Mysterious plague exacts increasing toll on birds of prey

The parasitic Hippoboscid or louse fly is suspected by some naturalists to play a role in raptor deaths.

A pair of great horned owls peer with wide eyes around the edge of their shelter, like children sneaking a peek at a TV after their bedtime.

But maybe they're simply looking at what just hit them. Unknown numbers of raptors - or birds of prey - are falling to an unprecedented plague this summer.

“I have this sense of a tsunami coming through the midsection of this country,'' said Dr. Patrick Redig, director of the Raptor Center at the University of Minnesota. “I don't think it's a stretch to assume the number of birds killed by this is in the thousands.''

This curious owl pair is among 21 Great Horned Owls still alive at Nature's Nursery in Lucas County. They are the lucky ones. Eighteen other raptors delivered to the wildlife rehabilitation center since Aug. 12 died.

Across the country, in a line that stretches from Maryland to Iowa, wildlife centers are crushed by the demands of the formerly mighty raptors.

The predators come in with heads bobbling, their balance shot, and sometimes seizuring. Few rehabilitation centers have been as lucky as Nature's Nursery. Most birds die.

“We received 67 birds. Seven are still alive,'' said Mona Rutger, director of Back to the Wild in Castalia. “The ones that are recovering are permanently impaired.''

The outbreak has many speculating that we're seeing a new face of West Nile virus. But if that's the case, what's changed? The virus has been in the United States since 1999. Until this year, raptors were no more than occasional casualties.

So far, there are only theories, some with disquieting features that suggest potential changes in the virus' transmission pathways. One theory suggests a new insect carrier. Another possibility is that raptors catch the virus from their prey.

While great horned owls seem to be the most at risk, red tail hawks, Cooper's hawks, kestrels, merlins, and even bald eagles have fallen.

“The big question is, what has been the impact on the wild bird population,'' said Dr. Kathy Converse at the National Wildlife Health Center in Madison, Wis. But that's just one of many questions no one can answer yet.

Dr. Converse is trying to address a more urgent problem at the moment: Why are these big birds dying? Her laboratory is overwhelmed with requests for help. It receives 100 bird carcasses a day for analysis. Only a complete examination of the dead animal's organs, tissues, and fluids will reveal what's killing these birds.

She frames her results thus far carefully:

“A few of the raptors have tested positive for West Nile virus,'' she said. But a positive test does not confirm that West Nile killed these raptors.

Testing positive for antibodies to West Nile only indicates the bite of an infected mosquito. Many bird species, such as the ubiquitous house sparrow, catch West Nile and never miss a wing beat. Most humans also show no symptoms.

But even when a bird sickens from West Nile, the virus alone may not be lethal. Another unrelated germ or toxin may team up with West Nile to weaken the host. For instance, the Toledo Zoo veterinary staff thinks a combination of avian malaria and West Nile may be attacking the zoo's penguins.

Kay McKeever in Vineland, Ont., thinks she knows how West Nile arrived to kill 74 of the 234 owls she breeds for release to the wild. And she's not blaming mosquitoes.

She faults a bird parasite called the Hippoboscid fly. It's actually a group of flies also known as louse flies or flat flies. These insects feed on blood in the quill of emerging feathers of large birds.

“We had millions of flat flies. We've never, ever before seen anything like this,'' she said. She blames a mild winter that allowed most of the fly pupae to live.

“I think the Hippoboscid may have played a role, although we're in the middle of trying to investigate that'' said Dr. Bruce Hunter, the wildlife pathologist at the University of Guelph who's worked with Ms. McKeever's Owl Foundation for about eight years.

Dr. Hunter said it might be possible for the flies to carry West Nile on contaminated mouth parts. He thinks it's less likely that the virus can live inside the flies the way it lives in mosquitoes.

But he doubts these flies bring the virus to wild owls.

“In this facility, there's a heavy concentration of captive owls used in a propagation program,'' he said. Wild owls are dispersed, and therefore less vulnerable.

Yet a number of wildlife rehabilitation facilities have noted an increase in Hippoboscid flies on the owls and hawks they've rescued.

“There have been an inordinate amount of Hippoboscid flies. We have never seen such an infestation [on raptors],'' said Maggie Wright, manager of the Lake Metroparks Wildlife Center in Kirtland, east of Cleveland.

“I noticed early on, all spring and summer, an unusual infestation of them on incoming birds,'' said Ms. Rutger, of Back to the Wild.

“Even in advance of the appearance of West Nile, we noted a heavy infestation of Hippoboscid flies,'' said Dr. Redig in Minnesota.

But no one knows whether these observations have any meaning.

“I know that it is a suggestion from some of the wildlife biologists,'' said Dr. Emi Saiko, West Nile disease surveillance coordinator at the National Wildlife Health Center. “We can't discount it, but we don't really know. I don't think anyone's had the time to look at it yet.''

At Nature's Nursery in Lucas County, Laura Ponceby-Zitzelberger said she has noticed no increase in Hippoboscid fly infestation.

While bird deaths have been a hallmark of West Nile since it arrived in the United States in 1999, it's not clear West Nile has always killed birds.

One other West Nile outbreak is identified with significant bird mortality, and that was in Israel in 1998. Research has since revealed the U.S. strain of West Nile is genetically identical to the strain that caused the outbreak in Israel.

Most of the bird victims in the United States are in a family that includes crows and blue jays. It appears any crow infected with West Nile dies from it. And this brings up another theory of why raptors are dying now. Great-horned owls in particular eat crows.

“Sick and moribund crows would be easy pickings,'' Dr. Redig said.

One study suggests such disease transmission is possible. Dr. Robert McLean with the National Wildlife Research Center in Fort Collins, Colo., put infected crows in a cage with uninfected crows. All the crows died.

“They excrete a lot of virus,'' Dr. McLean said. A previous study showed that infected crows caged separately would not pass the virus, suggesting that at close quarters, the virus passes orally, not through the air.

But even when West Nile fails to hurt birds, they are always part of the picture in the advance of the virus. Bird-feeding mosquitoes spread the disease among birds, and birds carry the virus when they migrate.

While it's not certain how West Nile first crossed the Atlantic to invade the United States, the viral spread since 1999 has followed a zigzag pattern that tracks bird migratory paths, said Dr. McLean. He and others predict birds will carry the virus south into Central and South America, if they haven't already.

While it could be months and years before the larger scientific questions about West Nile are answered, centers such as Nature's Nursery and Back to the Wild hope for some relief soon. Increased bird illness threatens to hobble them.

“Last year we had 1,713 rescues. As of today, we're at 1,733, and last year was our biggest year ever,'' said Ms. Ponceby-Zitzelberger.

“It's putting a real drain on us.''