Scientists fear that a bird and mosquito-borne virus that has killed four and sickened 33 in New York state is spreading southward with the season's bird migrations and are warning health officials to be alert for the strange crow deaths and other signs that have heralded the outbreak here.

West Nile fever, a rare and often encephalitic virus that had never before been diagnosed in the Western Hemisphere, is spread from birds to mosquitoes to humans. The first human cases here were diagnosed in mid-August, when birds began their north-south migrations for the fall. In addition to spreading the virus here, birds--known as the virus's reservoir hosts--have probably taken it with them to points farther south, though so far no cases of infection have been reported outside the New York area.

"Our guess is that probably the bird migrations took the virus south with them," said Duane Gubler, director of the vector-borne infectious disease division of the federal Centers for Disease Control and Prevention. He said the CDC was encouraging "states and local health departments in the southern U.S., as well as in Central and South America to intensify their surveillance for this virus."

The discovery of West Nile fever here has spawned an epidemiological mystery, for scientists have no clue how the virus got to this hemisphere. The virus is endemic in parts of east Africa, Asia and the Middle East, and it broke out as recently as three years ago in Eastern Europe. But birds generally do not migrate across the Atlantic Ocean, except for the odd occasions when birds are lost and "get up in the air currents and end up in a different continent," Gubler said.

More likely, he said, is that an imported bird brought the virus here, or that a human infected with it traveled to this region. In either case, with the mosquito as the vector, the virus soon spread. The first New York case was reported on Aug. 8.

Originally, state health officials and the CDC thought they were dealing with St. Louis encephalitis, which has broken out before in the eastern United States, including a few cases in New York during the late 1970s. In examining this latest outbreak, it made sense, virologists say, to look at known strains. And being a strain that was unknown in this hemisphere, the West Nile virus--though very similar to St. Louis--just wasn't considered.

But last week, after matching genetic specimens of the human viral deaths to viral deaths in birds stricken at the Bronx Zoo, the CDC reclassified the virus as the far more rare West Nile fever. West Nile can also lead to encephalitis, or inflammation of the brain, but often does not and thus is less severe than St. Louis. Fever and severe headache are its basic symptoms.

While birds of many kinds are believed to be carriers of the virus, its presence in crows has proved a particularly telling sentinel for the outbreak.

"More crows are dying than anything else, possibly because crows are more susceptible to the disease," said Nicholas Komar, a vertebrate ecologist with the CDC. After a 1950 outbreak of West Nile virus in Egypt, studies there showed crows to be particularly susceptible to the virus, Komar said.

By late today, state health officials reported that 732 dead birds have been found since the viral outbreak began seven weeks ago and 64 percent of them were crows. Tests conducted on a small number of those birds so far have turned up West Nile in five. In New Jersey, state health officials reportedly have sent three dozen dead crows to the CDC for testing. A dead crow has been diagnosed with West Nile in Connecticut.

Though the CDC had not received reports of abnormal incidence of dead crows in any other region, Komar warned that "a cluster of dead crows may be an indicator that the virus has spread."

West Nile fever first was diagnosed in Uganda in 1937 and later became endemic in Egypt, India and other parts of Asia. An outbreak centered in the Romanian capital of Bucharest in 1996 yielded about 400 confirmed cases of the most serious form of the fever, including encephalitis, which killed about 40 people, according to press reports of that time.

New York City and surrounding jurisdictions affected by this latest outbreak--such as Westchester and Nassau counties--continue their programs of insecticide spraying to kill the region's mosquito population.

Mosquitoes that escape the Malathion spray will become increasingly scarce as the weather changes. By the fall's first frost, mosquitoes will have largely disappeared, Gubler said.

But questions about the virus they bear will keep virologists busy for months to come.

"The important thing to remember: We don't even know when this was introduced," Gubler said. "It could have been introduced last year and we just missed it in our [mosquito] surveillance. It could have been introduced in one of the southern states and introduced into New York in the spring of this year."