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Author: E. Mitchell
How sewage can lead to detection without infection
An introductory video (4:53m). Transcript available here.
Background
What is wastewater?
Wastewater is any water that has been contaminated by human use, including the water that is collected from businesses and homes, such as sewage.
What is epidemiology?
Epidemiology is the study of health and disease in populations -- it looks beyond single individuals to investigate patterns of health and disease across an entire community.
What is Wastewater Based Epidemiology (WBE)?
Wastewater based epidemiology (WBE) is the use of wastewater to investigate patterns of disease within a community that contributes to that wastewater. For example, it can be used to evaluate the prevalence of the new coronavirus (SARS-CoV-2) at a population level to estimate the number of cases per capita in a given community (Polo et al, 2020).
How does SARS-CoV-2 have anything to do with wastewater?
While SARS-CoV-2 is known as a respiratory virus, viral particles can also be found in the gastrointestinal tract and, ultimately, in feces of infected individuals. These particles can then finds their way into wastewater via municipal sewage systems (Polo et al, 2020).
Source: Polo, D., Quintela-Baluja, M., Corbishley, A., Jones, D. L., Singer, A. C., Graham, D. W., & Romalde, J. L. (2020). Making waves: Wastewater-based epidemiology for COVID-19 - approaches and challenges for surveillance and prediction. Water research, 186, 116404. https://doi.org/10.1016/j.watres.2020.116404
Source: Google Sites licensed images
SARS-CoV-2 nasal swab testing
Purpose
How is wastewater epidemiology helpful in tracking SARS-CoV-2 infections?
Although typical nasal swab testing has become much more available than it was in the early months of the pandemic, many people may still not have access to testing due to distance from testing sites, lack of payment, etc. Furthermore, it is known that many people who are infected with SARS-CoV-2 are asymptomatic and thus are unlikely to get tested, meaning that case counts based on individual testing are likely to far underreport the actual number of cases in a community. With sophisticated enough WBE tools, with one test alone, the amount of SARS-CoV-2 circulating in an entire community could be assessed (Polo et al, 2020).
Monitoring SARS-CoV-2 levels in wastewater could potentially provide a better estimate of the true number of cases in a given population, as wastewater from both symptomatic and asymptomatic individuals would be analyzed (Polo et al, 2020). Thus, tracking SARS-CoV-2 particles in wastewater could be used to better monitor existing outbreaks.
Furthermore, it could be used to detect new outbreaks more quickly than relying on individual testing. The CDC states that tracking SARS-CoV-2 through wastewater has been able to detect increases in cases days earlier than individual testing. It also allows a snapshot of infection at the sewershed level, which could be more informative than the county-level that is reported by many public health departments (Centers for Disease Control [CDC], 2020).
WBE can also be used for targeted surveillance of higher risk communities to trigger increased individual testing -- see the page on Targeted Wastewater Surveillance for more information (CDC, 2020).
Overall, WBE holds potential as a cost-effective method for more accurate detection of the amount of SARS-CoV-2 present in a community and, if successful, could be combined with testing for other viruses in the future (Polo et al, 2020).
Common Questions
Can an individual catch COVID-19 from coming into contact with wastewater?
WBE detects the amount of SARS-CoV-2 RNA in wastewater, which are the molecules that make up the virus' genome and only one part of the whole virus particle. RNA by itself is not infectious. There is little data on whether full virus particles that would be capable of infection can be found in wastewater but according to the CDC, there have been no reports of wastewater treatment plant workers contracting COVID-19 due to wastewater exposure. The CDC currently advises that normal wastewater plant worker protections including engineering controls, administrative controls and PPE should be enough to protect workers from the possibility of infectious particles within wastewater (CDC, 2020).
Has WBE been done before?
Wastewater Based Epidemiology was first utilized in 2005 to track illicit drug use (note-- given this surveillance history of an issue that has disproportionately impacted communities of color this is an ethical consideration that should be evaluated regarding WBE). WBE has been used to investigate polio, hepatitis A and norovirus outbreaks but It has not yet been used as a predictive aid for emerging outbreaks (Polo et al, 2020). Research on the use of WBE for SARS-CoV-2 tracking is currently being conducted in many locations, including across 16 wastewater treatment plants in Virginia (Gonzalez et al, 2020) and many ongoing studies are currently in process, including through OHSU in Portland (Robinson, 2020).
