Scientists from the French National Centre for Scientific Research (CNRS) have reported a recent increase in the amount of SARS-CoV-2 RNA detected in the wastewater upstream of the main wastewater treatment plant (WWTP) serving the Montpellier metropolitan area in southern France, informs News Medical.
Wastewater based epidemiology has been established as an effective approach to monitor the evolution of viral circulation in populations. About half of COVID-19 symptomatic patients shed viral RNA in their stool. A recent study has shown that an asymptomatic patient can have a negative throat swab test but a positive stool test. Thus, whether symptomatic or not, people infected with SARS-CoV-2 may release viral RNA into wastewater.
The findings have been published as a preprint available on medRxiv. Researchers collected wastewater samples starting on May 7th, 4 days prior to the end of the lockdown in France, and then again at 8 to 11 day intervals until June 25th, to monitor the levels of viral RNA up to 45 days after lockdown ended. During that period, the virus was circulating in the area, and its incidence was low (less than 20 new patients diagnosed per day).
While the overall number of both newly diagnosed and hospitalised COVID-19 patients decreased since April 1st, wastewater data shows more viral RNA in wastewater from the later sampling dates (June 15th and 25th). Thus, the increase in viral RNA detected at the WWTP does not correlate with the number of newly diagnosed patients, and the authors conclude further epidemiological investigation is necessary to explain the results.
The researchers note that they are not able to determine whether the increased amount of vital RNA detected in the more recent samples is caused by an imminent increase of infected people in the area, or could be due to intrinsic viral RNA variations linked to uneven virus shedding (different patients releasing different amounts, or a single patient releasing different amounts as the disease evolves). Other factors that could come into play are movements of people while on vacation, underestimation of the prevalence of the disease, or uneven spread locally. They conclude future investigations will help establish whether wastewater surveillance can be a strong predictive tool for future disease outbreaks.