AVM is considered the most significant cause of unknown eagle mortality in US history. The AVM disease is caused by a neurotoxin that affects coots and waterfowl by ingesting plant material and associated epiphytes. Eagles are affected through ingested diseased prey (e.g. coots). The working hypothesis is that the suspect Stigonematales species is producing a novel neurotoxin. This coupled with the fact that the AVM-suspect cyanobacteria, a stigonematalan species, grows more densely on the hydrilla leaves than on any other aquatic plant may be the reason why hydrilla is the common denominator at all sites with the highest incidence of the AVM disease.
To date, there are 17 reservoirs (Arkansas, Georgia, North Carolina, South Carolina, and Texas) where AVM has been documented. AVM has been documented in large reservoirs, and small farm and neighborhood ponds. AVM has already adversely affected local breeding populations of eagles and has the potential to produce a regional impact if the disease continues to spread with new invasions of exotic aquatic macrophytes and cyanobacterial epiphytes.
Further research will be necessary to determine if and to what extent grass carp are affected by the suspect toxin, and whether they may be a viable means of vegetation control in reservoirs affected by AVM. The results from the swine feeding study indicate that swine were not affected by the Stigonematalan cyanobacterium. However, mammalian susceptibility warrants further research because humans ingest wildlife from affected reservoirs. Our results suggest that population level effects in coots were not apparent but bald eagle productivity at the reservoir has been severely impacted by AVM.
Feeding trials conducted with extracted toxin indicated that the unknown compound is water soluble. Future laboratory studies will focus on characterizing the unknown toxin using chemical analysis guided by testing on the new cell line bioassay (Wiley, et al 2009). Additionally, we want to learn what promotes the cyanobacteria to produce the toxin. This will be investigated by evaluation of the field conditions in all locations when disease is occuring and laboratory studies exposing the cyanobacteria to varied environmental conditions (temperature, pH, nutrient levels).
Although the etiology of AVM is not entirely understood, the disease poses an undeniable threat to the health of waterfowl and predatory bird, namely bald eagle, populations in the Southeast. Continued research, monitoring, and management actions will be necessary to better understand the epidemiology of AVM and protect vulnerable species.
The following podcast is a Warnell School of Forestry Presentation April 22, 2010. It provides an overview of emerging research on neurotoxic algae and ongoing research into the AVM toxin.