In what could be a cause of concern for the national capital, the World Health Organisation (WHO) has found the polio virus in the city's sewage. The Delhi government is on alert and has increased surveillance and immunisation of susceptible children.

The WHO, along with the Delhi government, carried out the environmental surveillance under the National Polio Surveillance Project (NPSP) from April 2010 till December.

The surveillance was done at five places along the Yamuna - Bhalaswa Lake, Red Cross Hospital in Seemapuri, Swarn Cinema in the trans-Yamuna area, Wazipur JJ Colony and Batla House in Okhla.

According to the WHO, the presence of wild polio virus was found before the Commonwealth Games in October when a lot of migration was taking place during the preparation for the mega event.

"Both Type 1 and Type 3 polio viruses were detected in the sewage samples from Delhi during 2010," Hamid Jafari, project manager of WHO-NPSP, said.

Mr. Jafari said: "The environmental surveillance is very strategic since Delhi is a major hub of migrant labourers and also because of its proximity to the historically polio endemic and high-risk areas of Uttar Pradesh."

"It is a cause of concern for us because if there is polio virus circulating in the environment then there is a risk of infection in children," said C.M. Khanijo, officer on special duty at the pulse polio cell of the Delhi government.

"We have written to the health ministry to increase the surveillance sites from five to over a dozen areas so that samples can be collected from upstream areas," he added.

Delhi did not report any polio case last year while four cases were reported in 2009 and five in 2008.

"No wild polio case has been detected in Delhi environmental samples since mid-August 2010, providing additional evidence for the remarkable progress towards polio eradication achieved in India," Mr. Khanijo said.

The surveillance can promptly detect polio virus circulation in the sampled area and serve as a supplement to the main polio virus detection strategy that involves testing of stool specimen from cases with acute flaccid paralysis.

The surveillance in Delhi can provide information about polio virus transmission in both adjoining Uttar Pradesh and other areas from where migrants come to the capital.

The national environmental surveillance was started in Mumbai in 2001.

Mr. Jafari said: "Both Delhi and Mumbai, most frequently visited by migrant labourers, are high?risk areas for repeated introductions of polio virus and are therefore covered in all sub-national immunisation rounds along with polio endemic states of Uttar Pradesh and Bihar."

There are two types of polio viruses prevalent in India -- P1 and P3. The transmission of the most dangerous virus P1, which caused 95 percent of polio cases in India till 2006, was at a record low level in 2010.

India, Pakistan, Afghanistan and Nigeria are four polio-endemic countries in the world.

India has reported a drop of 94 percent in polio cases with only 42 cases being reported in 2010 as compared to 741 in 2009.