The mosquito-borne viral outbreak that has killed four people and sickened at least 33 others in the New York City area has been reclassified as a rare African and Asian virus that never before has been diagnosed in the Western Hemisphere, city and state officials said today.

The federal Centers for Disease Control and Prevention in Atlanta said an earlier series of diagnoses that pinpointed the viral outbreak as St. Louis encephalitis was wrong. Instead, genetic testing has teased out a different viral strain that is similar, though not identical, to West Nile fever. West Nile fever typically is less dangerous than St. Louis because its symptoms do not necessarily include the brain and spinal infections known as encephalitis.

The diagnostic turnabout came last week with the discovery that a mysterious spate of Bronx Zoo bird deaths was caused by a West Nile-like fever. This prompted more testing of the human encephalitis cases.

In New York City, 14 confirmed cases of what previously had been called St. Louis encephalitis, including three fatal ones, were reclassified as West Nile, along with 11 additional cases that were under investigation. In Westchester County, north of the city, eight West Nile cases have been diagnosed, including one death. An additional 174 cases are being reexamined both for West Nile and Kunjin, a close viral kin from Australia. Though stricken birds have turned up in Connecticut, no human cases have been reported there.

The New York metropolitan area, and parts of Connecticut where dead crows were diagnosed, now have become laboratories for scientists tracking the course of a virus that first surfaced in Africa 60 years ago, became endemic to Asia and surfaced in Europe three years ago but never had been seen in North or South America.

City and health officials played down any new risk factors from West Nile. Both strains of the virus are passed from birds to mosquitoes to humans, so the region-wide program of insecticide spraying that has been underway here for three weeks will stymie the spread of either virus.

"From a scientific point of view, this is a bombshell," said Tracey McNamara, head of pathology at the Wildlife Conservation Society at the Bronx Zoo, whose dead birds led to the diagnosis of both animal and human West Nile.

"But from a human health point of view the mosquito spraying was appropriate three weeks ago and is still appropriate now," she said.

John Roehrig, chief of the arbovirus disease branch of the CDC's vector-borne infectious diseases division in Fort Collins, Colo., which conducted the tests on the New York outbreak, attributed the discrepancy in the diagnoses to the fact that the New York outbreak bore the markers of the St. Louis virus, which has occurred in the region before. He said New York City officials were alerted early on to the possibility that the diagnosis could change with further analysis.

City officials urged residents to continue taking precautions against mosquito bites at all times of the day or night. Though health officials assume that the West Nile virus is primarily transmitted by the Culex pipiens mosquito that is active from dusk to dawn and common in urban areas, they cannot know for sure because the disease is new here.

Because the symptoms can vary widely, from brain swelling, high fever, coma and convulsions to a mere headache, rash and slight temperature, Roehrig said it is possible that the pool of infected people is far higher than the 33 current cases. Those with mild symptoms may simply assume they've got a common bug and just ride the symptoms out, he said.

West Nile fever originally was diagnosed in Uganda in 1937. The first known outbreak of the virus came in 1950 in Egypt. Other outbreaks followed, especially in Asia and India, and the most recent one happened in 1996 in the Romanian capital, Bucharest, with 393 confirmed cases.

Roehrig's division studied the Romanian outbreak, which virologists believe was caused when migratory birds introduced the virus to the region. He said it is not yet clear how the virus got to New York, but it could be from bird migration or from virus-carrying imported birds that infected the area's mosquito population.

In what may be a sentinel for West Nile outbreaks, the first signs of something amiss came with a mysterious spate of dying crows. It happened in July and early August in the Bronx near the zoo. Then on Aug. 8, in what seemed a separate development, the first human encephalitis case was reported.

How the Fever Spreads

1. A migratory bird probably carried the virus to New York. The bird holds a high level of the virus in its blood.

2. Mosquitoes that feed on infected birds carry the virus and spread it to other birds.

3. Humans contract the virus from mosquitoes, but cannot transmit it to other humans.

The Disease

* The symptoms of West Nile fever can result in inflammation of the brain and damage to the nervous system or a mere headache, rash and slight temperture.

* Fever and headache appear within eight to 10 days of infection.

SOURCES: Centers for Disease Control and Prevention, Associated Press

CAPTION: A truck sprays an insecticide intended to kill mosquitoes in Manhattan. Nine cases of mosquito-borne West Nile fever have been confirmed in the New York City area.