* Man collapsed at Lagos international airport
* Lagos is a city of 21 million people
* Ebola has killed 660 people across West Africa (Adds man collapsed in airport, details from health ministry, byline)
By Chijioke Ohuocha
LAGOS, July 24 (Reuters) - A Liberian man in his 40s is being tested for the deadly Ebola virus in Nigeria's commercial capital of Lagos, a mega-city of 21 million people, the Lagos State Health Ministry said on Thursday.
Ebola has killed 660 people across Guinea, Liberia and Sierra Leone since an outbreak began in February, straining their flimsy healthcare systems despite international help.
If confirmed, the case would be the first on record of one of the world's deadliest diseases in Nigeria, Africa's biggest economy and, with 170 million people, its most populous country.
Nigeria also has some of the continent's least adequate healthcare infrastructure for a nation of its size, despite access to billions of dollars of oil money as Africa's biggest producer of crude.
The special adviser on public health to the Lagos state government, Yewande Adeshina, told a news conference the man had collapsed on arrival in Lagos airport from Liberia on Sunday. He was rushed to hospital and put in an isolation ward, she said.
"The patient was admitted and detained on suspicion of possible EBV (Ebola) infection, while blood sample collection and testing was initiated," she said in her statement.
Samples had been sent to a World Health Organisation (WHO) laboratory in Dakar, she said, adding that "results are pending."
The Ebola outbreak started in Guinea's remote southeast and has since spread across the region's poorly controlled borders. Symptoms of the highly infectious disease are diarrhoea, vomiting and internal and external bleeding.
Adeshina said Lagos state authorities had requested a flight manifest and would be contacting all the other passengers to warn them of the risk. They would also be tracing any places the passenger had been, and had already distributed protective clothing to health workers, she said. (Writing by Tim Cocks, editing by Jeremy Gaunt, Larry King)