Covert and dangerous: on the eradication of polio / Helen Branswell

Polio can be eradicated - provided that scientists find the last carriers of the virus

The effort to eradicate the polio virus in the last decade is likened to the ultimate version of the game "hit the mole" (a game in which "moles" pop up randomly from holes and you have to hit them on the head with a wooden hammer): when it seems that the virus has been beaten to a pulp, here it pops up in another area. Now that the eradication of polio worldwide seems within reach, a stealthy threat has emerged in sources of infection hidden from superficial view.

The last refuge of polio viruses is people with weakened immune systems called "chronic secretors". These people received as children orally a vaccine composition of weakened viruses and they produce live viruses for years and spread them in their feces and respiratory tract. Healthy children produce antibodies in response to the vaccine that prevent the virus from multiplying, thus giving them immunity from infection with the virus. Chronic secretors, on the other hand, cannot complete the process and instead rapidly produce a constant and continuous supply of viruses. The weakened viruses, in the oral vaccine, can mutate and regain the characteristic of the wild virus: the ability to paralyze the people it infects. When the news came to wider consciousness in the mid-90s, it shocked researchers.

Philip Maynor, deputy director of Britain's National Institute for Biological Control and Standards, describes this biomedical panic scenario: the spread of the natural polio virus stops, countries reduce vaccination campaigns, a "chronic excreter" kisses an unvaccinated baby and the baby is taken to a nursery. "And whoop," he warns, "the babies drool on each other and the virus spreads everywhere. This is a scenario where the polio virus returns to a developed country," and this may happen in developing countries as well. Although it was once widely believed that individuals with a weak immune system could not survive for a long period of time in low-income countries, the conditions are changing as the health systems of those countries improve. In 2009, an 11-year-old boy with a weak immune system contracted polio five years after being vaccinated orally. Only then did the researchers recognize him as a chronic excreter.

Chronic secretors are only discovered when they develop paralysis after years of unknowingly spreading the virus. Fortunately, such cases are rare. Ronald W. Sutter, a scientist at the World Health Organization who leads the research policy in the Global Polio Eradication Initiative, says that the initiative is aimed at developing drugs that prevent the spread of the weakened virus in the vaccine. A few select options are already in the pipeline.

However, drugs can only solve the problem if the chronic excretors are identified, which is a serious challenge. For years, scientists in Finland, Estonia and Israel have been monitoring the presence of traces of the virus in the sewage of various cities. Viruses spread by chronic secretors are detected in many samples, but efforts to locate the secreting individuals have failed. It is possible that these insidious distributors are not necessarily patients with a poor immune system that can be monitored during their visits to immunologists, but people who do not even know that they have problems with the immune system and are not under medical surveillance. "We know for sure that the sword of Damocles is hanging over their heads," says Setter. It also hangs over the heads of all of us.

The article is published with the approval of Scientific American Israel