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.