A Tel Aviv University researcher warns the deadly polio virus is still a threat, contrary to the widespread belief in Western countries the disease no longer exists.

Dr. Lester Shulman of the university’s Sackler Faculty of Medicine and Israel’s Ministry of Health has spent years tracking isolated cases of live poliovirus infections.

The illness has often been discovered in countries that were believed to be polio-free, he notes in research published recently in PLoS ONE.

When the live-virus version of the vaccine, referred to as Oral Polio Vaccine (OPV), evolves, Shulman says it can act like wild poliovirus — and continue the threat of contagion.

Israel is one of the few countries that practice environmental surveillance for polio, beginning in 1989. Technicians check designated sites along sewage systems every month for evidence of the virus, which allows for early detection before paralytic cases occur.

Researchers have spent the past 10 years trying to trace the origin of the strain that infected two individuals in central Israel, Shulman says. They tracked the strains to the sewage system, and have been working to trace the origin from that point.

Because Israel maintains “herd immunity” from the disease — a 95 percent immunity — the wider population has not been threatened.

Shulman and his colleagues traced a wild poliovirus discovered in sewage from the Gaza district to a village in Egypt, using the genomic structure as a marker. “From the sequence of the genome, you can match it with known sequences reported by labs throughout the world,” he explains.

The professor recommends that public health agencies continue the policy to maintain “herd immunity” to prevent the spread of wild and evolved vaccine strains of the virus. Shulman also warns that it is important to continue environmental surveillance of sewage systems.

In addition, he recommends that health authorities switch from OPV to an Inactivated Polio Vaccine (IPV) instead.