In the winter of 1994, in just 50 days, 29 bald eagles died unexpectedly around Lake De Grey in the United States. One year later, in the winter, 26 bald eagles died abnormally at the same location.
The death continued to spread, even spreading to other birds in the same area. What is even more bizarre is what these birds looked like before they died: they seemed to be suddenly possessed by something strange, and could no longer control their bodies properly, and flew straight into the rocky cliff.
The bald eagle (Haliaeetus leucocephalus) is the national bird of the United States, and the bird of prey on the front of the US national emblem is it. | Andy Morffew / Wikimedia Commons
In the brains of out-of-control birds, people have found strange lesions that have never been seen before: large and small circular vacuoles appear in the white matter of their brains.
Who killed the American national bird? Scientists tried to chase down the real culprit, but it was not until 25 years later that the mystery was finally solved .
Bizarre death continues to spread
These two large-scale deaths of bald eagles occurred in Lake De Grey and its surrounding areas. This is a large artificial lake with a history of 22 years, located in Arkansas in the southern United States. As the incident happened suddenly, there was no detailed autopsy report for the previous two incidents.
A similar bizarre death occurred in a Bald Eagle in 2014 | Mitch Lane / Utah Division of Wildlife Resources
When scientists were worried about the cause of death of bald eagles, they discovered that this bizarre death phenomenon had even spread to other birds in the same area . In March 1996, the eagle flocks in Arkansas exhibited abnormal physiological behavior. They flew out of their habitat and directly hit the rocky cliff; three rescued eagles all died within 24 hours.
A series of unusual behaviors have also been observed in American Coots: they are tired of flying, have uncoordinated limbs, sway when flying, and stagger when walking. Scientists tried to investigate the bizarre behavior of the American Coot, but failed.
A brain disease that has never been seen before
At the end of 1996, the corpses of 24 bald eagles were sent to the Wildlife Conservation Center for testing. Researchers found that these dead bald eagles are in very good physical condition. They have developed pectoral muscles and ample subcutaneous fat; but their stomachs are empty, with only some disgusting green viscous liquid, occasionally mixed with gray feathers, which shows that Before they died, these dead bald eagles hadn’t eaten for a while.
The most amazing discovery is hidden in the brains of bald eagles-all dead bald eagles have large and small circular bubbles in the white matter of the brain spinal cord. Some appear independently, and some are like a bunch of bubbles. The grapes are connected in series. Most of these bubbles are hollow, and a few of the remaining membrane structure fragments can be observed.
The arrow shows the hole in the brain spinal cord | Reference 
This is a strange brain disorder that has never appeared before. Scientists later named it “Vacuolar Myelinopathy ” (Vacuolar Myelinopathy) . From the reservoirs where the cases appeared, the researchers recovered sediment and dead bird carcasses for preliminary chemical analysis. It turned out that compounds known at the time to induce similar diseases in mammals and birds, such as hexachlorophenol, triethyltin or brommethacin, were not detected in these samples.
In other words, what induces this new disease is a completely new unknown .
Tracking down the real culprit, twists and turns
Unable to directly find the substance that caused the death, the researchers had to return to the sudden death of poultry that spanned several years. After all-round back, they found several characteristics: almost all dead birds have occurred in the winter, deaths were first discovered in the surrounding De Gray Lake, and these species death seems there is a correlation in the food chain – —The American Coot is one of the food of the Bald Eagle.
The diseased American cocks act abnormally and are more likely to be preyed. Bald eagles themselves have the habit of eating decay and are likely to eat infected and dead prey. When researchers feed animals that have died from diseases to healthy animals, all kinds of organisms, including amphibians, reptiles, and fish, can be infected. This shows that the disease may be spread through the food chain, and the bald eagle may have died suddenly after eating a sick American Coot.
Using the food chain as a clue, scientists found “Suspect No. 1” -Hydrilla verticillata . This is a common aquatic plant in Lake De Grey. It was introduced to the United States in the middle of the last century and is an invasive alien species. It grows very fast and is extremely rich in artificial water bodies. It is one of the food for fish and some herbivorous birds. In winter, food is scarce, and vegetarian hunters gathered in the black algae ponds have successfully attracted overwintering bald eagles.
