Poliovirus silently (and not so silently) spreads

23 July 2013

Poliovirus by Jason RobertsPoliovirus has been found in sewage in Israel. The virus detected is not vaccine-derived poliovirus; it is wild-type 1 poliovirus, the strain that occurs naturally in the wild and which the World Health Organization is trying very hard to eradicate from the planet.

As part of the global effort to eradicate poliovirus, environmental samples from many countries are routinely examined for the presence of the virus. Wild type poliovirus was detected in 30 sewage sample from 10 different sites, collected from 3 February to 30 June 2013 in Israel. No cases of paralytic disease have been detected in that country. This is not a surprising finding because only roughly one in 100 individuals infected with poliovirus develop paralysis.

During poliovirus infection, the virus replicates in the gastrointestinal tract and is shed in the feces. Most of the poliovirus-positive sewage samples were from southern Israel. Finding the virus at multiple sites suggests that the virus has been in the population for an extended period of time and that multiple individuals are shedding virus.

Why is wild type virus circulating in Israel? Poliovirus vaccine coverage in Israel is high but it is never 100%, and non-immunized individuals are hosts for the virus. Another explanation is related the the use of inactivated poliovirus vaccine (IPV) in Israel. Immunization with IPV, which is administered intramuscularly, does not protect the alimentary tract from infection. Therefore poliovirus can replicate in the intestine of immunized individuals, but once the virus reaches the blood its spread is blocked by anti-viral antibodies.

An interesting question is whether there is wild type poliovirus in the United States. I suspect that if we looked for poliovirus in our sewage, we would find it. However we no longer carry out surveillance of sewage for poliovirus and therefore we do not know if it is present.

Genetic analysis of wild type poliovirus from Israel suggests that it is related to the strain found in December 2012 in sewers in Cairo, Egypt. That virus in turn is closely related to virus from Pakistan, one of three countries from which wild type poliovirus has not been eradicated (the others are Nigeria and Afghanistan).

There is an ongoing outbreak of poliomyelitis in the Horn of Africa, with 65 cases in Somalia and 8 cases in Kenya. The virus causing that outbreak came from Nigeria. Somalia and Kenya had been free of polio since 2007 and 2011, respectively.

WHO has concluded that the risk of further international spread of wild type poliovirus from Israel as moderate to high. As long as there are individuals who are not immune, there will be a risk of poliovirus infection, in part due to the silent infections caused by the virus.

  • Alan Dove

    I wonder if chronic low-level contamination with wild polio is going to become a common pattern now, in contrast to the peak and decline cycle that typified polio epidemics in the pre-vaccine era. There are probably a few immunocompromised hosts available just about anywhere who can become chronic virus secreters without developing symptoms.

  • http://rybicki.wordpress.com/ Ed Rybicki

    Sinister…but if we can replace all vaccination with live virus by use of killed, AND keep vaccination rates high – then we may kill this thing.

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