Avian brain disease puzzles scientists

A brain disease called avian vacuolar myelinopathy has affected at least 58 bald eagles in Arkansas

March 30, 1999
Web posted at: 9:45 AM EST

A mysterious brain disease is killing birds in the southeastern United States and scientists can't find the cause.

Birds with avian vacuolar myelinopathy typically fly erratically or are unable to fly completely. According to USGS wildlife disease specialist Dr. Kimberli Miller, birds may crash land, swim tipped to one side with one or both legs or wings extended, or be in the water on their backs with their feet in the air.

"On land," said Miller, "birds appear intoxicated -- they stagger and have difficulty walking and may fall over and be unable to right themselves." Affected birds, however, are usually alert and still may bite when handled.

Miller said that the only consistent finding in affected birds is the microscopic change in the nervous system.

Despite extensive testing by USGS, SCWDS, and others, the cause of the disease and the route of exposure is still unknown.

"All of the diagnostic, field and laboratory efforts indicate the cause is most likely a toxin, either one that is naturally occurring or manmade," Miller said.

In addition to toxicology tests, USGS pathologists have tested the tissues of dead birds for bacteria, viruses, parasites and none have been found. In humans and other mammals, similar lesions have been associated with genetic disorders, certain types of chemicals or toxic plants. Tests for these chemicals in the affected birds have been negative or inconclusive.

"This is very frustrating for our scientists," said Dr. Robert McLean, director of the USGS National Wildlife Health Center. "We have examined more than 4,000 bald eagle carcasses from around the country to determine the cause of death, and have conducted thousands of wildlife mortality investigations on many other species, and we are accustomed to identifying and resolving these problems. With this disease, however, despite all our efforts and despite the extensive involvement of leading scientists from diverse disciplines and numerous organizations, we have yet to solve the puzzle about the exact cause of the disease."

McLean noted that "vexing" questions about the disease still need to be answered. "Is it emerging and spreading to new locations and new species, or has it been around for a long time and just now being recognized because more people are aware of the problem? If it is an emerging disease, finding out what is causing it may be just the tip of the iceberg."

USGS wildlife specialists have discovered the disease in two species of ducks found dead at Woodlake, N.C. It was previously found only in bald eagles and American coots.

The disease affects the brain and spinal cord by damaging the myelin sheath that insulates the nerve fibers. It is diagnosed by microscopic examination of very fresh brain and spinal cord tissue.

Dr. Nancy Thomas, the USGS pathologist who first described the lesion, explained that "in affected birds the disease appears as open spaces in the white matter of the brain." When the coating surrounding the myelin is damaged, Thomas said, "communication in the nervous system is impaired, causing a bird to become uncoordinated or paralyzed".

Dr. Thomas used an electron microscope to determine that the spaces are caused by separation of the myelin layers that surround nerve fibers. Using the same techniques, Dr. John Fisher, a SCWDS pathologist, confirmed the lesion in a North Carolina mallard and ringnecked duck, and a Strom Thurmond Lake bald eagle.

For more information, contact Kim Miller, USGS, (608)270-2448, email: kim_miller@usgs.gov.

Copyright 1999, Environmental News Network, All Rights Reserved

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