Washington (AFP) - The Ebola outbreak in Nigeria is almost over, US health officials said Tuesday, in a rare sign of authorities turning the tide on the highly contagious disease that has killed more than 3,000 in West Africa.

But in a fresh setback to the global fight against the virus, the United States almost simultaneously said an Ebola case had been diagnosed there for the first time in a man who became infected in Liberia and traveled to Texas.

The virus's incubation period is 21 days and after two of these periods have passed without any new cases, officials can declare an outbreak over.

Therefore, since there have been no new cases in Nigeria since August 31, the country should be able to announce a formal end to its outbreak on October 12, a spokesman for the US Centers for Disease Control and Prevention told AFP.

Meanwhile, the last three people monitored due to potential exposure to an Ebola patient will end their 21 days of follow-up for signs of symptoms later this week.

"The last three patient contacts will exit their 21-day follow-up on October 2 -- strongly suggesting the outbreak in Nigeria has been contained," the CDC said in a statement.

While Nigeria may be able to soon declare victory over Ebola, its outbreak was far smaller than in nearby Guinea, Sierra Leone and Liberia, all also in West Africa.

The death toll from the world's worst Ebola epidemic has claimed 3,091 lives in five West African countries out of 6,574 infected, according to the World Health Organization (WHO).

Eight people died of Ebola in Nigeria out of 20 confirmed cases, according to the WHO. The Nigerian government has said seven people died and 19 were infected.

The outbreak in Nigeria began July 20 when Patrick Sawyer, a dual US-Liberian citizen, boarded a plane to Lagos, a densely populated city of 21 million people.

Ebola is spread through close contact with the bodily fluids of an infected person. The illness causes diarrhea, vomiting, fever and fatal bleeding in some cases.

Story continues

"By the time it was recognized that the patient carried the Ebola virus, he had exposed 72 people on commercial aircraft, at the airport, and at the hospital," the CDC said.

The US agency credited Nigerian authorities with taking quick actions to isolate patients and set up an incident management center for a coordinated response.

"Although Nigeria isn't completely out of the woods, their extensive response to a single case of Ebola shows that control is possible with rapid, focused interventions," said CDC Director Tom Frieden.

Official forecasts of the end of Nigeria's outbreak vary.

The Nigerian government has previously said the last confirmed case was discovered September 8, suggesting the end would come later in October than foreseen by the CDC.

Typically, the WHO is the formal authority for declaring an end to any outbreak.

Speaking last week at the UN General Assembly in New York, Nigerian President Goodluck Jonathan said -- prematurely, according to medical experts -- that his nation was free of Ebola.

"We can confidently say that today Nigeria is Ebola free," Jonathan told the largest diplomatic gathering in the world on September 24.