Prof Carl Heneghan & Tom Jefferson

Studying sewage could help solve a coronavirus mystery

Studying sewage could help solve a coronavirus mystery
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There are plenty of mysteries about how coronavirus spread around the world so quickly. But could we shed some light on this by looking in an unusual place? Several studies have been doing just that: tracing the emergence of covid-19 by investigating frozen faeces samples from sewage. This analysis cannot tell us where the virus originated from, nor can it tell us whether the recovered micro-organisms are still infectious. But they can give us ideas about how long we have been living alongside a virus which has so far killed more than half-a-million people.

Coronavirus has been found in sewage from several countries predating the detection of the first confirmed cases in those areas: in Barcelona, in March 2019; Santa Catalina, Brazil in November 2019; and Milan, in February 2020. As more stool samples are assessed there are only likely to be more of these revelations.

So why was coronavirus in sewage before the virus was known to exist in those places? It seems unlikely that the virus spread through sewage, given the existence of modern sanitation systems. Instead, there might be a more straightforward answer.

Coronaviruses and a clutch of other known respiratory viruses do not spontaneously appear or disappear; they are likely to be already with us. At some point, they mutate into their pre-clinical form (i.e. before they start causing symptomatic illnesses) and spread unrecognised at a low level. The relatively slow rate of mutation of some of these viruses points to the possibility that this coronavirus has been around undetected for decades

These viruses then spread, at different concentrations, via bats or other animals or on surfaces. Our current lack of knowledge about coronaviruses means they go undetected – that is until we start looking for them. When a cluster of people suddenly fall ill with strange symptoms, it takes a suspicious and competent doctor, like Zhang Jixian, in Wuhan, to work out that something is afoot. As with many things, it is only when humans are affected that we start paying attention.

There is ample evidence that viruses which are not yet harmful are everywhere around us. A study in two nurseries in Copenhagen found viruses on most surfaces, like toys, pillows and sofas. The most common viruses found were coronaviruses. These were followed by bocaviruses and adenovirus (which is spread through infected faeces). Although some of the viruses found were alive and viable, many were remnants of dead viruses. As a result, illnesses in the children attending the nurseries involved were few and far between. So once we accept that viruses are with us – and sometimes for a lot longer than a decade – then what becomes important is discovering why all of a sudden covid-19 caused isolated and unconnected global outbreaks.

Why would Wuhan become the focal point in late December and Codogno in Italy in mid-February? The idea that this disease was spread simply by people travelling between those places cannot fully explain why the first cases identified in Codogno and Vò Euganeo had little or no connection or link to travellers from south east Asia.

If traces of coronavirus were in the sewage system of Barcelona a year before, then it also follows that neither of the two Italian cases can be the European source of the outbreaks. So what is going on?

If this coronavirus really did rear its head in different places far away from each other simultaneously, it wouldn’t be the first time something like this has happened. There are other well-documented instances of synchronous outbreaks of the same disease, thousands of miles apart. In 1918, this happened in the USA with Spanish flu, and it's hard to blame mass air travel.

At the moment, there is no answer as to what caused coronavirus’s sudden virulence and infectivity. But while the world’s attention is now focused squarely on the pandemic and vast medical resources are concentrated on finding out more about coronavirus, the simple truth is that this is still too little, too late. 

When Italian researchers reviewed the water habitat of the then-known coronavirus on the eve of the pandemic, they found only twelve separate similar studies since coronaviruses were first identified in the 1960s. Caught in the warp of other real or perceived emergencies, the scientific community has not devoted resources to studying viruses, their ecology, history and adapting capacity. And now, we’re all paying the price.

Written byProf Carl Heneghan & Tom Jefferson

Carl Heneghan is professor of evidence-based medicine at the University of Oxford and director of the Centre for Evidence-Based Medicine Tom Jefferson is a senior associate tutor and honorary research fellow at the Centre for Evidence-Based Medicine, University of Oxford

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