I am currently on a 3 week stint on the inpatient consultation service, necessitating a brief blog post this week.
If you haven’t read my previous post, How vultures, cattle, and a minority religious sect are interconnected, I ask you read that post before this one.
I wanted to follow-up my Vultures… post, with a story from the United States about a similar mystery: an emerging, occult disease affecting another raptor, the bald eagle.
In 1994, unexpected mortality was noted among bald eagles (Haliaeetus leucocephalus) in Arkansas’ lakes, including De Gray Lake.
Necropsies were conducted on dead birds at the USGS National Wildlife Health Center in Madison, WI. Pathologist Dr. Nancy Thomas described the characteristic pathologic finding: intramyelinic edema. Swelling (or edema) was noted within myelin sheaths of the nerve tracts. Myelin sheaths are the insulation encompassing the nerves, somewhat like the brightly colored plastic insulating copper wire in your electronic devices. You mess with myelin and, in essence, the brain short-circuits. In the case of these birds, lesions were found primarily in the cerebellum (a part of the brain that coordinates movements) and in the optic tectum (an area of the midbrain that coordinates movement to visual inputs.) As would be expected from deficits in these areas of the brain, afflicted birds had trouble flying, including an inability to land on a perch. Birds behaved as though drunk. They eventually appeared blind and would collide with objects. Most birds died, though some were able to recover. The syndrome was termed Avian Vacuolar Myelinopathy and abbreviated AVM. (Link)
No existing infectious agent or toxin was known to create this unique pathologic lesion. The search was on to identify a new culprit killing the eagles.
Over the next three years, American coots (Fulica americana) were also noted to be affected. Necropsies on these birds showed similar pathology. The coots became easy prey for eagles, so one explanatory hypothesis was a toxin or infection transmitted up the food chain from coots to eagles. And, indeed, Fischer and colleagues, in a small feeding study, were able to demonstrate this using 6 red tailed hawks (Buteo jamaicensis). Five red tailed hawks were fed coots affected by AVM (ie, tainted meat). One red tail was fed coots not apparently affected by AVM (ie, USDA Grade A). All five hawks fed the tainted coots were found to have AVM lesions on necropsy, whereas the one red tailed hawk fed the Grade A coots did not develop the pathologic findings of AVM. Though a small study, this did suggest transmission of the syndrome via the food chain.
Dodder et al (2003) examined potential environmental toxins as a cause of AVM. They compared sediments from affected sites and non-affected sites for specific chemical compounds including polychlorinated biphenyls, octachlorodibenzo-p-dioxin, polycyclic aromatic hydrocarbons such as retene, penta- and hexa-chlorobenzene, oxychlordane, p,p’-DDE, and dieldrin. There was no significant difference in the concentrations of these chemicals between sites, arguing against these chemicals being the cause.
Afflicted birds caged in proximity to unaffected birds did not acquire the syndrome, arguing against an infectious pathogen.
Where AVM was found in birds, the waterways were choked with invasive, exotic aquatic plant species. The lakes, rivers, and reservoirs were dominated by several species:
Eurasian watermillfoil (Myriophyllum spicatum) is a species native to Europe, Asia, and North Africa that was introduced into US waterways sometime between the 1880s and 1940s. It is uncertain whether it arrived as a hitchhiker in ship’s ballast water or if it escaped from the aquarium trade.
Brazilian elodea (Egeria densa) was imported from South America and was introduced in US waterways in 1893. All of the plants found in the US are male, but they spread when by fragments of the plant hitch a ride on boats.
And, finally, Hydrilla (Hydrilla verticellata). Yes, what sounds like a 1960s Japanese monster of the likes of Godzilla and Mothra, is an invasive aquatic plant from Asia. The plant was imported to the US as part of the aquarium trade but was subsequently introduced into US waterways in the 1950s.
All three of these invasive species are transported from one waterway to another primarily as hitchhikers on boats and boat motors. They outcompete native aquatic plants, overgrow the water table and choke the water column of oxygen.
Cyanobacteria (also known as blue-green algae) often live in dense mats of submerged vegetation, affiliated with plants such as these aquatic species. And cyanobacteria are known to produce neurologic toxins. But studies did not reveal any blooms of known toxic cyanobacterial species.
Ultimately, attention focused on a novel cyanobacterial species growing on the underside of Hydrilla leaves. This previously undescribed species was found in all 19 sites where AVM has been described in birds across six southern states (Arkansas, Texas, Florida, Georgia, South Carolina, and North Carolina).
Toxicologic testing was performed on chickens and farm raised mallards fed field-collected Hydrilla containing the identified cyanobacterial species and AVM was reproducibly produced. This has clinched the cause of AVM.
This cyanobacterium is completely new, representing not only a new genus and species, but a new family as well. (Wilde, et al, 2014)
The organism has been named Aeokthonas hydrillicola; Aetokthonos is Greek and translates to ‘eagle-killer’ and hydrillicola is Latin for ‘lives on hydrilla.’ This novel toxic cyanobacterium is a new threat associated with exotic, invasive species. This is another reminder that invasive species continue to wreck havoc in environments where they do not belong. I would direct people’s attention to the “Get Habitattitude” campaign of the US Fish and Wildlife Service/ NOAA’s Sea Grant/ Pet Industry Joint Advisory Council. Non-native plants and animal pets should not be released into the environment or flushed down the toilet (a la Nemo, the fish star of the Disney film). And boaters need to clean all recreational equipment of potential exotic hitchhikers. See this MN DNR website for helpful instructions.
Ominously, the A. hydrillicola toxin has been shown to not only affect birds but also fish (grass carp) and herbivorous turtles. And it is expected to continue to expand its range across the south. Time will tell what impacts it will have on waterfowl and raptors.
REFERENCES AND FURTHER READING:
Dodder, NG, B Strandberg, T Augspurger, & RA Hites. 2003. Lipophilic organic compounds in lake sediment and American coot (Fulica americana) tissues, both affected and unaffected by avian vacuolar myelinopathy. Science of the Total Environment 311: 81-89.
Fischer, JR, LA Lewis-Weis, & CM Tate. 2003. Experimental vacuolar myelinopathy in red-tailed hawks. Journal of Wildlife Diseases 39: 400- 6.
Wilde, SB, et al. 2014. Aetokthonos hydrillicola gen. et sp. nov.: Epiphytic cyanobacteria on invasive aquatic plants implicated in Avian Vacuolar Myelinopathy. Phytotaxa 181(5): 243- 60.