A deadly Islamist insurgency in northern Nigeria has harmed efforts to eradicate polio in the region, the WHO says, with a resurgence of the potentially paralysing virus reversing gains.
Nigeria, one of only three countries still considered to have endemic polio, alongside Pakistan and Afghanistan, has accounted for 72 of the 123 polio cases recorded so far this year, a World Health Organisation report said.
The resurgence of the contagious virus also threatens to spread to neighbouring nations such as Niger, it said.
The report, the WHO's latest weekly update on the global polio situation, said insecurity in Nigeria's northeast, where the Islamist insurgency has been based, was harming efforts to attack the virus.
"Insecurity remains a challenge in parts of the country, notably in Borno and Yobe states, particularly in the capital cities of both of these states," said the report issued last week.
Islamist militant group Boko Haram has carried out increasingly sophisticated bombing and shooting attacks in northern and central Nigeria, with northeastern Borno and Yobe states considered its stronghold.
A polio surveillance official at the Nigerian office of the WHO said the security challenges affect the whole region and had slowed vaccination efforts.
"The report was only being specific on Borno and Yobe on the security factor, but the situation is the same throughout the north," the WHO official said on condition of anonymity because he was not authorised to speak publicly.
"Polio immunisation coverage has dropped in areas where Boko Haram attacks occur because immunisation personnel would not want to go into such areas for safety reasons."
He said this partly explained the upsurge in new polio cases, though other factors, including spotty support from local officials and conspiracy theories about the vaccine among the population, have also been cited.
The country's north has been particularly hit by the "wild poliovirus type III" strain—one of three types of polio.
"Nigeria is the only country in the world to report wild poliovirus type III (WPV3) in the past four months," the report said. It added later: "Northern Nigeria is now the global epicentre of WPV3 transmission."
Polio cases in Nigeria dropped to 21 in 2010 from a staggering 388 cases in 2009 due to a massive effort to immunise the population.
But officials reported an uptick to 62 cases in 2011, and the 2012 numbers so far show that the situation has only worsened.
Nigeria has faced severe difficulties in dealing with the virus in the past.
In 2003, Kano, the most populous state in the north, banned polio immunisation for 13 months following claims by Muslim clerics that the vaccine was laced with substances that could render girls infertile as part of a US-led Western plot to depopulate Africa.
The ban caused mass rejections of polio immunisation by parents in the region, which led to the spread of the virus to other parts of the continent that were previously considered polio-free.
The WHO report also expressed concern over an upsurge in polio cases in Katsina state on the border with Niger, which could lead to the virus spreading to Nigeria's northern neighbour, where no new cases had been registered so far this year.
Katsina has recorded 14 new polio cases this year, eight of them in the past three months, the report said.
"The possibility of conducting immunisation campaigns across the border in Niger is being explored, to minimize the risk and consequences of polio spread from Katsina," said the report.