Over 50 eagles found dead from a new disease in Arkansas in the late 1990s mystified wildlife ecologists. More than two decades later, scientists believe they have discovered the source of vacuolar myelinopathy, now referred to as “the eagle killer.”

Vacuolar myelinopathy is a neurological disease most commonly seen in bird species such as eagles and coots. The brains of the infected organisms develop lesions in the white matter of the nervous system, causing a loss of motor function. Mortuary circles call this “Swiss cheese brain.”

From its first recorded instance near DeGray Lake in Arkansas during the winter of 1994-1995, researchers, including aquatic scientist Susan Wilde, conducted field and laboratory studies to determine how species contract this disease. Through laboratory and field experimentation, Wilde and her team have recently concluded that a toxin produced by cyanobacteria is growing on invasive plant species within water bodies in the southeastern United States.

Wilde, an associate professor at the University of Georgia, worked with an international team from various scientific backgrounds. Their findings, published in Science, discovered the epiphytic cyanobacteria (Aetokthonos hydrillicola) grow on Hydrilla verticillata, a non-native plant species found in bodies of water. Wildlife, such as fish, birds, and amphibians, eat these plants and consume the neurotoxin that leads to vacuolar myelinopathy.

Vacuolar myelinopathy is not limited to the waterfowl that feed on these plants, explained Anton J. Reiner, a professor of anatomy and neurobiology at the University of Tennessee Health Science Center.

“It’s not a uniquely avian brain disease. It winds up looking like a uniquely avian brain disease because you have to consume a lot of it,” said Reiner.

Vacuolar myelinopathy also affects fish and amphibians that end up eating these cyanobacteria-infected invasive plants. Fish, amphibians and waterfowl that feed on Hydrilla become infected with vacuolar myelinopathy. Higher-level predators such as coots and eagles consume the smaller animals, thus continuing the spread of the disease throughout the entire food web.

“So-called ‘apex predators’ like eagles, for example, wind up being especially susceptible to [vacuolar myelinopathy] because of their diet,”Reiner said.

The disease manifests approximately five days after exposure when clinical signs of loss of motor control function become visible in avian species like eagles and coots. Infected coots will float on the surface of the water on their backs and spin in circles, making them easy prey for eagles and other large avian predators. Infected eagles will sit on tree branches with their wings drooping downward. According to Reiner, there are instances of eagles flying into the sides of mountains while infected with vacuolar myelinopathy.

“Every eagle I’ve ever seen with symptoms [of vacuolar myelinopathy] has died,” said William Bowerman, professor and chair of the Department of Environmental Science & Technology at the University of Maryland.

Bowerman is a long-time collaborator of Wilde’s on vacuolar myelinopathy, and his current research revolves around eagles as indicators of climate change and contaminants around the Great Lakes. He explained that birds are quality gauges of how the environment responds to various stressors, so the conservation of avian species from vacuolar myelinopathy impacts everyone.

“[Vacuolar myelinopathy] killed tens of thousands of waterfowl and over 100 bald eagles. So, it’s a new, emerging disease, and it could be linked to pollution and also may be somewhat of an indicator of climate change,” said Bowerman.

Wilde and her team discuss spreading awareness and advocacy as a means to combat vacuolar myelinopathy in their recently published research. Early in the paper, they state that the shift from the previous name of the neurological disease, “avian vacuolar myelinopathy,” to its current title “vacuolar myelinopathy” is necessary as it is not only limited to avian species. Wilde and her team state that they are not yet sure of the potential impact vacuolar myelinopathy has on humans and recommend that further research be conducted.

“We want people to know the lakes where this disease has been documented and to use caution in consuming birds and fish from these lakes,” Wilde told the American Association for the Advancement of Science.

While there is still plenty of research to be done on vacuolar myelinopathy, Wilde and her team’s discovery of the link between the cyanobacterial toxins and the lethal neurological disorder helps lie to rest the source of the perplexing avian deaths. Bowerman hopes that Wilde’s work will serve as a reminder of the importance of conserving a clean environment.

“It’s just important to understand that our organisms in the environment tell us what’s going on,” Bowerman said. “If the birds and the mammals are healthy, then people are healthy.”