What Killed the Eagles? EPA Researchers Help Solve 25+ Year Mystery

A bald eagle.

For the past 27 years, scientists have struggled to understand the cause of unprecedented bald eagle deaths in the southeastern U.S. After decades of exhaustive efforts to pinpoint the cause, EPA researchers helped determine that the eagles contracted a neurological disease from ingesting a toxin produced by a species of cyanobacteria.

In 1994, scientists in DeGray Lake, Arkansas, found that mass numbers of bald eagles were dying from a disease that caused significant physical impairment. The diseased birds suffered from poor coordination while walking, swimming, and flying and often died from drowning, starvation, or injury.

By 1998, similar impairments were found in waterfowl and other birds of prey at 10 sites across six states. Scientists examined the brain tissue in these dead birds and found lesions in the brain and spinal cord. These lesions around the myelin sheath, or white matter, impact communication to nerve cells, impairing sensory, motor, and cognitive functions. The scientists termed this disease avian vacuolar myelinopathy (AVM).


The cause of AVM continued to elude scientists for decades while its occurrence spread throughout freshwater reservoirs in the southeastern U.S. The source of this disease remained a mystery until scientists started to focus on environmental conditions in AVM-positive water bodies.

Recently EPA scientists, along with academic researchers, helped identify that AVM-infected birds ingested a neurotoxin that is produced by a species of cyanobacteria, which grows on an invasive aquatic plant. The scientists conducted field sampling and lab studies of several water sources, aiming to create conditions that produced the toxin so they could determine the cause. They used mass spectrometry imaging, a tool to visualize the spatial distribution of molecules, to confirm that the cyanobacterium, Aetokthonoshydrillicola, does not grow in laboratory conditions. Instead, the scientists proved that A. hydrillicola only grows on the invasive plant, Hydrilla verticillata, at confirmed AVM sites. These AVM-positive sites occur in human-made water bodies.

However, the scientists also detected an unusual feature in the toxic bacteria: the presence of bromine. Bromine is a chemical element and it can be an indicator of pollution when found in the environment. While bromine can occur naturally, it’s scarcely found in freshwater. Instead, it’s usually introduced into nature through anthropogenic, or human-caused products, such as pesticides, dyes, gasoline, and plastic casings for electronics. Bromine is also rarely found in cyanobacteria. This unusual occurrence of bromine led scientists to suspect that the cyanobacteria produced toxins because of bromine-based herbicides used by lake managers to try and control the invasive weed, H. verticillata.

To understand the toxic concentrations of this cyanobacteria, scientists studied bromine content in H. verticillata, reservoir sediments, and from AVM-positive and AVM-negative reservoirs throughout different seasons. They found that the toxic levels were more concentrated during late autumn, when surface water temperatures cool, water layers mix, and the invasive plant deteriorates. The scientists believe that this seasonal shift provides a bromide-enriched environment which triggers the cyanobacteria’s toxic production.

The scientists also conducted studies to understand how the bromine-toxin is absorbed through infected birds’ bodies. They studied the tissues of diseased American coots, a type of waterfowl that eagles prey on, and found that the toxin is absorbed through the gut and accumulates in the body. After further tests, the scientists found that the toxin not only affects birds, but also fish, frogs, insects, and worms. This shows that the toxin accumulates in tissues after animals ingest hydrilla coated with the bromine-produced cyanobacteria, and the toxin moves up through the food chain as animals consume diseased prey. Because of this widespread affliction, AVM is now called vacuolar myelinopathy (VM).

Although researchers uncovered what’s killing the eagles, how bromine is getting into the lakes remains a mystery. Scientists will need to conduct more research to understand factors that affect the distribution and toxicity of invasive aquatic plants, as well as natural and anthropogenic sources of bromine.

“EPA has been primarily focused on screening environmental samples for the presence of the toxin. Since the findings were published, several programmatic and regional offices have reached out for potential collaborations,” said EPA scientist Matt Henderson, who contributed to this research. “We still need to sample how the toxin affects mammals and other organisms in food chains to understand how it accumulates in various food webs.”

These findings, recently published in Science, illustrate the role of cyanobacteria as potentially dangerous toxin producers and the need to better understand how harmful algal blooms can alter ecosystems and effect environmental health. They also highlight the risks of bromine and why scientists need to work with lake managers to inform them of the dangers of bromine-based chemicals. Scientists can help by providing alternative management options that prioritize eco-friendly, long-term solutions.

This research was a collaborative effort with U.S. and international scientists. EPA scientist John Washington assisted Henderson with dividing A. hydrillicola and H. verticillata extracts for initial bioactivity testing. Roles that scientists contributed to outside of EPA include coordinating the project, collecting samples, performing laboratory cultures, developing methods to quantify the toxin, and using light and transmission electron microscopy, among others.

Learn more at the following links:

Hunting the Eagle Killer: A Cyanobacterial Neurotoxin Causes Vacuolar Myelinopathy

Endangered Species: Save Our Species Information – Bald Eagle

Safer Chemicals Research

Prepared by the EPA.