(Reuters) - The polio virus has been found in sewage samples near Sao Paulo, one of the venues for the soccer World Cup in Brazil, but no human case of the disease has been reported so far, the World Health Organization said on Monday.

The virus discovered in Sao Paulo sewage collected in March at Viracopos International Airport, and reported by Brazilian health authorities last week, is a close match to a strain isolated in a case in Equatorial Guinea, the WHO said.

"(The) virus has been detected in the sewage only ... To date no case of paralytic polio has been reported," it said in a statement.

Polio invades the nervous system and can cause irreversible paralysis within hours. There is no cure for the disease but it can be prevented by immunisation. Children under age five are the most vulnerable.

The Brazilian Health Ministry said the presence of people carrying infectious agents from other parts of the world was to be expected as a result of increased international travel. "It is important to clarify that this detection does not mean any change in the epidemiological situation of Brazil or a threat to the elimination of the disease," it said in a statement.

Brazil has been polio-free since 1989 and the Americas region was declared free of wild polio transmission in 1991, according to the WHO, which is spearheading a global campaign to eradicate polio.

Brazil's last national immunisation campaign was conducted a year ago and coverage in Sao Paulo state has been higher than 95 percent, the WHO said, adding: "The high immunity appears to have prevented transmission."

The U.N. agency said it assessed the risk of further international spread of polio virus from Brazil as "very low", and from Equatorial Guinea as "high".

Equatorial guinea