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 in the U.S. for more than 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 some contamination.
In 1994 there was a mass extinction of bald eagles in Arkansas. Before death came, predatory birds would lose their navigational skills, crashing into trees or even losing the ability to fly. And when the scientists examined their brains post-mortem, they found different lesions and holes inside, making it look like the brain had been eaten. Finally, it was determined that eagles had contracted the water from preyed waterfowl and often had similar symptoms before death. The disease was known as avian vacuolar myelinopathy (AVM).
Although scientists suspected that AVM was infectious in some way, the exact culprit remained unknown for years. Along the way, more outbreaks of AVM occurred throughout the southeastern U.S. near lakes and other freshwater sources. By the early 2000s, a clear connection had been established between the spread of a so-called invasive aquatic plant Hydrilla verticillata and AVM. In 2015, researchers from the University of Georgia always evidence that a specific species of cyanobacteria (photosynthesizing bacteria) that grows in this plant was responsible for AVM. The group put the name of the previously undiscovered species Aetokthonos hydrillicola, translated from Greek and Latin into “eagle killer, living in the hydrilla”.
Cyanobacteria are also called blue-green algae because of the color they give off when grouped in large quantities (despite the nickname, they are not true algae, an imprecise term given to many species of aquatic plants). They often are dangerous to animals, including humans, because of the toxins they can produce. But when scientists at the University of Georgia and elsewhere tried to study A. hydrophilic in isolation, they encountered a problem: the bacteria that grew in the laboratory were not harmless to birds. They only seemed to be dangerous when they grew 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 together with researchers from Germany and the Czech Republic to unravel the final pieces of the AVM puzzle. His work indicates so A. hydrophilic it only produces the toxin that causes AVM when it is also found around bromide, the negatively charged version of the bromine element.
Once they discovered this connection, the researchers were finally able to induce this toxin from their laboratory samples. A. hydrophilic and found that it could kill birds in the same way that AVM does in the wild. Genetic analysis of the bacteria also uncovered the specific pieces of DNA that allow it to produce the toxin. His new discovery was called aetokthonotoxin (AETX), translated as “poison that kills the eagle”.
“We have confirmed that AETX is the causative agent of [vacuolar myelinopathy]”, The researchers wrote in a summary of their findings.
While the exact method of killing behind AVM could be resolved, there are still persistent questions. That is, where exactly does the bromide that feeds the production of this toxin come from, and why does it seem that AVM is only located in the US? Bromide exists naturally in many places, but is also seen in many synthetic chemicals that could reach the aquatic environment. In particular, it can be found in certain herbicides that are used to control the spread of Hydrilla plant around water treatment facilities and other places. Therefore, it is possible that, in trying to get rid of a problem, we help create an independent environmental crisis.
Further research will be needed to confirm the role of these herbicides and other human-created bromide sources in causing AVM outbreaks, but the authors already recommend that they not be used to control Hydrilla populations for longer. Because this toxin can build up in other animals besides birds, such as reptiles, fish, and amphibians, it is also possible that it can make mammals, including humans, sick.
Toxic blooms caused by algae (including blue-green greens) have already become more intense all over the world over the last few decades, and warming temperatures are likely to only make the situation worse. And while so far only AVM outbreaks have been seen in four states, the scale of the problem is likely to be larger than officially documented.
Revealing the identity of this eagle killer is definitely cause for celebration, but stopping him will be a whole new challenge.