The testing of sewers in Paris has revealed a pattern of viral coronavirus concentrations that coincided with outbreaks of cases across the city, a new study has found.

The discovery was made after wastewater sampling at five sewage plants in the French capital over a four-week period from March to April.

The technique could be a cost-effective way to sound the alarm that an outbreak is imminent.

Higher concentrations of virus in the wastewater means more infected people around a particular sewer system.

In Paris, researchers found “higher concentrations” of viral RNA – the genetic material from the virus – in certain sewers just days before 10 March, the first day that Paris reported coronavirus deaths, according to the report.

“This visibility is also going to help us predict a second wave of outbreaks,” said Sébastien Wurtzer, a virologist who took part in the study at Eau de Paris, the city’s public water utility told the journal Science.

The flow of faeces and urine into sewers is continual so it can offer data in almost real-time from infected people shedding viral RNA from Covid-19.

A recent study in the Annals of Internal Medicine found that samples of patients’ phlegm and faeces tested positive for traces of Covid-19, even after swabs from their noses and throats came back negative.

An aerial view shows the deserted Champs-Elysee Avenue in Paris during a lockdown imposed to slow the spread of the coronavirus disease earlier this month REUTERS/Pascal Rossignol/File Photo) (REUTERS)

Other researchers around the world, including one group in the Netherlands, have detected Covid-19 in wastewater, The Independent reported last month.

On 5 March, the KWR Water Research Institute in Nieuwegein detected genetic material from the coronavirus at an Amersfoort wastewater treatment plant.

When the results were gathered, no cases had been reported in Amersfoort. The Netherlands reported its first case on 27 February in the south of the country, an hour’s drive from where the testing was done.

The French study, which has not yet been peer-reviewed, first appeared in medRxiv on 17 April.

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Testing of wastewater has been used for detecting the poliovirus as well as discovering the use of illegal drugs.

An EU study from March 2019, analysing sewage samples from 68 cities in 23 European countries, showed a rise of amphetamine, cocaine, MDMA and methamphetamine from previous years, the BBC reported

Paul Bertsch, from the Commonwealth Scientific and Industrial Research Organisation in Australia, told Science one of the benefits of testing the sewershed was that it provided information on the virus associated with a large number of coronavirus infections.

This could be important as public health officials say some coronavirus patients have been asymptomatic or shown very mild symptoms.