Doctor Lester Shulman of Tel Aviv University's Sackler Faculty of Medicine and the Israeli Ministry of Health has spent years tracking isolated cases of live poliovirus infections, often discovered in countries that are supposedly polio-free. When the live-virus version of the vaccine, called Oral Polio Vaccine (OPV) evolves, he says, it can act like wild poliovirus and continue the threat of contagion.
Medical professionals widely believe that after the wild virus is eradicated, resources dedicated to polio immunisation can be redirected. But this is not so, he says. He recommends that public health agencies take a three-pronged approach: Vaccination policies to maintain "herd immunity" (a 95 per cent immunisation rate for polio) should be maintained to prevent the spread of wild and evolved vaccine strains of the virus; environmental surveillance of sewage systems should continue; and a switch to Inactivated Polio Vaccine (IPV) instead of OPV should be implemented.
While the eradication of polio is seemingly within reach, this is not the time to relax, Shulman warns. Most countries only investigate the possibility of poliovirus outbreaks when paralytic cases appear in the human population. But this does not take into account a potential problem posed by the live virus vaccine. Over time, the vaccine can mutate, and even a 1 per cent genomic change in the virus permits the virus to behave like a wild poliovirus. If a population is not sufficiently immunised, this spells trouble.
Israel is among the few countries that practice environmental surveillance for polio, beginning in 1989. Checking designated sites along sewage systems every month for evidence of the virus allows for early detection before there are paralytic cases. For the past decade, the researchers have been trying to trace the origin of the strain that infected two individuals in Central Israel. They tracked the strains to the sewage system, and have been working to pinpoint the origin. Fortunately, because Israel maintains herd immunity for the disease, the wider population has not been threatened.
Shulman says that in the lab, each strain of the virus can be identified from its genomic structure and traced back to the region from which it originated. "From the sequence of the genome, you can match it with known sequences reported by labs throughout the world," he explains. For example, he and his colleagues traced a wild poliovirus discovered in sewage from the Gaza District to a village in Egypt.
MEDICA.de; Source: American Friends of Tel Aviv University