ATLANTA -- Federal health officials say an injectible West Nile virus vaccine for birds prevents the deaths of crows. But they say an oral form did not work as well.

WEST NILE VIRUS Prevention Transmission History Overview Insecticide Tips Virus In Horses RESOURCES CDC Facts Case Count CDC Hot Line: (888) 246-2675

Crows and other birds can carry West Nile virus. Although it's impossible to vaccinate every bird in America, the researchers think injections could save some. They were thinking of threatened and endangered species in zoos, or small populations in the wild.

The virus is estimated to have killed millions of birds in the Western Hemisphere. A researcher from the Centers for Disease Control and Prevention hopes that an oral vaccine can be developed to keep birds from getting the virus from mosquitoes. He said this may reduce virus spread enough to cut the number of humans who become infected with West Nile virus.

As of Wednesday, the CDC reported 1,442 human cases of West Nile virus in 34 states and 21 deaths. Colorado accounts for nearly half the total cases and six deaths.