Freetown (AFP) - The latest flare-up of Ebola in Sierra Leone has ended with no confirmed cases of the virus now in West Africa, the UN said Thursday, as the country celebrated a "joyous" milestone.

"The World Health Organization joins the government of Sierra Leone in marking the end of the recent flare-up of Ebola virus disease in the country," a statement said.

The global health body made the announcement 42 days -- or two Ebola incubation cycles -- since Sierra Leone's last Ebola patient tested negative for the second time.

"I want to seize this joyous moment to thank all Sierra Leoneans for your contribution to the success we are celebrating today," Sierra Leone's Minister of Health and Sanitation Dr Abu Bakarr Fofanah said in a televised speech.

The epidemic infected a reported 28,600 people across the three hardest-hit nations, half of which were recorded in Sierra Leone.

In total, the virus has claimed 11,300 lives since December 2013, although a significant number of deaths are believed to have gone unreported.

The WHO has recorded almost 4,000 deaths in Sierra Leone since the outbreak began.

- 'Still at risk' -

On Thursday the world health body was also careful to warn that a recurrence of the deadly tropical disease remained a possibility.

"WHO continues to stress that Sierra Leone, as well as Liberia and Guinea, are still at risk of Ebola flare-ups, largely due to virus persistence in some survivors, and must remain on high alert and ready to respond," the statement said.

The health body's representative in Sierra Leone, Anders Nordstrom, said it was "critical that we remain prepared."

The virus can stay in semen for at least nine months after a patient has recovered, six months longer than previously thought.

Scientists are working to establish how long it can persist in other bodily fluids and tissues such as the spinal column and the eye, and for how long it could remain infectious.

"Until the virus is completely cleared from the survivor population -- and that may take the remainder of the year or more -- we have to anticipate and be prepared for additional small outbreaks," a WHO representative told AFP.

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Sierra Leone was declared free of Ebola transmission on November 7 last year, but two cases have emerged since then.

In early January, a 22-year-old woman was taken ill near the Guinean border and died days later, and her aunt was infected soon afterwards.

The WHO refers to these isolated cases as "flare-ups" but maintains the original "chains of transmission" have been stopped in Guinea, Liberia and Sierra Leone.

The rapid containment of the January cases was down to "investments made in rapid response teams, surveillance (and) lab diagnostics," the WHO said, as well as greater prevention and control measures developed in the last 18 months.

- Devastating consequences -

The pathogen, one of the deadliest known, is spread through contact with the bodily fluids of an infected person showing symptoms such as fever or vomiting, or the recently dead.

The epidemic was first reported in Sierra Leone in May 2014, when more than a dozen women contracted Ebola at the funeral of a healer who had been treating patients from Guinea.

At the peak of the outbreak that year, Sierra Leone and its neighbours Liberia and Guinea were reporting hundreds of new cases each week, with social order on the brink of collapse.

Authorities in Sierra Leone locked down entire villages and banned sporting events and social gatherings at Ebola's height, with schools shut for eight months.

The government drew criticism from the international community for confining millions to their homes -- measures deemed punitive and self-defeating.

With life back to normal, taxi driver Sulaiman Banya told AFP in the eastern city of Kenema he shared "the joy of all Sierra Leoneans over today's declaration," adding he hoped the disease "will now give us some respite."

While the primary cost of the outbreak has been in human life, the crisis has also wiped out development gains in Sierra Leone, which was devastated by 11 years of civil war ending in 2002.

Ebola slashed growth to four percent in 2014 and the economy contracted by a massive 21.5 percent in 2015, according to official figures.