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It was a mystery that went unsolved for more than 25 years..
Across the southeastern United States, bald eagles were dying.
This time, it wasn’t DDT. But no one knew what it was.
Finally, in March of 2021, researchers announced that they had found the killer:
a toxin from a recently discovered species of bacteria.
And the scientists hope that this knowledge can better protect eagles
and the ecosystems they depend on.
But how these scientists uncovered this mystery is better than any murder TV
show that you’ve ever seen, so get ready for some twists and turns.
It all begins in 1994 near a reservoir in Arkansas called DeGray Lake.
Soon after the eagles had arrived to spend the winter,
they started turning up dead. Two years later, it happened again.
This time, another overwintering species was affected as well.
Some duck-like birds called coots, a prey item of eagles, by the way,
were having trouble swimming and flying.
In necropsies, the non-human version of an autopsy,
the bodies of the affected birds looked mostly healthy.
But in their brains, the white matter looked spongey.
The axons, the skinny tubes that carry messages
between brain cells, were swollen.
And their fatty myelin wrappers were riddled with microscopic, fluid-filled bubbles.
Researchers ruled out some possibilities,
like prions, or misfolded proteins, and viruses.
And by 1998 they classified it as a new brain disease:
Avian Vacuolar Myelinopathy, or AVM.
No one had ever seen anything like it in wild birds
But it did look a lot like the damage in mammals from certain neurotoxins.
Like, from the rat poison bromethalin.
Bromethalin works by blocking mitochondria
from producing energy in the form of ATP.
Neurons use a lot of ATP to set up their signals.
And when this breaks down, they sort of fall apart.
Trouble was, testing for these and other toxins turned up nothing.
Meanwhile, the problem was growing.
AVM began showing up across the southeastern US.
Over eight winters, it killed at least a hundred bald eagles and thousands of coots,
and it sickened members of several other bird species.
There was growing evidence that the cause was a natural toxin
not a chemical that humans had dumped into the environment.
But despite years of trying, researchers couldn’t identify it.
Then, in 2005, we have our first breakthrough.
Scientists linked the poisonings to artificial bodies of water
where a certain invasive weed was growing.
And where a certain type of cyanobacteria was growing on the weed.
The nuisance weed is called Hydrilla — or waterthyme.
And these lakes had a lot of it.
Hydrilla also grew in similar lakes without these cyanobacteria.
And even though the same types of birds spent time there,
they were not getting sick. So all signs were pointing to the bacteria.
No one had seen this species before, but others like it are notorious
for making potent toxins, including ones that affect the brain.
You may have heard about toxic algal blooms that poison wildlife
and close beaches?
Well, that is actually cyanobacteria,
sometimes called blue-green algae, and they can be nasty.
Researchers suspected these new cyanobacteria were making a toxin
that accumulated as it moved up the food chain.
This is sort of like what happens with other toxins like mercury or DDT,
the pesticide infamous for thinning bird eggshells.
Like, waterbirds eat the weeds.
Then eagles eat the waterbirds.
Especially the sick ones, because they’re easy to catch.
Since eagles are at the top of the food chain,
they likely get the highest dose of toxin.
Which may have made AVM especially deadly for them.
But it affected other critters in these lakes too,
like snails, tadpoles, fish, and turtles.
Problem was, when researchers grew the cyanobacteria in the lab,
itself, turns out, no easy task — they didn’t make any toxin.
So even though there was all this evidence that they were the killer,
it was all circumstantial evidence.
Now to be clear, it’s not that the scientists were missing something.
The bacteria seem to produce very low levels of the toxin most of the time,
and they need a secret ingredient to do it.
The final clue fell into place when researchers used a super fancy tool
called atmospheric-pressure matrix-assisted laser desorption/ionization
mass spectrometry imaging.
...is what it was called.
What is it?
Well, it uses a laser to vaporize the molecules on the surface of an object.
In this case, the objects in question were bacteria-covered weeds
gathered from the wild.
Then it weighs and analyzes the molecules.
What stood out was a molecule with five bromine atoms.
Sort of like the ones on bromethalin, the rat poison.
Now, bromine isn’t all that plentiful in living things.
And it is not something you’d necessarily think to add to culture medium
if you were trying to grow cyanobacteria in a lab.
But when the researchers fed the bacteria some bromine
in the form of potassium bromide, they made the toxin!
They named it AETX — short for aetokthonotoxin,
which means “poison that kills the eagle” in Greek.
Though, as it turns out, it can kill plenty of other animals too.
And with that, the mystery was solved.
They also made some important connections
to what was happening in the reservoirs.
Like, when bacteria are stressed, they crank out a hundred times more toxin.
And they get stressed by things like water temperatures dropping
and the water being stirred up.
Researchers think those same conditions precede bird poisonings.
Plus, it turns out Hydrilla accumulates bromine from the environment.
So the cyanobacteria probably suck bromine out of the weed.
And now we know we should probably stop using a certain
bromine-containing herbicide to try to control the Hydrilla,
because it could be feeding bromine straight to the bacteria!
We can also monitor bromine in the lakes
and look for ways to make it less available to cyanobacteria.
And finally, we can look for better ways to combat Hydrilla, the invasive weed,
and the toxic bacteria.
Meanwhile, by studying how the toxin works,
we might even be able to find a treatment.
Solving this mystery should help us protect not only bald eagles,
but these entire freshwater ecosystems and the people living near them.
Decades of detective work have finally paid off.
Scientists have to be comfortable with the unknown,
they tackle it all the time.
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