Dead birds a clue on West Nile spread

The final migration of dozens of wild birds ends every afternoon in the city of Davis, where they are delivered by truck, boxed in ice, to the state's West Nile virus experts.

Dead birds from eight additional California counties have tested positive for West Nile, scientists at the state lab announced Thursday, bringing the tally of affected counties to 15.

Also Thursday, the first human death this year in the state was announced.

Forty to 60 dead birds arrive every day at the lab, which is responsible for investigating mystery animal deaths in the state.

"If the bird hasn't been dead for too long, hasn't been out in the sun and isn't run over, we're interested," said Vicki Kramer of the Department of Health Services Vector-Borne Disease Section, which does bird testing.

The steadily expanding borders of the epidemic confirm that the virus, spread by mosquito bites, now has a secure foothold in California's bird population, will sicken more people and is unlikely to be eradicated anytime soon.

So far 980 California birds have been found infected; many others die quiet deaths and are never found, scientists said.

Mosquito bites also lead to human cases, which tend to follow bird cases by about two weeks. There have been more than 30 people infected statewide this year, and the first death was reported Thursday in Orange County.

Over time, outbreaks in the United States are likely to ebb and flow, as they have elsewhere in the world since the disease was discovered in Uganda in 1937, say scientists.

"It'll stay around and become part of the California terrain," said John Edman, director of the UC-Davis Center for Vectorborne Diseases.

The good news in New York, where the disease was initially reported in the United States in 1999, is that there is an apparent decline in both bird and human cases.

But the virus hits hard in previously unexposed regions, where there is no natural resistance, said Aaron Brault, assistant professor of pathology, microbiology and immunology at the University of California-Davis, who studies the virus.

In places where the disease is endemic, such as Africa, children are infected but readily recover and develop resistance. But because it hasn't been here before, older people and not just children are infected. Older adults are more likely to develop severe neurological illness and die.

In Davis, 21 dead birds have arrived from Santa Clara County at the California Animal Health & Food Safety Laboratory System. The 14th to arrive, a crow, was found to be infected by the virus, scientists announced Wednesday. Results of seven other tests are pending.

The birds are lifted out of their "biosafety boxes" and dissected in an airtight lab by a pathologist garbed in gown, goggles and gloves.

The birds' tiny kidneys -- where the virus is most concentrated -- are of greatest interest to scientists. Kidneys are shipped to the state's Center for Vector-Borne Diseases, also in Davis, where tests seek proof of the virus. It often takes several weeks for tissue to be shipped and then undergo multiple tests.

The glossy-feathered American crows are affected more than other birds, and no one knows why.

Nor do they understand how crows brought it here. Crows migrate only brief distances but at night gather in huge rookeries, Edman said. Scientists suspect that the virus is spread when youngsters get pushed out of their home turf by parents, then fly away in small flocks in search of new territory.

But other affected species do migrate. And even though mosquitoes die off in winter, the virus will be reintroduced in the spring as birds travel up California from Central America via the Pacific Flyway.

Over time, birds die off or develop resistance, then the number of infections drops. It rebounds several years later, Edman said.

Disturbingly, the virus in the United States belongs to a strain that is far deadlier to birds than West Nile strains in most other parts of the world, such as Africa and Australia, researcher Brault said.

The American strain came from Israel and kills birds within days, said Brault, who until recently worked for the U.S. Centers for Disease Control, where he studied the effects of three different strains of West Nile virus on small groups of crows.

It was thought to have been carried here in a mosquito that stowed away in an airplane from Israel, then was released at Kennedy International Airport in New York.

No one knows whether this strain is more dangerous to humans, too.

To report dead birds, which may be collected for testing, call the state hotline, (877) 968-2473.