Bald Eagle Mass Death of the 1990s

During the winters of 1994–95 and 1996–97, unprecedented mass deaths occurred among bald eagles at DeGray Lake, a manmade reservoir located near Arkadelphia (Clark County). First identified in 1994 at DeGray Lake, a neurological disease known as Avian Vacuolar Myelinopathy (AVM) was killing the eagles and American coots, which are herbivorous ducklike waterbirds. The disease had been identified, but its cause remained a mystery for more than twenty-five years.

Over the years, AVM occurred in additional locations and avian species, including mallards and great horned owls. By 2015, AVM had killed 160 bald eagles at twenty reservoirs in Arkansas, Georgia, North Carolina, South Carolina, Texas, and Florida. In 2021, scientists announced their breakthrough discovery: AVM is caused by a toxin known as a brominated cyanobacterial neurotoxin. They named the toxin Aetokthonotoxin (AETX), which means “the poison that kills the eagle.”

Inhabiting tree-lined lake, river, and sea shores, American bald eagles are large raptors that feed on fish, small animals, and carrion. Arkansas’s eagles are mostly winter migrants, favoring DeGray Lake, Lake Hamilton, Lake Ouachita State Park, and other large lakes and rivers.

By the early 1970s, once abundant bald eagles hovered near extinction due to habitat loss, shooting, and post-1945 reproductive failures caused by exposure to the now-banned pesticide DDT. By 2007, after being listed as an endangered species in 1978, bald eagles had recovered enough to be removed from the federal endangered species list. The number of Arkansas’s nesting pairs of bald eagles rose from one recorded in 1983 to 146 nests reported in 2008.

From late November 1994 through mid-January 1995, however, twenty-nine dead or dying bald eagles were found at DeGray Lake. During the winter of 1996–97, twenty-six bald eagles and a number of coots, a favorite prey of the eagles, died at DeGray Lake, Lake Ouachita, and Lake Hamilton. The sick eagles and coots exhibited extreme difficulty with swimming, flying, perching, and walking.

Because bald eagles are a federally protected species, the U.S. Fish and Wildlife Service initially investigated the eagle deaths as a matter of law enforcement. The Arkansas Game and Fish Commission led subsequent investigations undertaken by state and federal officials. Post-mortem examinations revealed microscopic lesions in the birds’ brains and spinal cords, but no evidence of viral, bacterial, parasitic, or prion infections was found. A toxicant was suspected as the cause of the disease, but known toxicants were not found in any of the bird carcasses.

In the early 2000s, Susan B. Wilde, who is an aquatic ecologist at the University of Georgia, and her team investigated the hypothesis, which was first suggested in 1996, that the eagles acquired AVM by ingesting AVM-infected prey. A nonnative and invasive aquatic weed known as hydrilla verticillata grew in DeGray Lake and the other reservoirs where AVM was found. Coots and other waterbirds fed on the hydrilla. In 2001, Wilde discovered a new species of cyanobacteria (a.k.a. “blue-green algae”) growing on the plant’s leaves. She named the new species Aetokthonos hydrillicola, which means “Eagle killer residing on hydrilla.”

Cyanobacteria produce toxins that can sicken wildlife and humans. Wilde’s suspicion that the A. hydrillicola produced a neurotoxin that sickened and killed the birds proved correct. In 2021, Wilde and German scientists successfully isolated Aetokthonotoxin. The A. hydrillicola produces AETX only when it is exposed to bromide, which the hydrilla accumulates in its leaves. Bromide pollution in lakes can come from several sources, including, ironically, the herbicides that are used to kill the hydrilla. Eagles and other raptors develop AVM when they ingest AETX through their prey’s gut contents. Other prey animals, including fish, aquatic turtles, and amphibians, are susceptible to AETX. Later, the name Vacuolar Myelinopathy (VM) came to be used because the disease occurs in animals other than birds.

Beginning in the early 2000s, sterile grass carp and the Asian leaf-mining fly (a.k.a. Pakistani fly), both of which can safely eat hydrilla, were used to eliminate the plant from DeGray Lake. (Grass carp do not transfer AETX to birds.) Arkansas has had no recorded cases of VM since 2002. Ongoing investigations seek to establish whether mammals are susceptible to AETX.

