Health

Occurrence and persistence of SARS-CoV-2 in wastewater

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can shed from infected hosts and enter the local environment via respiration and excretion – namely, in urine and feces. This has been reported to persist in the air, on high-traffic household objects, and recently in wastewater and sewage. The presence of the virus in wastewater could be of great concern, particularly in less developed countries where water treatment facilities are poor or missing. Many other viruses have been implicated in community-level transmissions and can persist within the water infrastructure, and early reports suggest that SARS-CoV-2 can survive in 5⁰C water for 25 days. In a paper recently published in Science of the Total Environment, the occurrence of SARS-CoV-2 in the household water supply, wastewater, and in environmental waters is reviewed based on over 120 reports collected from around the world. The applicability of wastewater epidemiology and various water treatment strategies are also assessed.

Wastewater epidemiology

SARS-CoV-2 tends to remain detectable in serum samples for 16 days, in the respiratory airways of COVID-19 patients for around 18 days, and in feces for 22 days. However, some studies have observed viral RNA shedding in feces for over 30 days following infection. Survival of the virus in the environment, on surfaces or in water, is highly dependent on local relative humidity, pH, exposure to sunlight and temperature extremes, and several other factors. Temperature is particularly a factor in an aqueous environment, with SARS-CoV-2 reportedly surviving for almost two years at 4⁰C in purified tap water and just days at around 20⁰C. Faulty sewage systems have been blamed for at least one SARS outbreak in Hong Kong in 2003, facilitating the production of virus-laden aerosol droplets in one hotel building when flushing the toilet, and many other viruses are routinely checked for at treatment facilities.

By accurately determining the presence of SARS-CoV-2 in wastewater, wastewater surveillance could potentially provide an estimation of the extent of COVID-19 infections in the community. This could allow additional information to be extrapolated, such as the rate of asymptomatic individuals. Of the many regions that have reported SARS-CoV-2 in wastewater around the world, a general trend between the number of local cases and the concentration of viral RNA found has been noted. The group notes that a wide variety of sampling methodologies were employed in the papers reviewed, leading them to call for greater standardization.

The wastewater samples must undergo several preparatory steps such as filtering, pH adjustment, and the addition of viral elution enhancers. The virus particles can be separated from other components of wastewater by centrifugation, filtration, precipitation, or by other recovery routes. PCR assays developed to test for SARS-CoV-2 in humans were used by all of the papers reviewed to multiply and identify the RNA present, and the group emphasizes the need for more rigorously standardized and routinely employed collection and detection procedures, agreeing that routine qPCR testing is currently the most suitable method.

The authors suggest that improved wastewater surveillance for SARS-CoV-2 could be an appealing alternative or addition to widespread testing of individuals, indicating the virus’s presence and degree of prevalence.

Demographic information could also be obtained by monitoring wastewater from individual communities, potentially allowing additional information regarding the pathogenicity of the virus to be realized. The first case of COVID-19 was reported in Italy on February 21st 2020, and the virus was then detected in wastewater in Italy on February 24th, suggesting that the presence of the virus in wastewater is highly indicative of very recent widespread infection. Indeed, wastewater epidemiology has exhibited a predictive capacity, with SARS-CoV-2 in some cases being noted in wastewater in advance of any cases being reported locally. Individual clinical testing on the scale required to predict an oncoming outbreak would be extremely costly in comparison, the authors argue, and estimate that a single infected individual in a population of two million could be detected with sufficient monitoring of wastewater.

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