We found the coronavirus in wastewater, and this could help us better monitor the epidemic

The Covid-19 epidemic, caused by the SARS-CoV-2 coronavirus, impresses both by the speed of circulation of the virus since December 2019 and by the number of cases reported: when the World Health Organization officially declared pandemic state, March 12, 2020, 125 cases had been confirmed. A month and a half later, on April 22, more than 2,5 million confirmed cases were counted.

THEemergence of this virus is reminiscent of that of SARS-CoV, the same family at the origin of the SARS (Severe Acute Respiratory Syndrome) epidemic between November 2002 and July 2003. It was eradicated in just a few months, not without having affected nearly 8000 people and 800 victims in 26 countries.

Comparing the epidemics of SARS and Covid-19 is particularly informative if we consider that the two viruses that cause them have many common characters and that they are quite close genetically. But the measures that had made it possible to contain the SARS epidemic - early detection and isolation of cases, quarantine of contacts, social distancing and in some cases collective quarantine - have so far proved insufficient for control the circulation of the novel coronavirus.

SARS and Covid-19: two major differences

At least two important factors distinguish the outbreaks of SARS and Covid-19.

In the case of SARS, the excretion of the virus by infected people, essential for its transmission, followed the onset of clinical signs. This characteristic made it possible to quickly identify and isolate patients (fever, respiratory syndromes) and to quarantine people with whom they had come into contact, before they transmitted the virus. In contrast, for Covid-19, viral shedding could precede the onset of symptoms, making transmission possible even before the sick are identified and therefore isolated.

In addition, if mild clinical forms and the existence of healthy carriers were reported during the SARS epidemic, it would appear that these people did not not significantly transmitted the virus. In the case of Covid-19, non-symptomatic subjects, in the incubation phase or not very symptomatic - in proportions which still remain to be specified - could transmit the virus.

Detect carriers with little or no symptoms

Silent-contagious carriers are difficult to identify outside of massive screening campaigns. However, they could introduce the virus into unaffected populations and contribute to the silent dissemination of the virus in the epidemic phase.

Virological tests (called direct tests) used to identify carriers of the virus are currently based on swabs taken from the nose using a swab. The samples are processed in order to extract and detect the viral genome, which consists of a Single stranded RNA. This RNA is then used to obtain the corresponding DNA, thanks to an enzyme that produces DNA from RNA. This DNA is then amplified thanks to another enzyme (by PCR - Polymerase Chain Reaction).

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This technique, called RT-PCR (Reverse Transcription PCR), is remarkably sensitive. However, the result of this analysis can be negative for several reasons. The subject may not be infected: this is a true negative result. But it can also happen that false negatives are obtained, even though the person tested is indeed infected. Indeed, it is possible that the sampling and / or the RT-PCR are not carried out in an optimal way. Another possibility: the subject is indeed infected, however the virus is little or not present in the nose at the time of the sample, but is at this time in other anatomical sites, such as the stool or the blood. This is possible if the virus migrates through the body as the infection progresses, for example.

It seems that, for the SARS-CoV-2 coronavirus, this scenario applies. Indeed, if the biological cycle of the virus begins and takes place in part in the upper and lower respiratory tract - the lungs in particular - several studies have also detected traces of the virus in the blood and, above all, in the stool. The presence of viruses in the digestive tract is consistent with description of gastrointestinal disorders during illness in some patients.

This discovery naturally raises the question of a possible fecal transmission, not formally established to date. But it has other implications.

In the absence of systematic, massive and repeated screening to identify carriers of the virus, it is urgent to propose other reliable indicators that would allow the entry and level of circulation of the virus to be assessed as early as possible. in populations. To be rolled out globally, including in low-income countries, these indicators must be easy to set up, ethically acceptable and financially sustainable.

Tests carried out on wastewater could meet these criteria.

The virus detectable in wastewater

The detection of the coronavirus in the stool has prompted several groups around the world to promote wastewater analysis to assess his circulation in populations.

Wastewater in fact corresponds to all water from homes and urban public facilities (hospitals, schools, etc.) as well as from certain industries (if it does not require specific treatment). This water is transported, via the “main sewer”, to the purification stations; they are treated there and then released into the environment.

