Precious wastewater

Tracking the Covid-19 epidemic through... wastewater. A valuable indicator that not only provides information on the level of circulation of the virus, but could also help anticipate a possible rebound in the epidemic.

Monitor the level of coronavirus circulation to better anticipate possible resurgences. This is a major challenge in the management of the Covid-19 pandemic. Number of positive tests, emergency room visits on suspicion of Covid-19, number of patients in intensive care... these are all indicators that are closely monitored to assess the circulation of the coronavirus. But there's another, perhaps less well-known but no less valuable indicator: the presence of virus residues in wastewater.

"The detection of SARS-Cov-2 in wastewater is done in many countries, it was notably set upin Paris at the start of the epidemic, so why not here?" To fill this gap, Franz Durandet, president of the start-up IAGE specializing in environmental biological analysis, and Pierre Becquart, researcher at the Mivegec* laboratory, approached the Syndicat Mixte du Bassin de Thau. Their aim: to set up microbiological analyses of wastewater in the region. The principle? Collect wastewater upstream of treatment plants to track down the coronavirus. " We actually measure the quantity of inactivated nucleic acids of SARS-Cov-2 using an extremely reliable technology: digital PCR", explains Franz Durandet.

Group screening

How does coronavirus get into wastewater? "It is excreted in the faeces of infected patients. Its presence in wastewater is therefore correlated with the level of circulation of the virus in the population", explains Pierre Becquart. This measurement is all the more interesting as it concerns the entire population in the area concerned. "Even if we carry out tests to identify carriers of the virus, we can't test everyone, and we mainly screen those with symptoms. On the other hand, everyone goes to the toilet, so the wastewater partly reflects the state of health of the entire population, in a sort of collective mass screening", says the researcher.

Can the quantity of coronavirus residues in wastewater be used to determine the number of infected people in the population? We can't make this extrapolation," answers Franz Durandet, "on the other hand, the evolution of the quantity of coronavirus detected correlates very well with the evolution of the epidemic. For example, at the end of May, we detected almost no residues in our samples, so this measurement represents a reliable indicator that gives a trend in the situation".

Anticipate

A reliable indicator with another major advantage: its early detection. The virus is excreted even before the first symptoms appear," stresses Pierre Becquart, "so if we see an increase in virus residues in wastewater, we can warn of a possible future rise in the number of cases that will have to be dealt with by the healthcare system".

Better prediction for better anticipation: a major challenge in the management of the Covid-19 pandemic. In a press release dated July 7, the FrenchNational Academy of Medicine stresses that microbiological analysis of wastewater can play a strategic role in the prospective and regular monitoring of virus circulation, and even recommends that such monitoring be made systematic for as long as the virus circulates in the population.

A technique that could become a widespread epidemiological surveillance tool: "We can detect several different types of virus on a single sample, so we could simultaneously monitor the evolution of several epidemics", explains Franz Durandet. An idea shared by the French National Academy of Medicine, which now recommends extending this systematic monitoring to other viruses such as myxovirus, rotavirus and respiratory syncitial virus. New developments in wastewater.

Cash in the bank

When did the coronavirus first appear? The answer to this question can also be found in wastewater. " Some countries, such as the USA, Spain and Italy, have wastewater banks: samples are taken every month and frozen, enabling them to be analyzed at a later date," explains Franz Durandet. A closer look at the samples in this collection revealed that SARS-Cov-2 residues were present in Barcelona as early as March 2019. The French National Academy of Medicine in turn recommends setting up a sample bank to retrospectively detect any new viruses or pathogens appearing in the population, thus pinpointing the start of the epidemic.

* Infectious diseases and vectors: ecology, genetics, evolution and control (UM -CNRS - IRD)