(Reuters) - When Liberian-American Patrick Sawyer collapsed at Lagos airport, he brought Ebola into a potentially ideal place for the deadly virus to spread - a vast, dirty, overcrowded city where tracing carriers and their contacts is a major problem.
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By the time they realized where he was from or what illness he had, Sawyer had had contact with dozens of people. Lagos has now had 10 cases of Ebola, an illness spread by contact with the fluids of an infected person.
As Africa's biggest economy, Nigeria has a better health system than the other west African countries which are among the poorest in the world, and Ebola doesn't spread through the air or water supply as with many other epidemic diseases.
But health experts - who are trying to overcome superstition and public ignorance about Ebola as well as the disease itself - say there is now only a short opportunity to find and lock down other infected people before the outbreak in the city of 21 million gets out of hand.
"Lagos is big, it's crowded. It would make in many ways a perfect environment for the virus to spread," said Nigerian epidemiologist Chikwe Ihekweazu, who runs website Nigeria Health Watch and worked on Ebola in South Sudan a decade ago.
"In the heart of Lagos, people live on top of each other, sharing bedrooms and toilets. In densely populated communities infection control becomes almost impossible to do well."
When Sawyer landed at Murtala Mohammed airport on July 20, none of the bystanders, airport staff or health workers who rushed to help him understood the danger they were in.
No one had the full body protection of mask, suit and gloves that are essential to prevent contagion, so his ill advised journey gave the world's worst Ebola outbreak a foothold in Africa's most populous nation.
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Sawyer was taken to First Consultants Hospital where, Idris said, he was treated "like any ordinary patient".
Hospital staff took blood samples, checked his temperature, treated his symptoms, which by then included severe vomiting, diarrhea and bleeding - all signs of late-stage Ebola.
"In the course of doing this, a lot of those health workers got infected," said Idris.
It took weeks to trace about 70 people who were primary contacts with Sawyer, multiplying the possible avenues of contagion in the meantime.
Tom Frieden, Director of the U.S. Centers for Disease Control and Prevention (CDC) which is helping to fight the West African outbreak, said he was "deeply, deeply concerned about the situation" in Lagos because of the city's size.
"If you leave behind even a single burning ember, it's like a forest fire. It flares back up," he told a U.S. House Committee on Foreign Affairs last week.
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Sawyer, who worked as public health manager at an iron ore mining project of steelmaker ArcelorMittal, caught the disease from his sister who died in Monrovia of Ebola on July 8.
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In many ways, Africa's top oil producer is in a better position than the other three affected countries. According to consultancy DaMina Advisors, it has one doctor per 2,879 people - compared with one per 86,275 in Liberia.
While public doctors are striking over pay, Nigeria has the money to mobilize enough health workers for now.
Recent progress in tracing Sawyer's contacts, though it got off to a slow start, gives some cause for hope, experts say.
Yet because it is a city of migrants, Lagos is a potential springboard for Ebola to spread across Nigeria. "There's a lot of mobility within the country. If infected people end up taking taxis to their villages, then we're in trouble," said Ihekweazu.
"PEOPLE ARE AFRAID"
Alarm bells first went off when the hospital ran I.D. checks on Sawyer two days after his arrival, and figured out belatedly that he was Liberian who had come from Monrovia, Idris said.
They quickly isolated him and tested him for Ebola. Yet even after that, nurses continued treating Sawyer without protection, so "the chances of infection again went up".
Tracing Sawyer's contacts aboard the flight then took time because the airline produced a passenger list only after a week.
Three weeks later, 177 primary and secondary contacts of Sawyer's have been traced and all are under surveillance, federal Health Minister Onyebuchi Chukwu said on Monday.
Chukwu also announced measures to contain the disease, including training health care professionals in surveillance of possible cases, putting port officials on red alert, and a public awareness campaign in multiple languages.
In its built-up metropolitan areas, Lagos has 20,000 people per square km (50,000 people per square mile), the state government says, about the same as other overcrowded cities such as Mumbai and Dhaka.
Sanitation is at least as bad as either of the other two, with most Lagosians urinating and defecating in the open.
Both walls leading to the entrance to the now closed First Consultants hospital have "Do Not Urinate Here" stenciled on them, though judging by the smell no one pays much attention.
Against one, an old woman sells a kaleidoscopic array of flip-flop sandals flowering out of a rusty wheelbarrow. A child hawks plastic bags of pineapple slices next to an open drain.
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"We've never had such a situation before, never had Ebola in a large, densely populated area like Lagos," said John Vertefeuille, leader of the CDC's response team for the city. The real problem "is identifying suspected patients ... and making sure that we find every contact", he told Reuters. "That's our priority."
Public education is another essential. Boyai Sanusi, who runs a bag-making shop on a street opposite the hospital, was shocked when he heard Ebola was on his doorstep, but confesses he has no idea how it is spread. "People are afraid," he said. "They don't know what to do."
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Comment re Red Highlighted Section
SURVIVAL OUTSIDE HOST: The virus can survive in liquid or dried material for a number of days (23). Infectivity is found to be stable at room temperature or at 4°C for several days, and indefinitely stable at -70°C (6, 20). Infectivity can be preserved by lyophilisation.
From http://www.phac-aspc.gc.ca/lab-bio/r.../ebola-eng.php
So a single infected person defecating in the open could generate an enormous number of secondary infections, via people treading in faeces etc.