Mysterious Disease Plaguing Bald Eagles

Aug. 6, 2001 — -- Soaring faster than 40 miles per hour, diving at more than 100 miles per hour and deftly riding columns of rising air, bald eagles are known as the masters of the sky.

But in recent years a mysterious disease known as avian vacuolar myelinopathy has been attacking these majestic birds' greatest skills. The disease leads to lesions in their white brain matter, and afflicted birds become awkward, disoriented, even clumsy.

"It's pretty striking when you see it," says Tonie Rocke, a wildlife disease specialist at the National Wildlife Health Center in Madison, Wisc. "The eagles become erratic in flight. On land they'll run and fall over. Some have been seen flying into cliffs or falling off their roosts."

The first dead eagle known to be found with the disease was picked up on Thanksgiving Day in 1994 by fishermen crossing DeGray Lake in Arkansas. By the end of that winter, 29 eagles had died from the disease. Since then dozens more eagles, as well as coots, ducks, geese and (new this year) great horned owls and killdeer have picked up the disease in 11 different lakes in Arkansas, Georgia and North and South Carolina.

So far this year the death count among bald eagles is 16 — a small, but troubling number considering the animal only recently staged a comeback to reach a population of an estimated 5,800 pairs from near extinction in the 1960s. And researchers are still baffled by the cause.

"We have some ideas, but we don't any answers," says John Fischer of the Southeast Wildlife Disease Study at the University of Georgia in Athens. "And we're concerned that it is occurring on a more widespread basis than we realize."

Fischer says bald eagle numbers are high enough now that the disease should not decimate the eagle population nationwide. Still, finding out what causes the disease will be key to keeping it in check.

What is known about the disease is this: In affected birds, lesions form in the myelin sheath that insulates nerve fibers of the birds' brains. The damage appears as open spaces in the white matter of the brain and causes disorientation, motor problems and, eventually, death.