When an American consultant named Patrick Sawyer stepped off a plane in Nigeria last month, he may have touched off an Ebola outbreak in the worst possible place — one of the world’s largest and most densely populated cities.
A Nigerian health commissioner said Tuesday that eight people who came into contact with Sawyer, including at least one doctor who treated him after he collapsed, are suspected to have contracted Ebola and have now been quarantined.
Authorities announced early Wednesday that a nurse had died of Ebola. Earlier, Nigerian officials acknowledged a slow response, and said health officials did not know when Sawyer landed that his sister had died of the disease.
Meanwhile dozens of people are being watched by Nigerian authorities for Ebola symptoms, and the government has asked for volunteers to hunt for others who may have been exposed to people carrying the deadly virus. Arriving airline passengers are being subjected to health screenings.
And epidemiologists — so-called disease detectives — are trying to track down anyone else who may have crossed paths with Sawyer, who died on July 25, five days after he walked off the plane in Lagos.
The Ebola outbreak in Africa, the worst in history, has mostly been in three countries — Sierra Leone, Guinea and Liberia, where Sawyer boarded the plane. But the growing number of cases in Lagos, the most populous city in Africa, presents new alarm.
"Lagos is probably our worst-case scenario in terms of where you want Ebola to spread," said Carl LeVan, an assistant professor at American University and a specialist on Nigeria.
The Centers for Disease Control and Prevention said Tuesday that it has four experts in Nigeria and plans to send more.
The most immediate work is tracking down who came into contact with Sawyer, a naturalized American citizen who was working in Liberia, and watching them for Ebola symptoms, including fever, nausea, vomiting and diarrhea.
Dr. Inger Damon, the CDC’s incident manager for the outbreak, called it an "extraordinary effort."
"We know how many flights, what planes go where, and if there is a need we can request the manifest information, but there are certain approaches of when that can be done and how that can be done," she said.
Nigerian authorities have given few details about the investigation. But Dr. Trish Perl, a professor of medicine and infectious diseases at Johns Hopkins, said that disease detectives work in figurative concentric circles, moving outward from the patient.
In Sawyer’s case, that means tracking down not just who was on the plane with him but who was seated next to him, in front of him and behind him, she said.
In West Africa, she said, there are cultural challenges to conducting such an investigation and preventing the further advance of the disease, including rampant rumors, mistrust of medicine and burial practices that can help the disease spread.
And in a city as densely populated as Lagos, "You just sort of having to take everything we’re trying to deal with and just magnify it by 800,” she said. "You get into crowded areas, it’s much easier for people to disappear."
Lagos has a population of about 21 million, roughly the same as the New York metropolitan area, and is a busy air travel hub for Africa. Someone already infected can get on a plane, as Sawyer did, and take the disease to another country.
Sawyer, 40, had lived in Liberia since 2008 and worked as a consultant for the government’s Ministry of Finance while his wife raised their young children in Coon Rapids, Minnesota. His sister had died of Ebola in recent weeks. The people on the plane with him were not quarantined.
"If they knew his history, they probably would have taken better precautions," Jide Idris, the state health commissioner for Lagos, told reporters on Tuesday.
Still, health officials have stressed that Ebola is spread through direct contact with bodily fluid, and that it is unlikely someone would get sick just from being on a plane with an Ebola-stricken passenger.
Perl said that guidance was "absolutely on target," but she said it was important to track down everyone on the plane to be sure no one else was infected, and to reassure the public about the Ebola outbreak.
British Airways on Tuesday suspended its flights to and from Liberia and Sierra Leone. And Saudi Arabia said that a man who took a business trip to Sierra Leone showed up at a hospital in Jeddah on Monday night with Ebola-like symptoms.
Delta and United Airlines fly to Lagos but have not suspended flights. American Airlines does not fly to Africa.
On the ground in Nigeria, part of the challenge for the CDC workers will be educating the public about Ebola and making sure procedures are in place to keep health workers from getting infected themselves.
Nigerian emergency management authorities have gotten better in recent years, but the outbreak will be a test for the troubled government of President Goodluck Jonathan, said LeVan, the American University expert, who recently published a book on Nigeria.
In the last few months, during the crisis after the abduction of almost 300 Nigerian schoolgirls by the terror group Boko Haram, the Jonathan government has struggled to avoid contradicting itself and confusing the public.
"If you do that with a public health threat, you can very easily exacerbate the risks and contribute to the spread," LeVan said.
Dr. Nancy Snyderman of NBC News contributed to this report. Reuters and The Associated Press also contributed.