Monitoring AVM sites for sick waterfowl and eagles
Most of the sick or dead birds recovered from AVM sites have been collected during periods of intense shoreline monitoring and by local reservoir users such as hikers,campers, and sportsmen. While some birds with AVM do not appear neurologically impaired, there are some characterics to watch for: stumbling on land, difficulty taking off from the water or land, wobbling in flight and loss of righting reflex in water. At its most advanced stage, AVM affected birds appear listless, have a loss of vision and some are even partially paralysed.
Searching for the common factor in all AVM sites
A survey of reservoirs from 2001-2005 investigated the potential involvement of plant-associated (epiphytic) cyanobacteria as the causative agent of AVM associated with bald eagle and other bird deaths in SC, NC, AR and GA lakes. In these reservoirs, there is very little water-column (phytoplankton) production, but aquatic plants such as hydrilla are abundant. Plant samples were collected from all sites where AVM positive birds have been recovered. (Wilde, et al. 2005).
Among the many algal species surveyed from the water, plants and sediment, eleven species/genera were capable of producing toxins (Table 1). The species listed in have been documented with the potential to produce a range of hepatotoxins (liver toxins), LPS (lipopolysaccharides — mostly dermal toxins) and neurotoxins (brain, nervous system toxins). The primary known cyanotoxins that have neurotoxic effects are anatoxins and saxitoxins. There are many recently discovered neurotoxins produced by cyanobacteria that have yet to be characterized. One unknown Stigonematalan species of cyanobacteria has been noted on the leaves of plants from all the AVM sites sampled during 2001-2005. This newspecies of epiphytic cyanobacteria covered 20-90% of the leaf and stem surface of the aquatic plants (primarily hydrilla). Additionally, this species was not or rarely present on plant samples collected from sites where AVM has not been documented. (Wilde, et al. 2005)
Linking invasive aquatic vegetation to AVM
The AVM-suspect cyanobacterium appears most closely associated with Hydrilla verticillata, but occurs on two other non-native species; Egeria densa, and Myriophyllum spicatum. Hydrilla is native to Asia, Egeria came from South America, and milfoil originated in Europe. While there were native aquatic species with cyanobacterial epiphytes, native aquatic plants never dominate the entire water column with dense biomass in the manner of hydrilla, egeria and eurasian watermilfoil.
AVM lakes with hydrilla include: DeGray Lake, Lake Ouachita (AR), Sam Rayburn Reservoir (TX), Lake Murray, Davis Pond, (SC) J. Strom Thurmond Reservoir (SC/GA) Lake Juliette, Emerald Lake, Lake Horton, Smith Reservoir, Lake Varner (GA), Coachmans Trail, Woodlake (NC)
AVM lakes with Egeria densa include: DeGray Lake, Lake Ouachita (AR) Lake Juliette (GA)
AVM lakes with Eurasian watermilfoil include: Lake Hamilton (AR), L Lake and Par Pond (SC), Lake Juliette (GA)
American coots are known to feed primarily on aquatic plants and algae, and thus may be at the greatest risk (among waterbirds) of contracting AVM by ingesting cyanobacteria. Bald eagles, in turn, are thought to contract AVM by preying on sick diseased coots. A conceptual model has been developed to show transmission from aquatic vegetation and associated epiphytic cyanobacteria to waterfowl to bald eagles.