Concerned about the spread of the West Nile virus, zoos across the country are vaccinating some of their most susceptible animals.

The Cleveland Metroparks Zoo is using a commercial vaccine for domestic horses to inoculate its equine species — zebras, donkeys and miniature horses.

And many zoos are taking prevention a step further, vaccinating both equine and bird populations, even though the vaccine has not been proven effective in birds.

"This is an emerging disease, so for any emerging disease, you study it intensively to figure out how bad it's going to be,'' said Dr. Dominic Travis, a veterinarian epidemiologist at Chicago's Lincoln Park Zoo who is also a national coordinator of zoo response to West Nile. "It has spread from New York to Colorado and Wyoming in three years, and that is of concern."

The St. Louis Zoo began vaccinating its most susceptible birds, as well as its equine species, about two weeks ago, said Dr. Eric Miller, the zoo's director of Animal Health and Conservation. Susceptible birds include crows, raptors, flamingos, parrots and birds that live outside.

The zoo has had four confirmed cases of West Nile deaths in birds, and the Audubon Zoo in New Orleans lost two flamingo chicks to the disease. A penguin also died at a zoo in Rochester, N.Y., and the National Zoo in Washington, D.C., reported that two flamingo chicks and a duck died from the virus, which the zoo also believes killed a dozen more of its birds.

Zoo and health officials have stressed that people shouldn't fear visiting zoos, because they're no more likely to be bitten by a virus-carrying mosquito at a zoo than elsewhere. People can't get West Nile from each other or by touching an infected animal.

But zoo officials worry the virus could endanger valuable animals.

"Many of the species at this zoo and others are endangered, and we don't want to be losing them to disease,'' Miller said.

The West Nile virus has struck in Africa, Europe and Asia for decades, but was discovered in the United States only in 1999. Since then, at least 31 people nationwide have died from the disease.

At the Bronx Wildlife Conservation Society in New York, where the West Nile virus was first discovered in 1999, senior veterinarian Bonnie Raphael has been vaccinating birds at high risk.

"We won't know if it works until we get through this season and maybe the next season, but it's really all we have aside from really, really good mosquito control," she said. "That's our No. 1 defense. We're just hoping the vaccine will be the icing on the cake.''

Some zoos with the room are keeping their animals indoors from dusk to dawn — prime mosquito time — and others are putting mosquito larvae-eating fish in their ponds, installing netting and using animal-friendly insecticides.

The vaccine, which is manufactured by Fort Dodge Animal Health, was approved last year for domestic horses and is considered effective on all equine species. As for the bird population, "we know that the vaccine has done no harm,'' Travis said.

With that in mind, many zoos figure they might as well try.

The Audubon Zoo began vaccinating all of its equine species and about half its bird species six weeks ago.

"We've started mainly with the ones who are most at risk — really young ones, really old ones, the really rare ones, and the ones that live outside,'' said Sarah Burnette, public relations director.

The Cleveland zoo, meanwhile, is relying on mosquito control to protect its birds. Staff members are bringing animals in for the night, placing mosquito traps on the grounds and spraying the buildings with bug repellant. They've also drilled holes in the bottoms of garbage cans so there isn't any standing water.

Chicago's Lincoln Park Zoo is not vaccinating birds, either.

"We're waiting for the data to prove a little bit more to us,'' Travis said.

But zoo directors are holding out hope that the vaccine, along with effective mosquito control, can keep the virus at bay.

"We're all kind of working in the dark with this as to if this is going to work or not,'' Raphael said.