In the twenty-first century, VM, which was present in thirty-one states in 2014, continues to be identified in new locations in the United States. Ongoing interdisciplinary research seeks to ensure that reservoir management protects the health of wildlife and humans.

For additional information:
Bowden, Bill. “’90s Eagle Killer Found to be Toxic Water Weed.” Arkansas Democrat-Gazette, March 27, 2021, pp. 1A, 4A. Online at https://www.arkansasonline.com/news/2021/mar/27/90s-eagle-killer-found-to-be-toxic-water-weed/ (accessed January 22, 2025).

Breinlinger, Steffen, Tabitha J. Phillips, Brigette N. Haram, Jan Mares, Jose A. Martinez Yerena, Pavel Hrouzek, Roman Sobotka, W. Matthew Henderson, Peter Schmeider, Susan M. Williams, James D. Lauderdale, H. Dayton Wilde, Wesley Gerrin, Andreja Kust, John W. Washington, Christoph Wagner, Benedikt Geir, Manuel Liebeke, Helke Enke, Timo H. J. Niedermeyer, and Susan B. Wilde. “Hunting the Eagle Killer: A Cyanobacterial Neurotoxin Causes Vacuolar Myelinopathy.” Science 371 (March 26, 2021): 1–7. Online at https://www.science.org/doi/epdf/10.1126/science.aax9050 (accessed January 22, 2025).

Fischer, John R., Lynn A. Lewis, Tom Augspurger, and Tonie E. Rocke. “Avian Vacuolar Myelinopathy: A Newly Recognized Fatal Neurological Disease of Eagles, Waterfowl, and Other Birds.” In Transactions of the 67th North American Wildlife and Natural Resources Conference. U.S. Fish and Wildlife Publications, 2002.

Fischer, John R., Lynn A. Lewis-Weis, Cynthia M. Tate, Joseph K. Gaydos, Richard W. Gerhold, and Robert H. Poppenga. “Avian Vacuolar Myelinopathy Outbreaks at a Southeastern Reservoir.” Journal of Wildlife Diseases 42 (2006): 501–510.

Gillette, Becky. “The Bald Eagle Comeback.” About You (AY), Little Rock, June 29, 2022. https://aymag.com/the-bald-eagle-comeback/ (accessed January 22, 2025).

Howard, John. “Officials Investigating Deaths of 14 DeGray Lake Bald Eagles.” Sentinel-Record, December 14, 1994, p. 1.

Martin, Sandi. “UGA Researchers Identify, Name Toxic Cyanobacteria Killing American Bald Eagles.” UGA Today, February 18, 2015. https://news.uga.edu/identify-name-toxic-cyanobacteria-killing-american-bald-eagles-0215/ (accessed January 22, 2025).

Murphy, Thomas M. Southeastern States Bald Eagle Recovery Plan. Atlanta, GA: United States Fish and Wildlife Service Southeast Region, 1989.

“Outdoors: Celebrating the American Bald Eagle.” Southwest Times Record, June 15, 2016. https://www.swtimes.com/story/sports/2016/06/15/outdoors-celebrating-american-bald-eagle/24331859007/ (accessed January 22, 2025).

Peacock, Leslie Newell. “Eagle Killer Identified.” Arkansas Times, March 27, 2015. https://arktimes.com/new/arkansas-reporter/2015/03/11/eagle-killer-identified (accessed January 22, 2025).

Wilde, Susan Bennett, Jeffrey R. Johansen, Henry Dayton Wilde, Peng Jiang, Bradley Bartelme, and Rebecca Smith Haynie. “Aetokthonos Hydrillicola gen. et. Sp. Nov.: Epiphytic Cyanobacteria on Invasive Aquatic Plants implicated in Avian Vacuolar Myelinopathy.” Phytotaxa 181 (October 10, 2014): 243–260.

Woods, Wallace H., III, Brigette N. Haram, Susan B. Wilde, and Jim Ozier. “Avian Vacuolar Myelinopathy (AVM).” UGA Warnell School Outreach, May 2016.

Zhang, Sarah. “What Killed these Bald Eagles? After 25 Years, We Finally Know.” Atlantic, March 25, 2021. https://www.theatlantic.com/science/archive/2021/03/humans-accidentially-created-death-trap-bald-eagles/618413 (accessed January 22, 2025).

Melanie K. Welch
Mayflower, Arkansas

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