ATLANTA (AP) _ The first study of a new West Nile virus vaccine for birds showed that an injected version prevented deaths, federal health officials said Thursday.
An oral version, however, was not as effective, the Centers for Disease Control and Prevention said.
Although it would be impossible to capture and inject millions of birds to prevent the spread of the virus, an effective vaccine could be used to protect threatened and endangered species. An oral vaccine would be better because it could be given to many more birds by spreading it with bait.
Four years after the virus first appeared in the Western Hemisphere, little is still known about West Nile. Health officials say the bird-vaccine study will add to existing knowledge and efforts to develop a human vaccine.
Researchers at the National Institute of Allergy and Infectious Diseases said human clinical trials of a vaccine under development are expected to begin at the end of the year.
The bird vaccine, if approved by the USDA, could be used in zoos or to protect small populations in the wild.
``There are collections of valuable birds out there that need to be protected,'' said Nicholas Komar of the CDC's arbovirus diseases branch. ``This vaccine may be the way to do it.''
Vaccines are generally made by using live or dead viruses. The new vaccine uses a third technique: It is made from pieces of virus DNA called plasmids that allow a bird's immune system to develop antibodies to detect and neutralize West Nile. An existing horse vaccine that is also used on birds is from a dead form of the virus.
The virus is estimated to have killed millions of birds _ including some in rare and endangered zoo collections _ since it first appeared in the Western Hemisphere.
Researchers gave nine fish crows the vaccine in shot form, eight crows in oral form and 20 crows received a placebo. Each group then was exposed to the virus. None of the crows that received the shot died; half the crows in the placebo and oral vaccine groups died.
A larger test, soon to be published by CDC officials, also confirmed that American crows _ which are very susceptible to West Nile _ had ``significantly improved'' death rates with the vaccine shot.
The DNA vaccine _ developed in the CDC's Fort Collins, Colo., labs _ does not prevent birds from carrying high levels of the virus. Mosquitoes transmit the virus to humans, horses or other animals after they bite infected birds, the virus' host reservoir. Lower levels of the virus would keep mosquitoes from spreading it.
``We've decreased the mortality rates and have decreased (virus levels) but not significantly enough to where it stops infecting mosquitoes,'' said Dr. Mike Bunning of the CDC.
If quickly approved by the USDA, the DNA vaccine could be available commercially within a year, Bunning said. In March, the DNA vaccine was used on 196 California condors _ the only ones that remain in the world.
``I think that (a vaccine) is really critical ... because for one thing if you're working with endangered species such as California condors ... you can't afford to lose any of them,'' said Judy Scherpelz, director of the Rocky Mountain Raptor Program, a Fort Collins nonprofit that rehabilitates ill and injured birds of prey.