Monrovia - Families hiding infected loved ones and the existence of "shadow zones" where medics cannot go mean the West African Ebola epidemic is even bigger than thought, the World Health Organization said on Friday.
Some 1 427 people have died among 2 615 known cases of the deadly virus in West Africa since the outbreak was first identified in March, according to new figures released by the WHO on Friday.
However the U.N. agency, which has faced criticism that it moved too slowly to contain the outbreak, said that many cases had probably gone unreported.
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Independent experts raised similar concerns a month ago that the contagion could be worse than reported because some residents of affected areas are chasing away health workers and shunning treatment.
Despite initial assertions by regional health officials that the virus had been contained in its early stages, Ebola case numbers and deaths have ballooned in recent months as the outbreak has spread from its initial epicentre in Guinea.
"We think six to nine months is a reasonable estimate," Keiji Fukuda, the WHO's Assistant Director-General for Health Security, said during a visit to Liberia, speaking of the time the agency now believes will be required to halt the epidemic.
An Ebola outbreak will be declared over in a country if two incubation periods, or 42 days in total, have passed without any confirmed case, a WHO spokesperson said.
Under-reporting of cases is a problem especially in Liberia and Sierra Leone, currently the two countries hardest hit. The WHO said it is now working with Medecins Sans Frontières (MSF) and the U.S. Centers for Disease Control and Prevention to produce "more realistic estimates".
Nigeria, the fourth country affected, confirmed two new cases on Friday, bringing the total number of recorded cases there to 14. The health minister said both patients caught the disease from people who were primary contacts of the Liberian man who first brought it to the economic capital Lagos.
The stigma surrounding Ebola poses a serious obstacle to efforts to contain the virus, which causes regular outbreaks in the forests of Central Africa but is striking for the first time in the continent's western nations and their heavily populated capitals.
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"As Ebola has no cure, some believe infected loved ones will be more comfortable dying at home," the WHO said in a statement detailing why the outbreak had been underestimated.
"Others deny that a patient has Ebola and believe that care in an isolation ward - viewed as an incubator of the disease - will lead to infection and certain death."
Corpses are often buried without official notification. And there are "shadow zones", rural areas where there are rumours of cases and deaths that cannot be investigated because of community resistance or lack of staff and transport.
In other cases health centres are being suddenly overwhelmed with patients, suggesting there is an invisible caseload of patients not on the radar of official surveillance systems.
On Friday, the WHO said it had drawn up a draft strategy plan to combat the disease in West Africa, and details would be released early next week.
David Nabarro, Senior United Nations System Coordinator for Ebola, who was travelling with the WHO's Fukuda in Liberia, said the strategy would involve ramping up the number of health workers fighting the disease.
"It means more doctors, Liberian doctors, more nurses, Liberian nurses, and more equipment," he said.
"But it also means, of course, more international staff."
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The affected West African countries were already struggling with few doctors and fragile healthcare systems before the Ebola outbreak. And health workers have been among the hardest hit by the disease.
The head of MSF, which has urged the WHO to do more, told Reuters in an interview on Thursday that the fight against Ebola was being undermined by a lack of international leadership and emergency management skills.
In a sign of spreading regional alarm, Senegal, West Africa's humanitarian hub, said it had blocked a U.N. aid plane from landing and was banning all further flights to and from countries affected by Ebola.
Gabon also announced on Friday its suspension of air and sea links to the four affected countries, following the lead of a number of regional nations who have defied WHO advice in an attempt to isolate themselves from the disease.
The World Health Organisation has repeatedly said it does not recommend travel or trade restrictions for countries affected by Ebola, saying such measures could heighten food and supply shortages.