NEW YORK (Reuters) - A doctor who worked in West Africa with Ebola patients was in an isolation unit in New York City on Friday after testing positive for the virus, becoming the fourth person diagnosed with the disease in the United States and the first in its largest city.

Dr. Craig Spencer, 33, was placed in a quarantined unit at Bellevue Hospital on Thursday, six days after returning from Guinea, renewing public jitters about transmission of the disease and rattling financial markets.

Three people who had close contact with Spencer, a physician for the humanitarian group Doctors Without Borders, were quarantined for observation - one of them, his fiancée, at the same hospital - but all were still healthy, officials said.

A woman named Morgan Dixon was identified on Spencer’s Facebook page as his fiancée.

Mayor Bill de Blasio and Governor Andrew Cuomo sought to reassure New Yorkers they were safe, even though Spencer had ridden subways, taken a taxi and visited a bowling alley between his return from Guinea and the onset of his symptoms.

"There is no reason for New Yorkers to be alarmed," de Blasio said at a news conference at Bellevue. "Being on the same subway car or living near someone with Ebola does not in itself put someone at risk."

Cuomo said that unlike in Dallas, where two hospital nurses treating an Ebola patient contracted the disease, New York officials had time to thoroughly prepare and drill for the possibility of a case emerging in the city.

"From a public health point of view, I feel confident that we’re doing everything that we should be doing, and we have the situation under control," he said. "There's no reason for New Yorkers to panic or feel that they have anything to worry about on the subway system, etc.," Cuomo said in an interview on CNN.

U.S. stock index futures dipped on Friday, with the benchmark S&P 500 index down 0.3 percent, as the first diagnosed case of Ebola in New York City raised concerns about the spread of the virus in the nation's most populous city.

The worst Ebola outbreak on record has killed at least 4,877 people and perhaps as many as 15,000, mostly in Liberia, Sierra Leone and Guinea, according to World Health Organization figures.

Four Ebola cases have been diagnosed so far in the United States: Liberian Thomas Eric Duncan, who died on Oct. 8 at Texas Health Presbyterian Hospital in Dallas, two nurses who treated him there, and Spencer.

Health officials emphasized that the virus is not airborne but is spread through direct contact with bodily fluids from an infected person who is showing symptoms.

After taking his own temperature twice a day since his return, Spencer reported running a fever and experiencing gastrointestinal symptoms for the first time early on Thursday. He was then taken from his Manhattan apartment to Bellevue by a special team wearing protective gear, city officials said.

Cuomo said Spencer checked into the hospital when he realized he had a temperature of 100.3 degrees Fahrenheit, "not 103 as has been reported," suggesting he may have caught the onset of symptoms early.

He was not feeling sick and would not have been contagious before Thursday morning, city Health Commissioner Mary Travis Bassett said. Owners of the bowling alley he visited said they had voluntarily closed it for the day as a precaution.

The driver of the ride-sharing taxi Spencer took was not considered to be at risk, and officials insisted the three subway lines he rode before falling ill remained safe.

"We consider that it is extremely unlikely, the probability being close to nil, that there would be any problem related to his taking the subway system," Bassett said.

The U.S. Centers for Disease Control and Prevention (CDC) will confirm Spencer's test results within 24 hours, she said. President Barack Obama was briefed about the New York Ebola case and spoke by telephone separately on Thursday night with Cuomo and de Blasio, the White House said.


Spencer's case brings to nine the total number of people treated for the disease in U.S. hospitals since August, but just two - Duncan's nurses - contracted the virus in the United States.

The New York case surfaced days after dozens of people who were exposed to Duncan emerged from a 21-day incubation period with clean bills of health, easing a national sense of crisis that took hold when his nurses, Nina Pham and Amber Vinson, became infected.

"I'm really concerned," said Kiki Howard, 26, a student who lives on a street near Spencer's home in Harlem. "There's a school at the end of the block. My main concern is for the safety of the children."

"I just moved here from Dallas three weeks ago," said Emma Clarke, a dancer who also lives nearby.

The city health commissioner said Spencer's apartment was isolated and sealed off.

"I see no reason for the tenants in the apartment building to be concerned," she said.

The health commissioner said Spencer completed work in Guinea on Oct. 12 and arrived at John F. Kennedy International Airport in New York on Oct. 17.

His Facebook page, which included a photo of him clad in protective gear, said he stopped over in Brussels.

Spencer has specialized in international emergency medicine at Columbia University-New York Presbyterian Hospital in New York City since 2011. Columbia in a statement said he had not been to work nor seen any patients since his return.

The CDC did not name Spencer but said he "participated in the enhanced screening" instituted for all travelers returning from Guinea, Liberia and Sierra Leone this month at five major U.S. airports - including Kennedy. The doctor "went through multiple layers of screening and did not have a fever or other symptoms of illness," the CDC added in a statement.

By Ellen Wulfhorst and Sebastien Malo

(Additional reporting by Natasja Sheriff, Barbara Goldberg, Jonathan Allen and Laila Kearney in New York and Bill Trott in Washington; Writing by Steve Gorman; Editing by Louse Ireland, Howard Goller and Bernadette Baum)