The impacts of hydrilla and other invasive species are many, but research in the past few decades sheds light on the potentially “deadly” impact of such aquatic plants. In 1995, researchers began to notice the sudden isolated deaths of various birds including the bald eagle around DeGray Lake, Arkansas. Although unknown at the time, the cause was Avian Vacuolar Myelinopathy (AVM), a neurological disease that affects various waterbirds and their predators.

AVM primarily affects American Coots and Bald Eagles in the Southeastern United States. The disease often causes birds to display neurological impairment including difficulty swimming, flying and a loss of the ability to “right” themselves. Birds often see rapid progression of the disease from the first symptoms to ultimate death in less than a week. Other than the obvious strange nature of affected birds, there are no other visible signs to the disease. Only microscopic investigation of the affected animals will reveal the true culprit (AVM) where lesions in the animals brain are a tell-tale sign. AVM has since found its way into four other states including Texas, Georgia, North Carolina, and South Carolina and to date is responsible for the death of over 100 bald eagles and thousands of coots.

So where exactly did this mystery disease come from? Could it be a man-made compound or a natural occurrence? Well researchers have ruled out any possibility of man-made compounds as the site specific, seasonal occurrence (late fall-winter) and rapid onset of the disease suggests actual environmental causes. Lab and field studies have determined that the cause is actually a toxin produced by a previously unknown cyanobacteria. The potentially deadly epiphytic (attached to plants) cyanobacteria is ingested when waterbirds like coots consume large amounts of vegetation. As coots begin to show symptoms of the disease, predatory birds like the bald eagle discover the easy meal and thus contract the disease themselves.

So what does hydrilla have to do with such a horrific disease? Well, researchers have shown that AVM only exists in water bodies with dense populations of invasive submersed plant species, mainly hydrilla. The toxin actually accumulates in such plants, which is the direct pathway to affected birds. Large amounts of submersed vegetation common in hydrilla infested lakes present a real threat to waterfowl and predatory birds in the southeast.

While the disease has only been shown fatal in birds, it also has produced lesions in fish and turtles which feed on the submersed plants. Herbivorous fish like the grass carp have developed lesions in both lab and field studies. Turtles in lab trials which fed on hydrilla with the cyanobacteria showed the same neurological impairments seen in birds. So far no mammals have been affected by the disease but the potential for public health concern is still very real until more tests can be completed.

Currently, one of the only management strategies to reduce AVM is to control invasive aquatic plant species like hydrilla on which the toxin producing algae makes it’s home. As scientists scramble to learn more about the disease, one thing is quite apparent. If you live near a waterbody affected by invasive aquatic plant species, it may be of great risk for the consequences of AVM. Lakes like Kerr Lake and Lake Gaston, with such large amounts of hydrilla, could be at great risk for the disease in the future. While impacts of invasive plant species are not often associated with life and death, the discovery of AVM and it’s causes proves to be one of which we should pay close attention. For more information, visit the University of Georgia’s AVM website or for AVM positive locations in the US, see “Web Links for Additional Information”. If you see strange behavior exhibited by waterfowl or other birds on the lake, please contact Brett Hartis at the information provided below.

Information, technical guidance, and photo provided by Dr. Rebecca Haynie and the Wilde AVM Lab of the University of Georgia.

Web Links For Additional Information:
University of Georgia’s AVM site
AVM Locations
Videos and Pictures of Affected Birds

If you have questions please contact your Aquatic Extension Associate, Brett M. Hartis, at (919)-515-5648 or email at bmhartis@ncsu.edu.