As some of the stricter COVID-19 lockdown restrictions are being eased in countries around the world, some, such as China and Germany, have already experienced a second wave of cases. But might there be a way to predict new epicenters of infection and nip them in the bud before they are able to establish themselves and spread further? Some scientists think so, and the answer lies beneath our feet – wastewater.
What can wastewater tell us about SARS-CoV-2 infections?
Wastewater monitoring, also referred to as wastewater-based epidemiology (WBE), has previously been used to detect outbreaks of norovirus, hepatitis A, measles, poliovirus and antibiotic-resistant bacteria. When someone becomes infected with SARS-CoV-2, the virus replicates inside their body. Some of that viral RNA which makes up that virus is excreted in our feces which then passes into wastewater when we flush our toilet. By detecting SARS-CoV-2 genetic material in wastewater, scientists are therefore able to determine if members of the community are infected.
Current estimates based on European and North American data suggest that 56.6 million to 11.3 billion viral genomes will be excreted into wastewater per infected person per day. This translates to between 0.15 and 141.5 million viral genomes per liter of wastewater generated.
The presence of SARS-CoV-2 in wastewater has also raised the question of fecal-oral transmission, although this has yet to be proved.
Quantitative RT-PCR has been used successfully to detect SARS-CoV-2 in wastewater samples. A paper-based analytical device designed to detect nucleic acids of SARS-CoV-2 in wastewater rapidly, cheaply and with a visible positive/negative result is also being explored. It is hoped this could provide a solution for non-expert users. Biosensors and ELISA-based methods have also been proposed.
Whichever technique is used, it is important that testing is sufficiently sensitive and specific to ensure that data points are representative. Monitoring from suitable locations and repeated testing over a period of time too are vital considerations to ensure the samples used are representative of the true picture.
Wastewater detection of polio infection achieved a sensitivity of 100 infected but mostly asymptomatic individuals in tens of thousands of individuals, demonstrating how sensitive wastewater detection methods can be.
One RT qPCR-based detection method for SARS-CoV-2 boasts a sensitivity capable of detecting a single infected individual among 114, increasing to 1 in 2 million people under optimum conditions.
What are the potential advantages of wastewater testing?
By testing wastewater samples, it negates the need for testing hundreds or thousands of individuals, reducing the burden on testing facilities and the associated costs. Wastewater detection could therefore offer testing capabilities in regions where testing of individuals is challenging for both logistical and economic reasons.
This approach enables epidemiologists to monitor and model the number of infections within communities, useful for outbreak tracking and improving disease modeling. In addition, if a potentially problematic area where infection appears to be increasing – the second wave - is identified, it enables scientists to then focus follow-up testing and remedial action in specific areas.
With large numbers of those infected with SARS-Cov-2 not displaying clinical signs, and others shedding live infectious virus prior to showing clinical signs, wastewater monitoring can offer a more representative snapshot of true infection rates within a population.
The U.S. have added SARS-CoV-2 to the range of health indicators that are continually tracked in wastewater by the Human Health Observatory (HHO), the largest national and international WBE network and sample repository.
Whilst the principle has been proved for other viral infections, with SARS-CoV-2 detection, there are however a number of questions that need to be considered and further research carried out. These include how much viral RNA is excreted in feces, at what stages during infection is it excreted, how does variation in wastewater volume (eg following a rainstorm, water usage variations) affect detection, how many people need to be infected in an area before a test is sensitive enough to detect it consistently? Factors such as wastewater temperature and seasonal impacts too must be taken into account when assessing the effectiveness of detection methods.
Wastewater experts are working hard to develop appropriate procedures for wastewater collection, storage and testing and approaches to interpret and utilize the resultant information effectively. This is a relatively new area of research for SARS-CoV-2 but is already showing a great deal of potential.