Scientists believe the virus made its way into Tampere's sewer system via human waste
Authorities in Tampere Finland (pictured here) are searching for someone who is a carrier for polio / (mdid/Flickr)
TAMPERE, Finland--Somewhere in Tampere, Finland, someone is excreting polioviruses. He or she has been doing it there since at least 2008, passing stools laced with polioviruses then flushing them into the sewers of Finland's third largest city.
Known in polio science circles as a long-term excreter, the unidentified individual is likely completely unaware of this quirk of his or her bowel. Merja Roivainen would very much like to find this person.
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Roivainen is the director of the intestinal virus unit at Finland's National Institute for Health and Welfare. There she oversees a program of environmental surveillance for polioviruses -- the routine testing of sewage samples from nine cities around the country. Tampere and its suburbs are home to about 300,000 people and a number of multinational companies, and it is one of the nine.
Finland's environmental surveillance program started in 1960 and for decades the country's sewage systems seemed to be poliovirus free. Bimonthly sampling turned up no wild polioviruses. (Scientists use the term "wild" polioviruses to distinguish between the viruses found in nature and the weakened viruses used in oral polio vaccine.) Because Finland uses injectable polio vaccine made with killed viruses, there were rarely any sightings of the altered live vaccine viruses that are hallmarks of oral polio vaccine use. Those viruses are called Sabin strains after the man who developed the oral polio vaccine, Albert Sabin.
Tests were also consistently negative for vaccine-derived polioviruses, a troubling byproduct of Sabin strains. These viruses, known by the nickname VDPVs, don't look exactly like wild polioviruses if you study their genetic codes. They have evolved from the Sabin strains, picking up mutations as they spread from human gut to human gut. But while they don't look exactly like wild polioviruses, they act like them. Just like wild polioviruses, vaccine-derived polioviruses paralyze a percentage of the people who become infected with them.
Finland's environmental surveillance program never found VDPVs. Never, that is, until late 2008. That December, sewage samples from Tampere yielded two VDPVs which had evolved quite substantially from the Sabin strains. "And since then we have found 13 sewage specimens positive for VDPVs. And they all have been collected from Tampere," Roivainen said. "Altogether, more than 70 VDPV strains have been isolated."
Science can estimate how long vaccine-derived viruses have been circulating--in other words, how long ago the vaccine dose that started the problem was given. They look at the genetic distance of the VDPVs from Sabin strains, then calculate the time based on the rate at which polioviruses are known to mutate. The person shedding the Tampere VDPVs has probably been generating polioviruses for 12 or 13 years, scientists believe.
Genetic analysis of the Tampere viruses suggests they have all passed through the gut of one person, likely someone whose immune system doesn't work properly. When healthy people get oral polio vaccine, their immune systems respond to stop the virus from replicating in their gastrointestinal systems. The antibodies generated in that process protect them against future infection. But in a small number of people the immune system doesn't shut down virus replication after vaccination, so they keep generating and excreting polioviruses.