By Dr. Brooke K. Mayer

In the midst of the second year of the COVID-19 pandemic, as we work through concerns over increased transmissibility of the Delta variant, awareness of microbial risks is heavy on many people’s minds. Although water and wastewater are not believed to play an important role in transmission of novel coronaviruses such as SARS-CoV-2 (the virus responsible for COVID-19), nor other enveloped viruses linked with recent outbreaks including SARS, MERS and Ebola, recent events have prompted interest in improved understanding of microbial routes of transmission, environmental fate and persistence in water matrices.[1,2] Such information contributes to our growing knowledge of the waterborne microbial universe and how it relates to each of us, wherever we are in the world.

In the US, in spite of great strides in preventing waterborne disease and having one of the safest drinking water supplies in the world, over seven million people get sick every year from diseases spread through water (including drinking water, recreational water and cooling systems).[3] As reported via the Centers for Disease Control (CDC) Waterborne Disease and Outbreak Surveillance System, microbes were responsible for roughly half of US drinking water-related outbreak cases in 2014, with parasites constituting the majority of those cases (Figure 1b). Legionella has been responsible for the majority of reported drinking water outbreaks in the US since 2009 (Figure 1a).[4] Note that outbreaks are defined as two or more cases epidemiologically linked by time, location of water exposure and illness characteristics.

Figure 1. Etiology of drinking water outbreaks and outbreak-related cases reported through the US Waterborne Disease and Outbreak Surveillance System, illustrated as A) number of outbreaks 1971- 2014 and B) percent of cases (N = 1,006) 2013-2014. Data were reported by Benedict et al. (2017).[4]

The CDC estimated that 17 waterborne pathogens were responsible for over seven million cases, 118,000 hospitalizations and over 6,000 deaths in the US in 2014. This equated to approximately one in 44 people getting sick from waterborne disease, with an associated $3.33 billion in direct healthcare costs for hospitalizations and emergency department visits.[3] Toward continuous improvement in our ability to mitigate these waterborne risks (as well as those from chemical contaminants), US EPA updates its Contaminant Candidate List (CCL; first implemented in 1998) approximately every five years. This list includes chemicals and microbes that are not currently regulated but which are known or believed to occur in public water systems. Accordingly, these contaminants are prioritized for research on their occurrence, health effects, treatability, etc., in support of regulatory determination.

US EPA’s multi-step process for identifying microbial contaminants for inclusion on draft CCLs is: 1) building the microbial universe, 2) screening to construct the preliminary CCL and 3) classification and selection for the draft CCL. Using this approach, the microbial universe is defined as any pathogen (bacteria, virus, fungi, helminth, or protozoa) that causes human disease. For the CCL 3 (published in 2008) and the CCL 4 (published in 2016), 1,425 microbes were included as part of the initial universe; reorganizations and the addition of five bacteria, seven viruses (including SARS-CoV-2) and two fungi brought the total microbial universe to 1,435 for the CCL 5. Subsequent screening to evaluate the potential occurrence of these microbial contaminants in drinking water honed the list to 35 pathogens for inclusion on the preliminary CCL 5. Further evaluation of these pathogens’ occurrence in water and their ability to cause adverse human health impacts led to the selection of the 12 microorganisms currently included on the draft CCL 5, as illustrated in Figure 2.[5]

While the microorganisms shown in Figure 2 are at the forefront of consideration for potential future regulation in the US, other areas around the world may be dealing with similar, or very different, waterborne microbial concerns depending on the condition of local water and sanitation systems. Across the globe (but also on smaller geographic scales, e.g., rural versus urban), sharp socio-cultural and economic inequalities persist that can limit access to improved sources of drinking water and sanitation systems. In fact, in 2020, 26 percent of the world’s population lacked safely managed drinking water services, 3.6 billion lacked safely managed sanitation services and 2.3 billion lacked handwashing facilities with soap and water at home. Figure 3 shows a breakdown of causes of death categorized by income group (based on gross national income, as defined by the World Bank). The data for both 2000 (Figure 3a) and 2019 (Figure 3b) show the inverse relationship between income group and proportion of deaths caused by infectious and parasitic diseases (of which water-related deaths constitute a fraction). Also evident from the figure, nearly all regions reduced the burden of deaths caused by infectious and parasitic diseases from 2000 to 2019, although the fraction increased slightly from 1.8 to 2.1 percent in high-income countries.

