By Laura MacInnis

GENEVA, Aug 10 (Reuters) - Four infants aged five months to 21 months have been infected with polio in Angola, a southern African country which was once clear of the crippling disease, the World Health Organisation (WHO) said on Friday.

Ten people have been stricken with polio since the beginning of 2007 in Angola, which had just two confirmed cases last year, raising concerns the virus could spread into neighbouring Democratic Republic of Congo, Zambia and Namibia.

The four new infections occurred in Angola's western Luanda and Benguela provinces, on the Atlantic coast, the United Nations agency said in a statement.

Polio, a highly infectious disease that proliferates in situations of poor hygiene, has been wiped out in most of the world but remains endemic in four countries -- Nigeria, India, Afghanistan and Pakistan.

Angola's polio cases are believed to be of Indian origin, but it is not clear when it was imported into the country, which had no incidence of polio from 2002 to 2004.

WHO spokeswoman Sona Bari said there was a high risk the virus could spread from Angola into the Democratic Republic of Congo, which is responding to its own outbreak, also traced back to India.

Some 5.9 million children under the age of five are due to be vaccinated on Aug. 31 in the latest of Angola's national immunisation days aimed at ridding the country of polio. It can take several doses of the oral vaccine to ensure immunity.

The Global Polio Eradication Initiative -- a joint project of the WHO, UNICEF, the U.S.-based Centers for Disease Control and Prevention and Rotary International -- last month cited "major operational challenges" in Angola, noting inadequate supervision and vaccinator training had "severely marred" previous campaigns.

"Campaign quality and sub-national surveillance are of concern in both countries, particularly Angola," Bari said.

Children under the age of five are most at risk from the virus causing polio, which can cause incurable life-long paralysis within hours.

Worldwide, there have been 345 confirmed cases of polio so far this year, concentrated mostly in India and Nigeria, compared with 1,999 cases in the full year in 2006 and 350,000 annually when the global eradication drive began in 1998.

In addition to Angola and the Democratic Republic of Congo, other previously polio-free countries battling infections from the virus in 2007 include Myanmar, Chad, Niger and Somalia.

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