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Abstract
Avian vacuolar myelinopathy (AVM) is a neurologic disease affecting waterbirds across the southern United states. The disease is linked to a novel toxic species of cyanobacterium (Aetokthonos hydrillicola) that grows on aquatic weeds. Submerged aquatic plants with invasive growth habits provide ample substrate for epiphytic A. hydrillicola to colonize and all sites with AVM contain submerged aquatic weeds supporting moderate to high densities of A. hydrillicola. Herbivorous waterbirds like American coots (Fulica americana) feed on aquatic plants, ingesting the cyanobacterium and toxin(s). Many of these birds are affected by the neurotoxic compound(s) and become easy prey for opportunistic raptors like bald eagles (Haliaeetus leucocephalus), which in turn suffer neurologic impairment and often perish. Additionally, six species of waterfowl, owls, and a shorebird have been diagnosed. These species utilize southern reservoirs as fall migratory stopovers and wintering grounds. This fall - winter period coincides with peak toxicity in the vegetation/cyanobacteria complex many waterbirds use to forage. Hundreds of bald eagles, American coots, and waterfowl have been diagnosed with AVM; however, the identity of the neurotoxin(s) and mechanism of disease are undescribed. This doctoral research explored the effects of AVM on avifauna on an ecological level and advanced our understanding of the character of the toxin(s) and methods to detect it. Four questions guided this work: (1) What is the species vulnerability range for AVM? (2) Are invasive aquatic plants creating traps for bald eagles by promoting AVM? (3) What is the compound responsible for AVM? and (4) Do invertebrate bioassays provide efficient tests for AVM toxin? We investigated avian use of the aquatic weed hydrilla (Hydrilla verticillata) on a southeastern reservoir during an AVM season to assess disease vulnerability. A number of waterbird species have not been diagnosed with AVM, but our qualitative risk assessment puts them at risk based on dietary link and use of hydrilla. We found evidence in bald eagle production and mortality data to support the hypothesis of an ecological trap on a southeastern reservoir. We isolated cytotoxic compound(s) to a semi-purified fraction and developed an invertebrate bioassay for use in AVM toxicity studies.