Sewage clue to silent polio scourge
Despite mass vaccination, polio is making a comeback. Sewage could become a critical early warning system. Paul Biegler reports.
Human excrement, much maligned as a carrier of disease, flexes its public health credentials as a weapon against polio in a study published in Science Translational Medicine.
The research, led by Yakir Berchenko of the Ben-Gurion University of the Negev in Israel, analysed polio virus in sewage collected during a 2013 outbreak of the disease centred on the Bedouin town of Rahat in the south of the country.
The result, the authors report, is a more accurate way of spotting a “silent epidemic” of polio. This could prompt early deployment of vaccines to halt the disease’s devastating effects.
Polio destroys nerves as they exit the spinal cord, leaving its victims, mostly children under five, paralysed. In severe cases the breathing muscles are disabled, leading to the searing images of rows of children in iron lungs that became familiar during the polio outbreaks of the mid twentieth century.
But polio has been a showcase of vaccination success, with the toll reduced from 350,000 cases in 1988, when the World Health Organisation launched its Global Polio Eradication Initiative, to just 74 in 2015.
The virus is, however, fighting back. It reared its head in 2013 across four regions previously declared disease-free – Egypt, Syria, the Horn of Africa and Israel – in a resurgence that presents something of a Gordian knot for vaccine policymakers.
There are two polio vaccines. The oral version stops person-to-person spread, but is made from a weakened version of the virus that actually causes paralysis in three out of every million people who take it.
The injectable alternative uses killed virus and is, therefore, safer. But it only prevents paralysis among the vaccinated, leaving open the possibility of the virus spreading unnoticed.
Children used to get both vaccines, but as polio prevalence dropped around the globe many countries, Australia included, ditched the oral one.
The rub is that with oral vaccination taken out of the picture polio can spread silently, only being detected when someone shows the telltale early signs of paralysis.
The current study combined known rates of disease and vaccination with the sewage data to develop a measure of how many people were actively shedding polio virus, a potential early warning sign of an epidemic.
According to the authors, the technique improves on existing environmental surveillance (ES), which relies on either waiting for a sentinel case of paralysis, or on a sewage analysis algorithm that produces little better than a ‘yes/no’ answer on whether the virus is present.
Noting that it took Israeli authorities two months to launch mass vaccination after the polio virus was first detected in 2013, the authors state that, “If the quantitative assessment of ES data had been available ... in Rahat [it] may have prompted swifter action.”
Commenting on the study, Maximilian de Courten, a global public health expert at Victoria University in Australia said, “This is an excellent study which reliably demonstrates what can be achieved with environmental surveillance.
“The ES approach is not new but, as the authors point out, provides a broad-based screening approach that would need to be followed up by other surveillance techniques.
“Importantly, however, in places where the infrastructure necessary for sewage capture is lacking – Syria, for example – their approach would be less effective.”