NEW YORK -- It's quite a scientific expedition -- testing birds up and down the East Coast to track the encephalitis virus that has killed five people and infected 38 others in the New York City area.

"We've got to look at all kinds of urban birds -- crows, pigeons, starlings and sparrows," said Andrew Spielman, an expert on mosquito-borne diseases at the Harvard School of Public Health.

The West Nile strain of the virus has been found in dead crows in the New York area. The U.S. Centers for Disease Control and Prevention is now collecting blood samples from city pigeons and also looking at ticks as possible carriers.

In New York's Queens borough, where the first cases of encephalitis were reported last month, the CDC and the city's health department planned to take blood samples door-to-door, with 300 households selected at random and on a voluntary basis.

"We hope to get a better understanding of the symptoms caused by West Nile virus, as well as possible risk factors for infection," said Ned Hayes, the CDC's medical epidemiologist.

Fearing the infection might spread with the fall bird migration, experts also are testing dead birds from Maryland to Florida. No sign of the virus has appeared in those birds, said Roger Nasci, a CDC entomologist.

The virus is believed to have been transmitted by mosquitoes that typically feed off birds. Six new cases were confirmed Wednesday, including a 79-year-old woman from the borough of Queens who died Sept. 11.

New York, New Jersey and Connecticut have been spraying pesticides to kill as many mosquitoes as possible before the first winter chills.

Dr. Neal Cohen, the city's health commissioner, said mosquito density has declined substantially, probably by 90 or 95 percent since the second citywide spraying a few weeks ago.

The encephalitis cases found in New York originally were blamed on the St. Louis strain. But late last month, some dead crows were spotted near the Bronx Zoo. Subsequently, 20 zoo birds died and tested positive for the African strain, said zoo spokeswoman Linda Corcoran.

The West Nile virus -- never before recorded in the Western Hemisphere -- probably entered the United States in infected birds.

Symptoms of the strain -- fever and headache -- are similar to those of the St. Louis strain but generally are milder. In rare cases, the virus can cause neurological disorders and death. The elderly, young and those with weakened immune systems are most vulnerable.

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