SARS-CoV-2 in water: A surveillance tool for infected populations

The presence of land-derived bacteria in the marine environment is an indicator of wastewater contamination and of human health at a population level. However, the survival of human viral pathogens in the marine environment and their use as indicators is poorly understood. Dr Helena Galvão of the University of Algarve, Portugal, investigated the survival of SARS-CoV-2 in environmental waters and its elimination in sewage treatment plants. Her findings help determine the potential contamination within the Ria Formosa lagoon.

Land-derived pathogenic bacteria are generally not able to survive long in the marine environment due to the stress presented by highly saline waters. However, it is not yet understood whether the same applies to viral pathogens. With the potential for human viruses to enter the marine environment through contaminated wastewaters, it is important to evaluate different viral pathogens’ survival in environmental waters and determine the impact this could have.

Professor Helena Galvão of the University of Algarve and colleagues considered the potential contamination of the Ria Formosa lagoon in southern Portugal with SARS-CoV-2 through wastewater discharges. As a result, the research team offer insights regarding the ability of the virus to persist in wastewater and the marine environment, its elimination in wastewater treatment plants, and what this can tell us about the population as a whole.

What’s in the lagoon?

The Ria Formosa lagoon system, located on the south coast of Portugal in the Algarve region, is characterised by an arid Mediterranean climate and a lack of significant freshwater inputs. As a result, the lagoon remains hypersaline throughout the year, having a high salt content relative to open coastal waters. The lagoon holds ecological importance, acting as a refuge and nursery area for a variety of marine species, and it is also a well-utilised area of economic value, particularly for the fishery, aquaculture, and tourism industries.

Due to old infrastructure and poor management of wastewater networks in the cities surrounding the Ria Formosa, there is an issue of wastewater overflowing into the surface drainage system and discharging into the lagoon.

This could represent a potential contamination risk to the public, with domestic wastewater contaminated with sewage being able to bypass wastewater treatment plants and enter the marine environment. As such, the potential presence and survival of human pathogens in the lagoon need to be evaluated to better understand the risk that wastewater contamination presents.

Wastewater as an indicator

Wastewater-based epidemiology (or WBE) is an approach based on analysing wastewater that can monitor the presence of drugs or pathogens within a population. Wastewater is a useful indicator for the presence of pathogens and allows a sample representing thousands of potentially infected individuals to be analysed at one time. In addition, wastewater analysis can provide early indication of potential infection hotspots.

Land-derived bacteria and those found in human faeces, such as Escherichia coli and intestinal Enterocci, tend not to survive for long in high-salinity waters like the Ria Formosa and can be used as an indicator for faecal contamination in these waters. However, there has been little investigation carried out regarding the survival of human pathogenic viruses, such as SARS-CoV-2, in environmental waters and what this could indicate.

What SARS-CoV-2 reveals

SARS-CoV-2 is the causal agent of COVID-19, characterised by a high infection rate and the possibility of being infected without symptoms. Galvão and colleagues assessed the survival of SARS-CoV-2 in environmental waters and in wastewater treatment plants as a predictor of human health and pandemic progression. They achieved this by taking samples from the inflow and outflow of several wastewater treatment plants in the Algarve, as well as from surface water at environmental stations within the Ria Formosa lagoon.

The researchers found that SARS-CoV-2 was not present in the surface water at environmental stations or in the treated water at the outflow of wastewater treatment plants, but it was detected in the untreated wastewater at the inflow. This suggests that wastewater treatment could effectively remove SARS-CoV-2 before its release into the marine environment, reducing the risk to human health for those that use the lagoon.

The highest presence of the virus was found in untreated wastewater at the Faro-Aeroporto wastewater treatment plant in March 2021. This coincided with a peak in the number of positive COVID-19 cases in the Algarve. However, despite this increase in the virus, the wastewater treatment plant outflow and environmental waters remained negative, meaning wastewater treatment remained effective in eliminating SARS-CoV-2, even when it was present in moderately high concentrations. As a result, the team’s findings confirm that monitoring the presence of SARS-CoV-2 in wastewaters in the Algarve region can be used as a means of detecting COVID-19 within the population.

Looking to the future

The research team’s findings indicate that there is a need for health authorities to monitor the survival of viral pathogens in both wastewater and environmental waters. Recreational waters in Portugal, like the Ria Formosa lagoon, are routinely monitored for bacterial contamination, in line with European Blue Flag and EU Bathing Water Directives. However, this monitoring does not currently extend to viral pathogens.

Galvão’s results regarding the increased presence of SARS-CoV-2 and concurrent peaks in COVID-19 cases in the region highlight the importance of monitoring the viral pathogens in the water in addition to the bacterial ones, especially in highly populated areas. This is particularly relevant in view of the potential for future pandemics.