Ebola patient in Liberia (file)
Ebola patient in Liberia (file) Reuters

Nigeria - Africa's most populous country - has been declared officially free of Ebola, the World Health Organization (WHO) said Monday. 

WHO may declare an Ebola outbreak over if two incubation periods of 3 weeks pass without any new cases. The last reported case in Nigeria was discovered 6 weeks ago on September 5. 

The virus spread to Nigeria when Patrick Sawyer, an American-Liberian citizen, was diagnosed there in July.

Nigeria immediately declared a national public health emergency.

John Vertefeuille, from the US Centers for Disease Control and Prevention (CDC), said that Nigeria took all the right steps to contain the outbreak.

"Nigeria acted quickly and early and on a large scale," he told AFP news agency. "They acted aggressively, especially in terms of contact-tracing," he added.

Patrick Sawyer later died of the disease, followed by seven Nigerians, including Dr. Ameyo Stella Adadevoh, who was the first to diagnose Sawyer. Dr. Adadevoh is credited with helping to contain the outbreak at its source.

Dr. Adadevoh's son, Bankole Cardoso, told BBC News that because Sawyer was diagnosed so quickly, Nigeria was able to trace all those who could possibly have contracted the disease from him.

"That was probably the difference between us and our West African neighbors," he said.

More than 4,500 people have died of the Ebola outbreak in West Africa, mostly in Liberia, Guinea, and Sierra Leone.

"The virus is gone for now. The outbreak in Nigeria has been defeated," WHO Nigerian representative Rui Gama Vaz told BBC News.

"This is a spectacular success story that shows to the world that Ebola can be contained but we must be clear that we have only won a battle, the war will only end when West Africa is also declared free of Ebola."

Senegal, another West African country, has also been officially declared Ebola-free. The announcement came from WHO on Friday.