New York, June 20 (IANS): The polio virus can continue to transmit silently for more than three years with no reported cases, says a study that suggests continued surveillance even after eradication of the disease.

To ensure that the disease is truly eradicated, aggressive surveillance programmes and vaccination campaigns must continue in endemic countries for years after the last reported case, the researchers said.

"Once we have eradicated polio -- or think that we have eradicated polio -- we probably should intensify the environmental surveillance to make sure the virus is not just lurking under the hood at very low levels," said Micaela Martinez-Bakker from the University of Michigan in the US.

"Using transmission models, we demonstrate that you can have sustained chains of silent transmission in populations for more than three years, without a single person ever showing up as a reported polio case."

"Polio eradication is about eradicating the virus. It is not about eradicating the disease paralytic polio," she said.

Pakistan, Afghanistan and Nigeria are the only countries where polio remains endemic - down from more than 125 countries in 1988.

The disease mainly affects children under five, and one in 200 infections leads to irreversible paralysis, according to the World Health Organisation, which reported 416 cases of polio worldwide in 2013.

Martinez analysed polio case reports from large-scale US epidemics in the pre-vaccine era, along with birth statistics and census numbers from every state.

This enormous data set provided a unique glimpse into the ecology of polio infection in the relative absence of human intervention.