Figure 3. World Health Organization (WHO) global health estimations of causes of death by region (classified using World Bank income groups) for years A) 2000 and B) 2019. The values shown in the figures represent the percent of total deaths attributable to infectious and parasitic diseases reported by WHO.[6]

Worldwide deaths specifically attributed to water, sanitation and hygiene (WASH) are geographically represented in Figure 4. With respect to diarrheal risk factors, 77 percent are due to inadequate WASH, 70 percent due to unsafe water, 54 percent due to unsafe sanitation and 34 percent due to lack of handwashing facilities.[7] The etiological agent associated with diarrheal deaths of children under five, a population that is particularly susceptible to waterborne diseases, is illustrated in Figure 5. As shown, the three most common etiologies included rotavirus, Cryptosporidium spp and Shigella spp. For comparison, Cryptosporidium is currently regulated in the US and Shigella sonnei is on the draft CCL 5, whereas rotavirus is neither specifically regulated nor under specific regulatory consideration in the US. This is further illustrated considering the differences between high and low income/socio-demographic index in Figure 5, where Clostridium difficil and Vibrio cholerae accounted for higher proportions of diarrheal deaths relative to the other microorganisms in higher income/socio-demographic index societies.

Figure 5. Relative frequency of 13 different etiological agents, grouped by global geographical socio-demographic index (SDI), associated with deaths of children under five due to diarrheal disease in 2015. The values shown in each box represent the estimated number of deaths modeled using a counterfactual approach. The figure was adapted from the GBD Diarrhoeal Diseases Collaborators report published by Troeger et al. (2017).[9]

As discussed above, the waterborne microorganisms of greatest concern may vary by region. Ashbolt (2015)[9] notes, however, that to some extent, categorizing pathogen risks across developing versus developed regions is somewhat artificial given, for example, failures or mismanagement of water and sanitation systems or higher risks of global disease transmission due to travel between regions. Accordingly, regardless of where you are in the world, there is a need for ongoing system-wide vigilance coupled with preventative rather than solely responsive water management.


  1. Silverman AI, Boehm AB. Systematic review and meta-analysis of the persistence and disinfection of human coronaviruses and their viral surrogates in water and wastewater. Environ Sci Technol Lett. 2020;7(8):544-553. doi:10.1021/acs.estlett.0c00313
  2. Bibby K, Casson LW, Stachler E, Haas CN. Ebola virus persistence in the environment: State of the knowledge and research needs. Environ Sci Technol Lett. 2015;2(1):2-6. doi:10.1021/ez5003715
  3. CDC. Waterborne Disease in the United States. Waterborne Disease & Outbreak Surveillance Reporting. Published 2020. Accessed August 29, 2021. surveillance/burden/index.html
  4. Benedict KM, Reses H, Vigar M, et al. Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2013–2014. MMWR Morb Mortal Wkly Rep. 2017;66:1216 1221doi:
  5. US EPA Office of Water. Technical Support Document for the Draft Fifth Contaminant Candidate List (CCL 5)-Microbial Contaminants.; 2021. Accessed August 28, 2021. https://www. pdf
  6. WHO. Deaths by cause, age and sex, by World Bank income group, 2000-2019. Global Health Estimates 2019 Summary Tables. Published 2020. Accessed August 29, 2021. https:// estimates
  7. Troeger C, Forouzanfar M, Rao PC, et al. Estimates of global, regional and national morbidity, mortality and aetiologies of diarrhoeal diseases: a systematic analysis for the Global Burden of Disease Study 2015. Lancet Infect Dis. 2017;17(9):909-948. doi:10.1016/S1473-3099(17)30276-1
  8. WHO. SDG 3.9.2 WASH deaths. Burden of disease – Burden of disease SDG 3.9.2 – Mortality rate attributed to unsafe water, unsafe sanitation and lack of hygiene (exposure to unsafe Water, Sanitation and Hygiene for All (WASH)). Published 2016. Accessed August 29, 2021. gho/data/indicators/indicator-details/GHO/sdg-3-9-2-washdeaths% 0A
    9. Ashbolt NJ. Microbial contamination of drinking water and human health from community water systems. Curr Environ Heal reports. 2015;2(1):95-106. doi:10.1007/s40572-014-0037-5

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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