黑藻（Hydrilla verticillata）| Yercaud-elango / Wikimedia Commons
However, the hypothesis that black algae is a killer of birds has shortcomings. Although all cases of bird infections occur in waters where black algae grows; however, not all black algae-rich areas have experienced bird deaths.
In 2005, the University of Georgia professor Susan Wilde (Susan Wilde) found the “No. 2 suspects” – one kind of leaves on the black algae previously unknown cyanobacteria (Aetokthonos hydrillicola) . They found that only where the cyanobacteria invaded Hydrilla verticillata would the birds become infected with vacuolar myelin disease. In this way, this cyanobacterium seems to be the real cause of vacuolar myelinopathy.
Cyanobacteria colonies on the leaves of Hydrilla verticillata displayed by fluorescence | Susan Wilde
When the case was investigated, the scientist thought he had found the real culprit. But when they hopefully brought the cyanobacteria from the crime scene back to the laboratory, they regretted to find that the cyanobacteria cultivated in the laboratory did not make the birds sick. New hopes were suddenly shattered, and the incident seemed to have entered a dead end again. “Not only the birds are crazy, we are also going crazy.” Professor Wilde said.
Who is the real culprit?
It wasn’t until 2011 that the research team’s doctoral student Steffen Breinlinger (Steffen Breinlinger) discovered a new substance on the cyanobacteria’s epiphytic Hydrilla verticillata , and the event turned around again. This substance is called AETX (aetokthonotoxin, taken from the Greek “poison to kill eagles”) by researchers . It is a pentabromobisindole alkaloid with 5 bromine atoms in its chemical structure; and cyanobacteria The standard medium does not contain bromine, so the cyanobacteria grown in the laboratory cannot produce this metabolite.
AETX contains 5 bromines | Reference Materials 
When they added bromine to the culture medium, the cyanobacteria successfully produced this bromine-containing metabolite; direct infection experiments with this substance also successfully caused the birds to develop vacuolar myelin disease. Researchers also found AETX in American bone-top chickens suffering from vacuolar myelin disease; this substance can be absorbed from the digestive tract and accumulated in waterfowl.
After studying the chemical structure, biosynthesis, biological activity and other characteristics of AETX, researchers recently published a paper in the journal Science, proving that this cyanobacteria metabolizes the toxin AETX, which caused the death of many birds such as bald eagles. The real murderer . From the mysterious death of the Bald Eagle in 1994, to the subsequent series of mass deaths of creatures, there is finally a scientific explanation.
De Grey Lake | Armyman / Wikimedia Commons
However, the real murderer is still surrounded by clouds-under natural conditions, why would cyanobacteria synthesize this bromine-rich metabolic toxin? Where does the bromine in the water come from?
Investigations have shown that the production of this metabolic toxin may be the result of a comprehensive induction of multiple conditions. A herbicide used to fight the invading black algae is likely to play a key role because it contains bromide, which may stimulate the production of cyanobacterial toxins. In addition, seasonal temperature drops can also induce cyanobacteria to produce metabolic toxins; power plants around artificial lakes, incompletely treated factory wastewater and other human social activity factors may intensify and induce the production of toxins.
After 25 years, ATEX, the real killer of bird brain diseases, has finally been unveiled by scientists. Standing at the end of the truth, looking back at the winter of 1994, in the Degre Lake full of black algae, human beings wanted to eliminate the invading aquatic plants, but accidentally caused a bird’s death tragedy. In the interlocking nature, sometimes a trivial move may also lead to an unexpected ending.
 Steffen Breinlinger, Tabitha J. Phillips, et al. Hunting the eagle killer: A cyanobacterial neurotoxin causes vacuolar myelinopathy. Science. 371, eaax9050 (2021). DOI: 10.1126/science.aax9050
 N. J. Thomas, C. U. Meteyer, L. Sileo, Epizootic vacuolar myelinopathy of the central nervous system of bald eagles (Haliaeetus leucocephalus) and American coots (Fulica americana). Vet. Pathol. 35, 479–487 (1998). DOI: 10.1177/030098589803500602
 What is killing bald eagles in the U.S.? https://www.eurekalert.org/pub_releases/2021-03/mh-wik031921.php
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