By Brooke K. Mayer, PhD, PE

Through their 2013 viral song “The Fox,” Ylvis made the world wonder, what does the fox say? Of course, the song also caused most of us to scratch our heads thinking about how the possibility that foxes say “ring-ding-ding-ding-dingeringeding” ever came to light. Nonetheless, the group created a sensation with the wildly catchy and potentially aggravating lyrics (depending on how many times your kids subsequently asked you, “What does the fox say?”). The leap we make here is to think about what other things we previously didn’t consider might tell us, specifically what does human waste say?

To be clear, I’m not advocating for you to put your ear to the porcelain bowl to see if you can hear a faint “Joff-tchoff-tchoffo-tchoffo-tchoff” chorus line. But even without the vocalization, we can learn a lot from poo. Nearly a year before the global COVID-19 pandemic brought the world to a screeching halt, Tamara Duker Freuman, registered dietician and certified dietitian nutritionist, wrote, “If you’ve ever been on a medical odyssey to obtain a diagnosis for bothersome digestive symptoms, chances are someone along the way has ordered a stool analysis test for you. And for good reason: The simple act of pooping into a cup and dropping the sample off at a lab can yield a lot of diagnostic data.”[1] Indeed, poo can have a lot to say. And since the onset of the pandemic, we’ve become acutely aware that the messages can extend beyond personal health (e.g., a bacterial or parasite infection in the digestive tract), such that poo can help us to track community spread of diseases such as COVID-19.

Along with numerous other changes, COVID-19 sparked an explosion of interest in wastewater surveillance and wastewater-based epidemiology.[2] The U.S. EPA defines wastewater surveillance as “a community-level approach for monitoring disease or chemical biomarkers that are excreted in human urine and feces and collected in sewers.”3 Related, wastewater-based epidemiology uses the identification, potentially with quantification, of these biomarkers in wastewater to reflect a population’s health in near-real time.[4,5] The approach is illustrated in Figure 1, where wastewater samples are commonly collected at the head of a public wastewater treatment plant and analyzed for SARS-CoV-2 ribonucleic acid (RNA) using molecular methods such as reverse transcriptase-polymerase chain reaction, and the resulting data is analyzed to glean public health insights. Compared to other outbreak surveillance techniques, this offers a noninvasive approach to infectious disease surveillance that decouples the information from ease of public access to health care facilities.[4] Data can be used as an early warning for outbreaks, as well as to gauge toward assuaging them.[5]

Figure 1. Illustration of wastewater surveillance and the waste­water-based epidemiology framework. Epidemiology is defined as “the study of the determinants, occurrence, and distribution of health and disease in a defined population.”[6]

The roots of wastewater-based epidemiology extend as far back as the 1940s, when Trask et al. (1942)[7] assessed the presence of poliovirus in sewage.[2] The approach gained traction in the early 21st century,[2] when wastewater-based epidemiology was hypoth­esized to provide a linkage between the presence of drug residues in wastewater and population usage,[8] for example by quantifying cocaine metabolites in wastewater.[9 ]The technique has since been widely used to enhance understanding of usage of chemicals such as pharmaceuticals and personal care products, as well as prevalence of biological markers, such as genetic material from pathogenic microorganisms such as poliovirus, hepatitis B virus, and norovirus, as well as antibiotic resistance genes.[3,4] These efforts have demonstrated that wastewater surveillance can be used to detect viral outbreaks earlier compared to clinical surveys.[3]

The most active wastewater monitoring networks have been in Europe, Australia, and the US.[4] Such efforts rapidly expanded to provide surveillance of SARS-CoV-2, the virus responsible for COVID-19. Although COVID-19 is a respiratory infection, its RNA can be shed in the feces of pre-symptomatic, symptomatic, and asymptomatic individuals (although there is no information to date that individuals have become sick with COVID-19 due to direct exposure to wastewater).[10] For example, researchers from the Netherlands and Italy began testing and detecting the presence of SARS-CoV-2 viral RNA in sewage in February 2020 even before the Netherlands reported its first COVID-19 case.[3] Such early efforts highlighted the potential for wastewater surveillance to help inform understanding of COVID-19 com­munity spread, including serving as an early warning system of increased virus spread,[3] thereby launching numerous COVID-19 wastewater surveillance programs. The University of California at Merced operates a COVIDPoops19 dashboard that tracks such efforts (Figure 2).

A major effort to coordinate and build capacity for SARS-CoV-2 tracking in the US was initiated by the Centers for Disease Control and Prevention (CDC) in September 2020 in the form of the National Wastewater Surveillance System (NWSS). As of October 2022, more than 1,250 NWSS sites in the US had begun waste­water surveillance efforts, leading to the collection of more than 90,000 samples from systems serving about 40% of the US population (more than 133 million people). Figure 3 shows NWSS data in a) aggregate percent change categories since January 2021 and b) percent change by site for a 15-day period in October 2022. Data such as this can help inform the public health re­sponse to the COVID-19 pandemic and are complementary to current and future infectious disease surveillance systems.[3] The CDC is continuing to explore how the NWSS can be used to detect and respond to other infectious disease threats, e.g., antibiotic resistance and food-borne diseases.

