New analyses confirm the absence of SARS-CoV-2 in shellfish but reveal traces in wastewater in the Grand Ouest area

Following the detection of the coronavirus SARS-CoV-2 in wastewater in Paris and the Great East area, Ifremer is now looking into the possible contamination of wastewater by the virus by carrying out a series of samples at three treatment plants in the Great West area. While the study of these samples confirms the presence of the SARS-CoV-2 genome in 9 out of 13 wastewater samples, new analyzes also confirm the absence of any trace of the virus in shellfish.

Following the initial results of seawater and shellfish analyses carried out by the Environmental Health and Microbiology laboratory (LSEM) in Nantes with the support of Ifremer's "Environment - Resources" laboratories (LER), Soizick Le Guyader’s team continued its investigations by analyzing wastewater sampled from three wastewater treatment plants in the Great West area and a new batch of shellfish from 3 coastlines.

The analysis of wastewater, a repository of human waste, gives a true snapshot of the range of microorganisms present in the human population. It is a relevant and compatible means of screening to estimate the proportion of the population infected and the actual level of virus circulation in a given area.

Traces of SARS-CoV-2 found in three treatment plants in Brittany and Pays de la Loire

Analyses of wastewater from regions severely impacted by the Covid-19 epidemic such as the Paris region (Wurtzer et al., 2020) revealed the presence of the SARS-CoV-2 genome. In this context, Ifremer wanted to know if traces of SARS-CoV-2 are present in the wastewater of other geographical areas less affected by the epidemic. The LSEM sampled three wastewater treatment plants in the Grand Ouest area (Pays de Loire and Brittany). The first two treatment plants are located in an urban area with a high population density, and the third is located in a less densely populated area close to the coast.

A total of 13 one-liter samples of untreated water were collected at the inlet of the 3 treatment plants (before treatment) between March 16 and May 12, 2020. Results: 

SARS-CoV-2 genome detected in 9 out of 13 wastewater samples collected

« Our time-phased analyses suggest that the virus is circulating at a lower rate than at the start of the epidemic », explains Soizick Le Guyader, virologist and head of Ifremer's Environmental Health and Microbiology laboratory in Nantes. « The 7 wastewater samples collected before April 24 are all positive for SARS-CoV-2. As of April 24, out of the 6 samples analyzed, 4 were found to be negative ».

Confirmation that there is no trace of SARS-CoV-2 in shellfish

Ifremer analyzed a second batch of shellfish from sites exposed to sources of human fecal contamination. The aim of its sampling strategy is to obtain balanced data for the different mainland coastlines. As a reminder, the first series had already concluded that there was no SARS-CoV-2 detected in the mollusks analyzed.

The Ifremer teams analyzed: one sample of clams, two samples of mussels, and 16 samples of hollow oysters collected along the Normandy, Brittany, Atlantic and Mediterranean coastline between May 4 and 11, 2020. Among these 19 samples analyzed:

No shellfish samples showed any traces of SARS-CoV-2.

« One of these shellfish samples was taken close to effluent from a treatment plant in which we had detected traces of SARS-CoV-2 before treatment. The fact that no trace of SARS-CoV-2 was detected there suggests that wastewater treatments are likely to reduce the risk of spreading the virus in the marine environment », notes Soizick Le Guyader.

Next steps: to monitor SARS-CoV-2 in wastewater nationally and over the long term

Ifremer recently joined the epidemiological wastewater observatory project called Obépine, the objectives of which are to pool the results of wastewater analyses carried out by laboratories throughout France in order to monitor the dynamics of the virus across the country.

« The presence of traces of SARS-CoV-2 in wastewater, revealed by our study, is not surprising in light of the results of previous studies carried out in Paris or in the Netherlands » , highlights Soizick Le Guyader. « Whilst the circulation of the virus in the population seems to have been slowed down by the lockdown, it is nevertheless important to continue our analyses in order to monitor the effects of the gradual lifting of these measures. Observing an increase in the virus's presence in wastewater could help prevent a possible second wave of the epidemic ».