Date of Award

5-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Toxicology

Advisor

Bowerman, William W

Committee Member

Johnson , Alan J

Committee Member

Rodgers , John R

Abstract

Avian vacuolar myelinopathy (AVM) is a neurologic disease of unknown etiology that has severely impacted waterbirds and raptors in the Southern United States since it was first diagnosed in 1994. Research studies have suggested that the causative agent is a neurotoxin produced by a previously undescribed Stigonematalan cyanobacterium which grows epiphytically on submerged aquatic vegetation. A suite of experiments were conducted to determine if: (1) triploid Chinese grass carp (Ctenopharyngodon idella) are an appropriate management strategy for AVM; (2) to measure mammalian susceptibility to cyanobacterium exposure; and, (3) to discern population-level impacts to American coots (Fulica americana) and bald eagles (Haliaeetus leucocephalus) on J. Strom Thurmond Lake (JSTL), during an AVM epizootic. In order to determine a cause-effect linkage, the existing body of AVM research was analyzed, and our working hypothesis was critically tested using Hill's epidemiology criteria, as adapted to ecotoxicology.
Grass carp that were fed hydrilla (Hydrilla verticillata) with the Stigonematalan cyanobacterium in field and laboratory trials developed microscopic lesions, in the white matter of the brain, that were similar to lesions seen in AVM-affected birds. Grass carp that were fed hydrilla without the suspect cyanobacterium did not develop these lesions, and chickens (Gallus domesticus) fed treatment and control grass carp tissues did not develop AVM. Young swine (Scus scrofa) were fed hydrilla with the Stigonematalan cyanobacterium, but did not develop clinical or histological symptoms of AVM.
Bald eagle productivity on JSTL declined by 50% from the previous season and 7 of 12 bald eagle carcasses recovered were positively diagnosed with AVM. AVM-attributed mortalities were severe in coots in localized regions, and coot carcasses were scavenged quickly, which may have caused an underestimation of mortalities. However, no effects in coots or other waterfowl were discernable at the reservoir-population level.
A strong cause-effect linkage was found between the presence of the suspect cyanobacterium in systems and incidences of AVM. A weaker correlation was present between incidences of AVM and declines in bald eagle productivity on JSTL, and no cause-effect linkage was evident between declines in coot populations and AVM on JSTL.

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