What are some important considerations for WBE?
Virology considerations: SARS-CoV-2 is an enveloped virus which typically does not survive as well in the environment as non-enveloped viruses (these are the types of viruses that typically cause gastrointestinal infections, as they can better survive the harsh, acidic process of digestion). Because of the increased likelihood of an enveloped virus like SARS-CoV-2 to degrade in the environment, If WBE is to be used, wastewater management facilities need to ensure that tests are being conducted prior to processing steps that could degrade the viral particles present in wastewater and make detection inaccurate. They should also be aware of the types of liquids present in collected wastewater, as presence of various non-water liquids (e.g. from factory or industrial waste) could lead to increased degradation of viral particles prior to reaching the facility (Polo et al, 2020).
SARS-CoV-2 particle. Source: Google Sites licensed images.
Example wastewater treatment plant. Source: Google Sites licensed images.
Furthermore, because detection of the amount of virus present in the wastewater is usually measured as a concentration, presence of rainwater in wastewater could dilute a sample and suggest falsely lower levels of virus in the community (Polo et al, 2020).
The location of testing within the flow from private home/ business to wastewater treatment plant is also an important consideration. Per current CDC guidelines, it is advised to test untreated water (at a plant or upstream of there -- see page on targeted wastewater surveillance) or primary sludge, which is produced after wastewater has been concentrated but before chemicals have been added that could further degrade virus RNA (CDC, 2020).
The manner in which a wastewater system is set up -- with sewer pipes, septic tanks, etc. -- may impact degradation of viral particles in wastewater and should be taken into consideration when conducting WBE (Polo et al, 2020).
Although WBE could be very beneficial for smaller communities where testing is less available or COVID-19 cases are less common, it could be more difficult to conduct in rural communities where there is not centralized wastewater collection (Polo et al, 2020).
What research still needs to be done?
How much virus is shed in feces and does this vary between individuals, particularly those who are symptomatic versus asymptomatic?
Is there another biological compound that is excreted by humans that could serve as a comparison assay for how large of a population is contributing to wastewater in a given watershed? (Polo et al, 2020)
Confirmative studies are needed to demonstrate that SARS-CoV-2 particles found in wastewater are not infectious and thus do not constitute a risk to either wastewater treatment plant workers or the general public (Polo et al, 2020).
WBE is a relatively new method, thus, possible ethical issues need to be explored, especially given the history of using WBE to survey for illicit drug use. If there is preexisting distrust of the healthcare community and/ or public health entities, communities may be suspicious of efforts to monitor disease in their community. This could be seen as an invasion of privacy and could lead to efforts to avoid utilizing public utilities. Thus, research should be conducted about how a community understands and/or feels comfortable with the concept of WBE prior to initiating surveillance (Polo et al, 2020).
Source: Google Sites licensed images.
References
1. Polo, D., Quintela-Baluja, M., Corbishley, A., Jones, D. L., Singer, A. C., Graham, D. W., & Romalde, J. L. (2020). Making waves: Wastewater-based epidemiology for COVID-19 - approaches and challenges for surveillance and prediction. Water research, 186, 116404. https://doi.org/10.1016/j.watres.2020.116404
2. Developing a Wastewater Surveillance Sampling Strategy. (2020, November 23). Retrieved November 23, 2020, from https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/wastewater-surveillance/developing-a-wastewater-surveillance-sampling-strategy.html
3. Information for Sanitation and Wastewater Workers on COVID-19. (2020, May 28). Retrieved November 23, 2020, from https://www.cdc.gov/coronavirus/2019-ncov/community/sanitation-wastewater-workers.html
4. Gonzalez, R., Curtis, K., Bivins, A., Bibby, K., Weir, M. H., Yetka, K., Thompson, H., Keeling, D., Mitchell, J., & Gonzalez, D. (2020). COVID-19 surveillance in Southeastern Virginia using wastewater-based epidemiology. Water research, 186, 116296. https://doi.org/10.1016/j.watres.2020.116296
5. Robinson, E. (2020, November 18). OHSU study will screen wastewater, test volunteers for coronavirus. Retrieved November 23, 2020, from https://news.ohsu.edu/2020/11/18/ohsu-study-will-screen-wastewater-test-volunteers-for-coronavirus
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