Wild polio virus has been detected in Israel. Routine environmental/water sampling in southern Israel (Rahat) detected the virus in May. It has now spread to the north. Since the first week of August, the Israeli government has mounted a campaign to administer oral vaccine (OPV) to all children under the age of 10. As of this week, over 180,000 children have been (re)vaccinated.

I must emphasize that no clinical cases of polio have been detected, despite enhanced surveillance. Rather, polio virus has been detected in water samples. That nonetheless poses a risk to any individual with inadequate immunity to the virus: either the non-immunized, or those whose immunity resulting from childhood vaccination has dwindled. And, the troubling question is how virus got into the water supply? This is because there is no known animal reservoir for the virus, and there are likely unsymptomatic carriers in the country.

Heightened wastewater surveillance has detected virus in 67 samples and 24 locations in Israel, according to the Global Polio Eradication Initiative (http://www.polioeradication.org/Dataandmonitoring/Poliothisweek.aspx).

It is not unheard to find polio virus isolates in the environment in developed countries with high vaccination rates (Israel’s is 94%). Indeed, there are sporadic reports of polio cases in developed countries. I remember that this was the case right before I left for a conference in Finland in 1985–and I quickly got a polio booster prior to departure. There was a well documented outbreak of clinical cases in the Netherlands in 1992.

A possible scenario explaining this unusual occurrence of polio in Israel is as follows. There are two polio vaccines that are available–the killed virus, injectable vaccine (IPV), and the oral vaccine (OPV), which is a live, attenuated virus vaccine. IPV, which was the standard vaccine given to children in Israel until roughly 1 decade ago, provides good protection to the individual, but still allows people to be adequate carriers of virus–poliovirus is an enteric virus, so the virus, when present, in the gastrointestinal system. The OPV provides mucosal immunity, and decreases shedding of the attenuated virus. To put it another way, IPV provides good protection for the individual, but, to oversimplify, does nothing to prevent passage of the virus from the individual to the environment. OPV, on the other hand, makes this far less likely because of its intestinal immunity. The current vaccination campaign uses only OPV.

The particular strain detected in Israel is identical to a strain detected in Egypt.

There is hope that polio will be eradicated in the near future. The only countries with clinical cases regularly detected are Afghanistan, Pakistan, and Nigeria. Recently, over 100 cases have been detected in Somalia. These are all countries where the public health infrastructure is chaotic, making it difficult to attain high immunization rates.