The EPA conducted interviews with 10 different wastewater surveillance programs in the US and reported that four key aspects were identified as being critical to program success: 1) collaboration, 2) flexibility, 3) transparent communication, and 4) adequate funding. For example, the state of Wyoming relied on outreach support from the Wyoming Association of Rural Water Systems to help educate utilities about wastewater sur­veillance, which led to voluntary participation in the state’s program. In Michigan, supply chain issues were overcome by opting to use a different analytical technique. Tempe, Arizona, leveraged strong relationships among public entities built on transparent sharing of information, e.g., at town hall meetings, to rapidly adapt their existing opioid wastewater surveillance program to COVID-19 surveillance. Finally, funding mechanisms such as the CARES Act and CDC ELC were instrumental in initiating and maintaining efforts across the case studies.[3]

The dramatic increase in wastewater surveillance efforts in response to COVID-19 has undoubtedly delivered meaningful public health benefits. However, these efforts are not a one-size-fits-all tool, and it is also important to recognize their limitations. Two potential limitations include securing finances to operate a program and actionability using the results. Specifically, wastewater surveillance programs require sustained (sometimes substantial) funding, including capital for setting up the testing lab, as well as operational expenses associated with collection and analyses. Second, although wastewater surveillance offers advantages of anonymous, unbiased data, acting on these pooled results can be challenging at the community level such that responses are not always viable across all settings.[2]

To help guide wastewater-based epidemiology efforts in light of these and other challenges, Safford et al. (2022)[2] offer the following recommendations:

1) Avoid redundancy between clinical testing and wastewater-based epidemiology testing.

2) Emphasize statistical thinking, data analysis, and data management.

3) Define action thresholds.

4) Monitor fewer sites more frequently.

5) Build on existing infrastructure and programs.

6) Be prepared to adapt.

7) Keep an eye on the future.

These suggestions inform the field about strategic approaches to “listening to” to the poo as a complementary strategy in an effective, multifaceted portfolio of surveillance efforts to im­prove public health monitoring and protection.

References

  1. Duker Freuman T. What Can a Stool Test Diagnose? US News World Rep. Published online 2019:1-9. https://health.usnews.com/health-news/blogs/eat-run/articles/what-can-a-stool-test-diagnose
  2. Safford HR, Shapiro K, Bischel HN. Wastewater analysis can be a powerful public health tool—if it’s done sensibly. Proc Natl Acad Sci U S A. 2022;119(6):1-5. doi:10.1073/pnas.2119600119
  3. Gutierrez S, Kazior K, Nepal S. A Compendium of U.S. Wastewater Surveillance to Support COVID-19 Public Health Response. https://www.epa.gov/system/files/documents/2021-09/wastewater-surveillance-compendium.pdf
  4. Sims N, Kasprzyk-hordern B. Future perspectives of wastewater-based epidemiology: Monitoring infectious disease spread and resistance to the community level. Environ Int. 2020;
  5. McMahan CS, Self S, Rennert L, et al. COVID-19 wastewater epidemiology: a model to estimate infected populations. Lancet Planet Heal. 2021;5(12):e874-e881. doi:10.1016/S2542-5196(21)00230-8
  6. Brachman PS. Epidemiology. In: Baron S, ed. Medical Micro­biology. 4th ed. University of Texas at Galveston; 1996.
  7. Trask JD, Paul JR, Riordan JT. Periodic examination of sewage for the virus of poliomyelitis. J Exp Med. 1942;75(1):1-6. doi:10.1084/jem.75.1.1
  8. Daughton CG. Emerging Pollutants, and Communicating the Science of Environmental Chemistry and Mass Spectrometry: Pharmaceuticals in the Environment. J Am Soc Mass Spectrom. 2001;12(01):1067-1076.
  9. Zuccato E, Chiabrando C, Castiglioni S, et al. Cocaine in surface waters: a new evidence-based tool to monitor community drug abuse. Environ Heal A Glob Access Sci Source. 2005;4:1- 7. doi:10.1186/1476-069X-4-20
  10. 10.CDC. National Wastewater Surveillance System (NWSS). Accessed October 30, 2022. https://www.cdc.gov/healthywater/surveillance/wastewater-surveillance/wastewater-surveillance.html
  11. UC Merced. COVIDPoops19. Published 2022. Accessed October 30, 2022. https://www.arcgis.com/apps/dashboards/c778145ea5bb4daeb58d31afee389082
  12. CDC. COVID Data Tracker. Published 2022. Accessed October 30, 2022. https://covid.cdc.gov/covid-data-tracker/#waste­water-surveillance

About the Author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds Bachelors, Masters and Doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.

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