these herbicides and other man-made bromide sources in causing AVM outbreaks, but the authors already recommend that they not be used to control Hydrilla populations more. Because this toxin can accumulate in animals other than birds, such as reptiles, fish, and amphibians, it is also possible to make mammals, including humans, sick.
Toxic blooms caused by algae (including blue-green algae) have already become more common intensive all over the world in recent decades, and warming temperatures will only make matters worse. And, although AVM outbreaks have been observed in only four states so far, the scale of the problem is probably greater than has been officially documented.
Revealing the identity of this eagle killer is certainly a reason to celebrate, but stopping it will be a whole new challenge.