A persistent outbreak of polio in Angola is now a matter of international concern and health authorities there must step up their efforts to stamp it out, the World Health Organization (WHO) said Friday.

The WHO's spokeswoman on polio eradication, Sona Bari, said an outbreak of the crippling virus, which started in 2007 after Angola had been polio-free for six years, now has international consequences if it is not stopped.

So far this year Angola has had 24 cases of polio -- a virus which attacks the nervous system and can cause irreversible paralysis -- adding to 29 in 2009 and 29 in 2008, she said.

It is the only expanding outbreak in all of Africa, spreading both within Angola and into the Democratic Republic of Congo, Bari said in a telephone interview. It's a high threat to neighboring countries.

The Democratic Republic of Congo has had 15 polio cases so far this year after having just three in the whole of 2009.

The WHO, UNICEF, Rotary International and the U.S. Centers for Disease Control and Prevention have been working together since 1988 to eradicate polio, but their initial target of the year 2000 proved over-optimistic.

Four countries -- Afghanistan, India, Nigeria and Pakistan -- remain endemic polio countries as they have been unable to stop it. Others like Angola have stopped it only temporarily.

As well as persisting in endemic countries, the virus can be imported from endemic areas to cause fresh outbreaks such as the one in Angola and another larger one in Tajikistan, which the WHO says has had 458 cases so far this year.

The WHO says Angola's inability to halt its prolonged outbreak is mainly due to poorly run vaccination campaigns.

Polio can only be stopped if every child is given the oral polio vaccine, and campaigns to date have sometimes missed more than a third of children in critical transmission areas such as Luanda, Bari said.

The WHO is launching a nationwide polio vaccination campaign in Angola on Oct 1-3 with another on Oct 29-31 aimed at covering 5.6 million children aged under five each time.

Bari said the campaigns would only really succeed if national and local authorities put their weight behind them.

According to latest WHO figures, the polio spread in Angola is in stark contrast to progress in other parts of Africa -- in particular to a 99 percent decline in cases in Nigeria, which is the only country in Africa that has never yet managed to stop polio altogether.

Nigeria has had just seven cases so far this year, down from 388 in 2009. The worst story in polio eradication was always Nigeria -- and now its the best story in Africa, Bari said.

There's not really any reason why Angola can't do this -- its population is less dense than northern Nigeria and it has fewer health and sanitation problems, so it's really a question of having proper supervision both at national and local levels.

If polio vaccination is made a national priority, like Nigeria did, you can see incredible progress.

The WHO said in March that Nigeria's impressive progress against polio had come in the year since religious leaders gave their backing to vaccination campaigns.

Pakistan India Afghanistan Angola