Mysterious Bald Eagle Killer Finally Identified

In a new study out Thursday, scientists say they’ve solved the mystery of a neurological disease that’s been killing bald eagles and other birds in the U.S. 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 may be churned out in the presence of certain pollution.

In 1994, there was a mass die-off of bald eagles in Arkansas. Before death came, the predatory birds would lose their navigation skills, crashing into 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 away. Eventually, it was determined that the eagles had caught the illness from the waterbirds they preyed on that often displayed similar symptoms before death. The condition came to be known as avian vacuolar myelinopathy (AVM).

Though scientists suspected 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 fresh sources of water. By the early 2000s, a clear connection had been made between the spread of an invasive aquatic plant called Hydrilla verticillata and AVM. By 2015, researchers at the University of Georgia provided evidence that a specific species of cyanobacteria—bacteria that photosynthesize—that grows 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 called blue-green algae for the color they give off when they group together in massive numbers (despite the nickname, they’re not true algae, a vague term given to many species of aquatic plants). They’re often dangerous to animals, including people, because of the toxins they can produce. But when scientists at the University of Georgia and elsewhere tried to study A. hydrillicola in isolation, they came across a problem: The bacteria they grew in their lab were harmless to birds. They seemed only to be dangerous when growing on the plant.

In this new study, published 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. Their work indicates that A. hydrillicola only produces the toxin that causes AVM when it’s also around bromide, the negatively charged version of the element bromine.

Once they discovered this connection, the researchers were finally able to induce this toxin from their lab-grown samples of A. hydrillicola and found that it could kill birds in the same way AVM does in the wild. Genetic analysis of the bacteria also uncovered the specific bits of DNA that allow it to make the toxin. They dubbed their new discovery aetokthonotoxin (AETX), translated to “poison that kills the eagle.”

“We 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 might be solved, there are still lingering questions. Namely, where exactly is the bromide that fuels the production of this toxin coming from, and why does AVM seem to be localized only to the U.S.? Bromide exists naturally in lots of places, but it’s also seen in many synthetic chemicals that could find their way into the aquatic environment. In particular, it can be found in certain herbicides used to control the spread of the Hydrilla plant around water treatment facilities and elsewhere. So it’s possible that, in trying to get rid of one problem, we helped create a separate environmental crisis.

More research will have to be done to confirm the role of these herbicides and other human-created sources of bromide in causing AVM outbreaks, but the authors already recommend that they shouldn’t be used to control Hydrilla populations any longer. Because this toxin can accumulate in other animals besides birds, such as reptiles, fish, and amphibians, it’s also possible that it can sicken mammals, including humans.

Toxic blooms caused by algae (including blue-green algae) have already become more intense worldwide over the past few decades, and warming temperatures will likely only worsen the situation. And while AVM outbreaks have only been seen in four states to date, the scale of the problem is probably larger than what’s been officially documented.

Revealing the identity of this eagle killer is definitely cause for celebration, but stopping it will be a whole new challenge.