In a new study released Thursday, scientists say they have solved the mystery of a neurological disease that has been killing bald eagles and other birds for more than 25 years. The disease appears to be caused by a poison produced by a type of blue-green algae that grows on an invasive plant – a poison that can be excreted in the presence of certain pollution.
In 1994 there was a massive die-off of bald eagles in Arkansas. Before death, the birds of prey would lose their navigation skills, crash into trees, or even lose their ability to fly. And when scientists examined their brains post-mortem, they found obvious lesions and holes inside that made it look like the brain had been eaten away. It was eventually determined that the eagles had contracted the disease from the waterfowl they preyed on and often showed similar symptoms before dying. The condition became known as avian vacuolar myelinopathy (AVM).
While scientists suspected AVM was somehow contagious, the exact culprit remained unknown for years. Along the way, more outbreaks of AVM occurred in the southeastern US near lakes and other fresh water sources. By 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 from the University of Georgia to provide evidence that a specific species of cyanobacteria – bacteria that photosynthesize – growing on this plant was responsible for AVM. The group named the previously undiscovered species Aetokthonos hydrillicola, translated from Greek and Latin to “eagle-killer, living on hydrilla.”
Cyanobacteria are also referred to as blue-green algae because of the color they give off when they flock together (despite the nickname, they are not true algae, a vague term given to many types of aquatic plants). They often are dangerous to animals, including humans, because of the toxins they can produce. But then scientists at the University of Georgia and elsewhere tried to study A hydrillicola Separately, they ran into a problem: The bacteria they grew in their lab were harmless to birds. They only seemed to be dangerous when grown on the plant.
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In this new study published In the journal Science, scientists from the University of Georgia teamed up with researchers from Germany and the Czech Republic on Thursday to unravel the last pieces of the AVM puzzle. Their work indicates that A hydrillicola produces the poison that causes AVM only when it is also around bromide, the negatively charged version of the element bromine.
Once they discovered this connection, the researchers were finally able to extract this toxin from their lab-grown samples of A hydrillicola and found that it could kill birds in the same way that AVM does in the wild. Genetic analysis of the bacteria also revealed the specific pieces of DNA that make it possible to make the toxin. They called their new discovery aetokthonotoxin (AETX), which translates to “poison that kills the eagle.”
“We have confirmed that AETX is the cause of [vacuolar myelinopathy], The researchers wrote, summarizing their findings.
While the exact method of killing behind AVM may have been resolved, there are still questions. Namely, where exactly does the bromide that fuels the production of this toxin come from, and why does AVM seem to be localized only in the US? Bromide is naturally found in many places, but is also found in many synthetic chemicals that may end up in the aquatic environment. In particular, it can be found in certain herbicides that are used to control the spread of disease Hydrilla installation around water treatment plants and elsewhere. So it is possible that by trying to get rid of one problem, we helped create a separate environmental crisis.
More research will need to be done to confirm the role of these herbicides and other man-made sources of bromide in causing AVM outbreaks, but the authors already recommend that they should not be used to Hydrilla populations no longer. Because this poison can build up in animals other than birds, such as reptiles, fish, and amphibians, it is also possible to make mammals, including humans, sick.
Poisonous blooms caused by algae (including blue-green algae) have already increased intense worldwide, and warming temperatures are likely to make the situation worse. And while AVM outbreaks have only been observed in four states so far, the magnitude of the problem is likely greater than what has been officially documented.
Revealing the identity of this eagle killer is certainly cause for celebration, but stopping it will be a whole new challenge.