A four-fold increase in polio has been reported in Nigeria, with the disease spreading to other countries, a World Health Organisation official says.Forty-three cases were reported in Nigeria this year, compared to 11 last year, the official, Thomas Moran, said.Curbing the polio virus in Nigeria is key to eradicating the crippling disease in Africa, he said.In 2003, northern Nigeria's Muslim leaders leaders opposed vaccinations, claiming they could cause infertility.Nigeria is one of four countries in the world - along with Pakistan, India and Afghanistan - where polio is still a major health risk.'Strong leadership'Mr Moran told the BBC the disease had also spread to neighbouring Niger, Mali and Ivory Coast."The success of polio eradication in Africa rests on Nigeria interrupting the virus," he said.Polio was affecting eight northern Nigerian states - two more than a few months ago, the head of Nigeria's National Primary Healthcare Development Agency (NPHDA), Dr Ado Muhammad, told the BBC.Mr Moran said the Nigerian government had shown "strong leadership" in the campaign to eradicate polio and the WHO had been carrying out large scale vaccination programmes to prevent the disease from spreading."The immunity profile of Nigerian children is far better [now], which limits the risk of international spread of the virus," Mr Moran said.He also stressed that the number of children affected remained low."You can call it a four-fold increase but it is still very low transmission in a country as large as Nigeria with almost 50m children under five," he said.At the Commonwealth summit last month, the leaders of Nigeria, Canada, the UK and Australia pledged millions of dollars towards the global effort to eradicate polio.In 2003, the northern Nigerian state of Kano backed Muslim religious leaders in opposing an immunisation programme, claiming it was a Western plot to make people infertile.Health experts say this led to many people becoming infected by polio.The clerics and the state government later dropped their opposition to the immunisation programme.In 2007, there was a rare outbreak of a vaccine-derived form of polio in northern Nigeria.It affected 69 children who had been vaccinated.