The West Nile virus, first spotted in this country in a sick crow three years ago, has now attacked at least 111 species of birds, including the bald eagle and the endangered Mississippi sandhill crane.
The spread of the virus has surprised and alarmed wildlife researchers because it has happened so quickly. Last year, West Nile had been detected only in about a dozen species of birds.
This year, hundreds of birds of prey, particularly red-tailed hawks and great horned owls, have been found dead in the upper Midwest, said Kathryn Converse, a wildlife disease specialist with the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis.
About 400 owls and hawks died in Ohio alone in what one wildlife official called "a major die-off." The carcasses were being tested for West Nile virus, which has been confirmed in several cases.
West Nile also has killed such birds in the wild as the ruby-throated hummingbird and Canada goose, and exotic and captive species such as the macaw and the Chilean flamingo, according to the Centers for Disease Control and Prevention Web site.
Of particular concern are the deaths of any endangered species, like the Mississippi sandhill crane, which numbers only about 120. It is one of six types of sandhill crane.
Since 1999, the virus has also killed at least one bald eagle, a threatened species, according to the CDC Web site.
"We don't know of any birds that can't be affected by the virus," Converse said.
It's impossible to know exactly how many birds have died from the West Nile virus, wildlife officials say, because the only way to confirm the virus in birds is to test them after they die.
Also, federal agencies like the CDC and Geological Survey rely on state and county health officers to report the bird deaths. But those officials are mainly interested in birds only as a tip-off that mosquitoes carrying the virus have shown up in their areas, so that people can be warned.
News that the virus is spreading in bird populations is frustrating for bird caretakers like James Mejeur, curator at the National Aviary in Pittsburgh, because the illness is hard to detect, treat and prevent.
Although veterinarians are experimenting with a vaccine approved for horses, the most effective way to prevent birds from getting sick is to control the mosquito population, Mejeur said.
Some institutions with captive bird populations install mosquito netting.
"It's manageable for us because the majority of our bird population is inside," said Mejeur, whose facility has lost three magpies and a crow this year. "But it is a tough time for zoos and other places that can't control the mosquitoes and have large populations of birds."
The horse vaccine has not been widely tested on birds, but the few facilities that have tested it found the birds were not harmed by it, Mejeur said.
Still, birds must be injected three times over a span of three months, which can be traumatic to wild populations, he said.
At the Raptor Center at the University of Minnesota, caretakers suspect the raptors may have the virus when they develop tremors, a blank stare and confusion. But other illnesses can cause similar symptoms, said Pat Redig, the center's director.
At that point there's not much veterinarians can do but give the animals fluids, antibiotics and special feedings that may help their immune systems.
But many raptors infected by the virus die after symptoms appear, said Redig. The Raptor Center has been studying and caring for eagle, hawk, owl and falcon populations since 1974.
There is hope that hawks, crows and other birds will become resistant to the virus over time.
"But we don't know how long that will take and how many raptors we'll lose in the meantime," Redig said.