Public health surveillance has traditionally focused on actual cases, either retrospectively through passive surveillance or prospectively through active surveillance. While sewage surveillance has historically been used to monitor toxic substances, in 1939 South Carolina successfully implemented active surveillance for poliovirus in wastewater. This marked the first evidence that poliovirus, excreted in stools, could be detected in wastewater, proving it to be an essential tool for anticipating potential polio outbreaks.
From a public health standpoint, diseases that are under-reported by traditional surveillance systems and result in widespread negative health outcomes (e.g. surveillance for flaccid paralysis in polio) should be prioritized in wastewater monitoring programs. These include illnesses with nonspecific symptoms, such as polio, influenza, and viral gastroenteritis, as well as those where patients often delay seeking care until symptoms become severe. Wastewater monitoring is also valuable for diseases where current diagnostic tests are inadequate, inaccessible, or costly, providing data that could inform vaccine campaigns or other interventions to improve health outcomes.
Among the various biomarkers and chemicals measurable in sewage to assess public health, viruses have shown the most promise. Additionally, growing evidence suggests that wastewater may be an ideal environment for detecting antimicrobial-resistant pathogens — a rising global public health threat.
It is important to highlight that several viral respiratory infections are also excreted through feces, making the investigation of these viruses in wastewater both practical and essential.
Why is sewage surveillance an essential tool for anticipating clinical cases? The outcome of exposure to a virus (or any other pathogen) can be categorized into three scenarios:
No Infection: The individual shows no colonization.
Subclinical Infection: The individual is colonized (either transitorily or permanently) but does not exhibit clinical symptoms.
Clinical Infection: The individual is infected and manifests the disease.
In scenarios 1 and 2, neither passive nor active surveillance can detect pathogens circulating in the environment, meaning that circulation may be occurring without detection, potentially leading to a latent outbreak.
Sewage surveillance enables us to proactively identify pathogens before any individuals develop the disease—such as the recent polio case in Gaza—and implement preventive measures. These measures can include mass vaccination, enhanced clinical surveillance, strengthening healthcare facilities, and educating healthcare workers, among others.
In addition, monitoring human pathogens in sewage is even more feasible because they can be excreted through various bodily fluids other than feces: skin, and hair during active infection. These materials enter sewage systems through waste elimination processes such as toilet flushing and personal hygiene activities like bathing and hand washing. Additionally, pathogens can be introduced into the sewage system through indoor cleaning (e.g., floor mopping, kitchen sink use) and outdoor activities (e.g., car washing, driveway cleaning, stormwater runoff). As a result, sewer systems collect pathogens from a wide area and transport them to a central facility for processing. This process can be effectively understood as outlined in the accompanying figures:
CLINICAL SURVEILLANCE:
SEWAGE SURVEILLANCE:
Over the past 30 years, numerous studies have demonstrated that wastewater testing offers an accurate population-level view of disease. Recent research further confirms that trends in wastewater concentrations of SARS-CoV-2 strongly predict clinical case counts.
However, several challenges remain for large-scale implementation, which have been thoroughly described by Drs. Aparna Keshaviah, Ruchir N. Karmali, Divya Vohra, Tessa Huffman, Xindi C. Hu, and Megan B. Diamond (“The Role of Wastewater Data in Pandemic Management” Washington, DC: Mathematica, 2022):
Barriers to implementation (United States):
Only 7% of local agencies that had not yet monitored wastewater possess the necessary resources to initiate this process.
The most common barrier to implementing wastewater surveillance is a lack of internal staff capacity.
Additionally, rural agencies reported more barriers to implementation compared to their non-rural counterparts.
Again, a global effort:
In the United States, the situation is already challenging; however, worldwide sewage surveillance faces even greater obstacles. These include the complexities of collecting wastewater from septic tanks in many developing countries, the willingness of sewage plants to share wastewater data, the budget and personnel required to collect large volumes of samples, and establish both regional and national laboratories, among other issues. Indeed, sewage surveillance is a distinct field of science that requires the harmonious integration of various factors, including political support, private sector involvement, and scientific expertise. This collaboration is essential for effectively detecting current and future outbreaks and pandemics, enabling prompt and effective responses.
