The American bald eagle (Haliaeetus leucocephalus) was once on the brink of extinction in North America. As a result of concerted conservation efforts, including the elimination of use of the pesticide DDT in the United States in 1972, the bald eagle population rebounded in North America and is no longer considered an endangered species. However, conservationists remain vigilant with regard to any disruptions affecting bald eagle populations. In particular, scientists have been concerned about ongoing episodes of unexplained bald eagle deaths in the southeastern United States in recent decades. Starting in the early 1990s, many dozens of eagles, mallards (wild ducks), and various other waterfowl began to die in mass-mortality events at many freshwater lake areas throughout Arkansas, Georgia, North Carolina, South Carolina, and eastern Texas. Although researchers have devoted almost 30 years investigating these mysterious bird die-offs, research efforts have failed to discover the underlying culprit responsible for the bird deaths—that is, until now. See also: Anseriformes; Aves; Conservation of species; Ecology; Ecosystem; Falconiformes; Lake; Pesticide
Eagles and other waterbirds that have died in the mass-mortality events in the southeastern United States since the 1990s were found to have a neurological disease known as avian vacuolar myelinopathy (AVM). This fatal condition, which causes microscopic holes (vacuoles) to appear in the white matter of birds' brains, initially disrupts the avian nervous system. Birds affected by AVM are incapable of coordinating their motor functions. Consequently, these birds cannot fly, have difficulty walking, and are subject to seizures. Scientists surmised that some type of neurotoxin, probably ingested as part of the birds' diet, must be affecting the animals. However, the toxin could not be identified until recent studies showed it to be a neurological agent—a structurally unusual alkaloid named aetokthonotoxin—produced by the cyanobacterium Aetokthonos hydrillicola. Importantly, A. hydrillicola is associated with an invasive aquatic weed known as Hydrilla verticillata, which is abundant in human-made freshwater lakes in the southeastern United States. See also: Alkaloid; Brain; Invasion ecology; Invasive species; Nervous system disorders; Toxin; Weeds
Cyanobacteria are microscopic bacteria capable of photosynthesis and are often dominant members of the phytoplankton in freshwater lakes. Under certain environmental conditions, cyanobacteria can produce toxins. Scientists note that A. hydrillicola manufactures the aetokthonotoxin that is the causative agent of AVM. Under typical laboratory conditions, though, this cyanobacterium does not produce the aforementioned toxin, hampering efforts of investigators to unravel the disease-causing agent. Instead, the cyanobacterium only creates the toxin when there is ample supply of the chemical element bromine. Scientists subsequently determined that the invasive H. verticillata weed provides high concentrations of bromine to A. hydrillicola, which grows in clusters on the undersides of the weed's leaves. Eagles and other waterbirds ingest the toxin-containing cyanobacteria when they feed directly on Hydrilla weeds or when the birds feed on other, smaller animals (for example, fish, amphibians, or snakes) that might have ingested the toxin previously. See also: Bromine; Cyanobacteria; Food web; Phytoplankton
Researchers are now working to remove the Hydrilla invasive species from freshwater lakes in the southeastern United States. By doing so, scientists believe that the root cause of the neurotoxin will be eliminated, thus preserving the lives of numerous eagles and waterfowl. Scientists also recommend integrated chemical plant management protocols that avoid the use of bromide-containing chemicals, as well as increased monitoring and public awareness for A. hydrillicola, to prevent the neurotoxin from bioaccumulating during transfer through the food web. See also: Freshwater ecosystem; Trophic ecology