Some networks are said to be “unitary” - rainwater and wastewater are mixed - while others are “separate” - rainwater and wastewater each have their own network. It happens that during exceptional climatic events (heavy rains or violent thunderstorms), certain networks overflow and cause degradation of the environment, which may not only be chemical, but also microbiological. It is nevertheless essential to recall here that the networks of wastewater and non-potable water are totally distinct drinking water distribution networks, the microbiological and chemical quality of which is closely monitored.

From the withdrawal of the water that will be distributed to the evacuation of wastewater, with a view to their treatment and then their discharge, the different stages of the “small water cycle”.
Water France

A well-known Parisian specificity concerns the presence of a so-called “non-potable” water network whose role is to allow the cleaning of the roadway, the sewers or the filling of various ornamental lakes. This network is directly supplied by surface water drawn from the Ourcq canal or the Seine, non-potable water in which traces of viral RNA have recently been detected, in very small quantities however.

Depending on their size, wastewater treatment plants can collect the faeces of a few hundred to several million people; consequently, the wastewater which results from it partially reflects the diversity of microorganisms lodged in our intestines, at the scale of an agglomeration.

Initial research results

A first study, carried out by Gertjan Medema and his collaborators in the Netherlands, demonstrated that the genome of the coronavirus can be detected in multiple wastewater sampling sites just days after the identification of the first human case of Covid-19 in this country. A similar study was driving in Massachusetts (USA).

The study initiated since March 5, 2020 in the Ile-de-France region, on 3 wastewater sampling sites, confirms this hypothesis. It also demonstrates, for the first time, that the quantities of viral genomes detected in wastewater are increasing in line with the number of hospitalizations linked to Covid-19 at the regional level. Preliminary results obtained even more recently on the same sites show a very significant reduction in the viral load in wastewater, an expected consequence of containment measures on the circulation of the virus.

These results prompt us today to propose regular monitoring of wastewater.

What the “collective macrobiota” could tell us

If the enrichment of wastewater with coronavirus is due to carriers with little or no symptoms - the most numerous - the regular and systematic search for the virus on these samples could make it possible to exercise essential and complementary monitoring of epidemiological approaches in human beings. human (counting of symptomatic cases - confirmed or suspected - and detection of the virus in sick subjects and their contacts, in particular by RT-PCR).

This strategy could be implemented in an integrated fight plan against the epidemic. In particular, it could be deployed as a priority in regions - in France and abroad - in which the prevalence of the virus is still very low (few cases reported) or even zero, in addition to epidemiological studies conducted in populations. It could also inform us of a modification in the dynamics of circulation of the virus: reduction of circulation linked to containment for example, resumption of the epidemic - often referred to as a “second wave” - signal of reintroduction into a region or region. he epidemic seemed to be under control, to name a few examples.

Ultimately, the current situation and ongoing research underscore the urgency of a national wastewater monitoring network, the benefit of which could be assessed during the monitoring of the current epidemic (participation in the deconfinement strategy). and apprehension of a possible second wave in particular), and more generally in the monitoring of all germs in faecal circulation - winter gastroenteritis viruses, bacteria resistant to antibiotics, etc.

The actors in charge of sanitation in France are already organized to monitor the quality of the effluents supplied to their facilities. This organization would make it possible to use the existing facilities to set up monitoring very quickly.

Finally, we could thus imagine, in the near future, using wastewater to assess the health of a “collective macrobiota” which would echo individual intestinal microbiota, the importance of which in human health is no longer in doubt. to prove.

Sebastien wurtzer (Paris water), Jean ‑ Marie Mouchel (Sorbonne University), Remy Teyssou (Armed Forces Biomedical Research Institute), Yvon Maday (Sorbonne University, co-founder of the Covid-IA initiative), Vincent Rocher (SIAAP) and Laurent Moulin (Eau de Paris) are co-authors of this article.The Conversation

Vincent Marshal, Professor of virology, Saint Antoine Research Center (Inserm), Sorbonne University

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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