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Avian Vacuolar Myelinopathy

Introduction

Avian Vacuolar Myeliopathy (AVM) is a fatal neurological disease that affects various waterbirds and raptors, but it predominately affects Bald Eagles and American Coots. Other bird species that have been affected by this disease include Killdeer, Buffleheads, Northern Shovelers, American Wigeons, Canada Geese, Great-horned Owls, Mallards, and Ring-necked Ducks . Avian Vacuolar Myelinopathy is a newly discovered disease that was first identified in the field in 1994 when dead Bald Eagles were found near DeGray Lake, Arkansas. Since then, it has spread to four more states and infested multiple aquatic systems including 10 reservoirs . The cause of death to the infected birds is lesions on the brain and spinal cord, but the exact causative agent is still being investigated by researchers .

Clinical Signs

Clinical signs have been recorded from research studies where individual birds were intentionally infected with the disease and from wild specimens and dead birds recovered from the field . Clinical signs listed are also those observed in American Coots.

• Raptors have been seen flying into objects, such as trees and rock faces.

• Waterfowl swim awkwardly, sometimes on their backs.

• Lack of coordination flying and walking, sometimes dragging wings or one leg and general limb weakness.

• Waterfowl crash land into water.

• Tremors of the head

• Weight loss

• Unresponsive to noise

• Beak and tongue weakness

• Decreased pain responses

Most of the Bald Eagles have been found deceased from October to March, while the death count peaks during mid-November through December. The waterfowl migrate to the area in October and November, where they consume the bacteria off of plants and become infected .

Causative Agent Proposal

Researchers currently believe that the probable cause of the disease is a strain of epiphytic cyanobacteria from the Stigonematales family . This particular bacteria grows very well on invasive aquatic plants, particularly on the invasive species hydrilla (Hydrilla verticillata) and will cover 20-90% of the aquatic leaf surface area in infected aquatic systems . These invasive hydrilla plants often take over any aquatic system to which they are introduced . Waterfowl then consume the hydrilla, indirectly ingesting the cyanobacteria growing on the plants, which make them express the clinical signs and symptoms described above. Some raptors, like Bald Eagles, prey upon the waterfowl and contract identical clinical symptoms from consuming infected tissues . The bacterium causes widespread bilaterally symmetrical vaculation of the white matter of the brain and spinal cord of infected birds, which in turn causes lesions to form in the brain and spinal cord .