Mysterious Disease Plaguing Bald Eagles
August 6, 2001
Soaring faster than 40 miles per hour (60 kilometers per hour), diving
at more than 100 miles per hour (160 kilometers 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, Wisconsin. "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."

Death Toll Rising

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 were known to have 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 worldwide. 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.

"In several cases with the coots, you'll see the birds swimming on their side with a leg trailing," says Rocke. "They swim in circles and on land, they'll run in circles."

Recent work has demonstrated that bald eagles contract the disease after preying on affected coots (a small, white-billed water bird). This was confirmed by feeding affected coot flesh to red tailed hawks that had already been injured in the wild. Although the hawks didn't reveal outward signs of the disease, once dissected, their brains showed the disease's tell-tale lesions.

The first known case of the disease was the bald eagle recovered in 1994, although the affliction may have emerged, at least in coots, much earlier. Residents in the Southeast have recently been reporting to scientists that they noticed coots around their lakes showing the strange symptoms starting in the early 1990s.

Besides this information, little else is known. As Fischer says, "In the absence of reliable data, everyone's opinion is valid."

Needle in a Haystack Search

Part of the problem is the disease is likely caused by an outside agent—not by a virus or bacterium that could simply be extracted from the brains of diseased birds. And searching for a cause in the varied and vast habitat of the birds is not easy.

"We're suspicious it might be a chemical from an algae or plant," says Rocke. "It could also be a man-made pollutant or an exotic plant agent. But we really have nothing to confirm these ideas."

One common factor scientists have traced is the fact that all the diseased birds have been found around man-made reservoirs. Rocke explains this finding is not terribly revealing since many of the freshwater lakes in the Southeast are man-made.

Also puzzling is that people are known to get brain lesions similar to the ones that stem from the bird disease. But unlike the eagles and coots, people generally recover. Fischer says the human problems are thought to be related to substances like hexachlorophene that were once used in baby soap.

Clues Leading Nowhere

Tests of diseased bird tissue have shown no signs of hexachlorophene or other chemicals that have been shown to trigger lesions in human brains.

Another discovery that has led nowhere is the finding that mammals feeding on a group of toxic plants have come down with similar symptoms. But the plants are only found in Australia.

"We've done chemical analyses for all the chemicals that cause similar lesions in mammals and never found them in the birds," says Rocke.

As pathologists scour water systems for causes, Fischer worries the disease may already be taking hold in new areas.

"Truth be told, this disease could have been around for much longer and it just wasn't noticed," says Fischer. "It could very well be occurring on a more widespread basis than we realize."

And right now, there is little scientists can do to stop it.

Copyright 2001

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