New York - As a New York City doctor tests positive for Ebola after volunteering in West Africa, health officials face the challenge of deciding how wide a net to cast for his possible contacts in the largest, most crowded city in the United States.
Dr Craig Spencer, an emergency doctor who was working with Doctors Without Borders in Ebola-stricken Guinea earlier this month, returned to the city last Friday.
Since then, city officials say, he visited a city park, had a meal at a restaurant, visited a Brooklyn bowling alley, took at least three subway trains and went for a 4.8km run.
A New York health official involved in the case told Reuters that the focus will be on finding people who have had close contact with Spencer.
Dr Mary Travis Bassett, the city's health commissioner, said at a press conference that Spencer had only come in close contact with two friends and his fiancée, all of whom seemed fine but had now been quarantined.
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Officials had spoken to a taxi driver who had given Spencer a ride on Wednesday but did not consider him at risk.
Officials will not undertake the likely impossible task of tracking down every last commuter who rode the same subway trains as Spencer because the chance any of them had caught Ebola was "probably close to nil," she said.
The worst Ebola outbreak on record has killed at least 4 877 people in West Africa since March, with a small number of infections detected outside Africa. The first patient to be diagnosed on US soil, Liberian traveller Thomas Duncan, arrived in late September and died on October 8. Two nurses who treated him fell ill.
Since then, US hospitals have been on high alert, with dozens of suspected cases evaluated for the deadly virus. Spencer is the first confirmed case in New York City.
The city will follow guidelines laid down by the US Centres for Disease Control and Prevention, which says that Ebola is transmitted by contact with the bodily fluids, such as vomit or sweat, of a person sick with the disease.
Dr Gavin Macgregor-Skinner, an expert in public health at Pennsylvania State University, helped advise officials in the densely populated Nigerian city of Lagos. Nigeria was declared Ebola-free on Monday.
Macgregor-Skinner said it will be important that public health officials cast as wide a net as possible for Spencer's contacts.
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"We believe the more people you had in contact tracing, the more confident people are that public health is doing the job it is set up to do," he said.
Experts said the fact that Spencer is an experienced doctor who had knowledge of dealing with Ebola was encouraging.
Doctors Without Borders said it had guidelines for staff returning from Ebola assignments that included regularly self-monitoring for signs of illness. A spokesperson for the humanitarian group did not respond to a request for more details.
"From what I understand, immediately upon getting symptoms, this patient isolated himself, so his circle of contacts is going to be very small," said Dr Amesh Adalja, a public health expert with the Infectious Diseases Society of America who is not involved in Spencer's case.
New York City's health department has a deep experience with containing illnesses, not least those that arrive through its busy airports. Before becoming director of CDC in 2009, Thomas Frieden ran the department.
"It's kind of bread and butter for New York to do that kind of work," Adalja said. "They are a model for the nation because they have a larger burden of the nation's tuberculosis cases."