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Seasonal Bird Migrations Could Spread West Nile Fever Southward West-Nile Fever Found in New York May Move South with Bird Migration

By Lynne Duke

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 based in Fort Collins, Colo.

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.

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 the late 1970s. In examining this latest strange 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.

West Nile fever first was diagnosed in Uganda in 1937 and later became endemic in Egypt, India and other parts of Asia.