Assessing health risks and disease burden from waterborne pathogens across multiple urban exposure pathways

Urbanization represents one of the most profound demographic shifts of the 21st century. According to United Nations projections, the global urban population is expected to nearly double by 2050, growing from 3.3 to 6.3 billion. This rapid urban expansion will result in 68% of the world's population residing in urban areas (United Nations 2018). Developing countries are expected to experience the most dramatic increases in urban residents. As urbanization accelerates and populations swell, the pressure on existing sanitary infrastructure intensifies, posing significant challenges for cities to deliver adequate services. This issue is particularly acute in densely populated urban slums, where water and sanitation infrastructure is often inadequate. The resulting environmental pollution, combined with overcrowding and poor sanitation, exacerbates the risk of waterborne diseases (Okaali & Hofstra 2018; Ronoh et al. 2020).

Bangladesh is a poignant example of these challenges. Despite notable progress in improving sanitation, with 61% of households now having access to improved facilities (JMP 2022), serious issues persist. Surface water contamination by fecal pathogens remains a major public health concern, particularly in urban areas. In Dhaka, the capital city, a large portion of wastewater is untreated, leading to the direct discharge of sewage and other pollutants into drains and surface waters. About one-third of Dhaka's population relies on pour-flush toilets that discharge directly into drains (WSUP 2018), while 30% use on-site septic tanks that often release effluent into drains, canals, and rivers (JMP 2022). This improper waste management has led to high levels of fecal indicator bacteria in Dhaka's open drains (Amin et al. 2019), exacerbating health risks through various exposure pathways (Harris et al. 2018; Islam & Islam 2020).

Dhaka city frequently experiences severe flooding and waterlogging due to its low-lying topography, high population density, and rapid urbanization. Flooding is often exacerbated by heavy monsoon rains, inadequate drainage infrastructure, and the city's proximity to the Buriganga River and other water bodies. Waterlogging occurs when these drainage systems are overwhelmed, leading to stagnant water on streets and in residential areas. Floodwaters often mix with sewage and waste, contaminating drinking water supplies and increasing exposure to fecal pathogens and the risk of waterborne diseases (Foster et al. 2021). Pathogens such as Vibrio cholerae (which causes cholera), Salmonella, and rotavirus can spread rapidly through contaminated water (Amin et al. 2020). The high density of the human population in Dhaka facilitates the transmission of these pathogens. Waterborne diseases like cholera, dysentery, and typhoid fever become more prevalent during and after floods, leading to outbreaks that strain the healthcare system. The economic consequences are profound, encompassing direct medical costs, lost productivity, diminished quality of life, and increased mortality. Addressing these issues is crucial for improving public health and mitigating the broader societal impacts of waterborne diseases in Bangladesh.

The primary transmission pathways for waterborne pathogens in urban environments include exposure to contaminated surface water during bathing or recreational activities and direct contact with polluted drains or flood waters (Van Abel & Taylor 2018; Dias et al. 2019). Such exposure pathways make communities vulnerable to waterborne diseases, highlighting the need for thorough assessments of health risks associated with contaminated surface waters (Dias et al. 2019; Amoueyan et al. 2020; Islam et al. 2021). This is especially critical in developing countries where microbial pathogens are encountered through diverse exposure pathways (Islam & Islam 2020). Despite the prevalence of these issues, a comprehensive understanding of the health risks associated with various exposure pathways remains limited.

A quantitative microbial risk assessment (QMRA) is a probabilistic modeling technique employed to evaluate health risks from exposure to waterborne pathogens. The QMRA is essential for assessing risks associated with the use of surface waters for activities such as swimming or recreation, particularly in communities frequently exposed to contaminated water. Previous QMRA studies have typically focused on single exposure pathways, such as drinking water (Petterson & Ashbolt 2016), reclaimed water (Zaneti et al. 2013), recreational waters (Girardi et al. 2019), irrigation water (Ezzat 2020), and sewage (Kozak et al. 2020). In Bangladesh, studies have similarly concentrated on single pathways, such as open drain water (Foster et al. 2021), drinking water production (Islam 2024), and river bathing (Islam & Islam 2020). A few studies have explored multiple pathways of fecal contamination (e.g., Wang et al. 2018, 2022; Raj et al. 2020), but, to date, no comprehensive QMRA study has quantified health risks from waterborne pathogens across multiple urban exposure pathways globally.

This study aims to fill this gap by quantifying the risks associated with waterborne pathogens through various exposure pathways in Dhaka city. By providing a comprehensive assessment of health risks from exposure to contaminated surface waters, this research will offer critical insights into the broader health impacts of waterborne pathogens in urban settings. Understanding these risks is crucial for developing effective interventions to safeguard public health and reduce the burden of waterborne diseases.

The QMRA was conducted using the common framework hazard identification, exposure assessment, dose–response analysis, and risk assessment.

In this study, the primary hazard identified is the presence of waterborne pathogens in source waters. We focused on three specific pathogens: V. cholerae, norovirus Genogroup-II (NoV-GII), and Giardia, utilizing mean concentration data (Table 1) from a previous study conducted by Amin et al. (2020) in Mirpur, Dhaka. These mean values were derived from extensive field measurements, including water samples (n = 151) collected from surface water, open drains, and floodwater across multiple locations and seasons. Pathogen concentrations were quantified using Polymerase Chain Reaction (PCR)-based methods and culture techniques. V. cholerae was detected through selective enrichment and PCR assays, while NoV-GII and Giardia were analyzed using Reverse Transcription Polymerase Chain Reaction (RT-PCR) and microscopy, respectively. The original study (Amin et al. 2020) followed standard laboratory protocols to ensure reliable pathogen detection and data quality.

V. cholerae was the most frequently detected pathogen in the water samples (100%), followed by NoV-GII (67%) and Giardia (50%). Giardia was more prevalent during the wet season, whereas NoV-GII was more commonly detected in the dry season. The geometric mean concentration of V. cholerae was generally higher in water sources compared to Giardia and NoV-GII (Table 1). Detailed descriptions of the methods and findings are provided in Amin et al. (2020). This robust dataset provides a solid foundation for our risk assessment, and the use of secondary data is justified due to the availability of these measured concentrations from the Mirpur study site.

The selected three reference pathogens represent one for each of the three pathogen groups V. cholerae (bacteria), NoV-GII (virus), and Giardia (protozoa). These pathogens are particularly relevant as previous studies have documented significant prevalence rates of infection among diarrheal patients admitted to hospitals in Dhaka: approximately 16% for NoV-GII (Rahman et al. 2016), 12% for V. cholerae (Das et al. 2013), and 8% for Giardia (Haque et al. 2005).

The SaniPath Exposure Assessment Tool survey (Raj et al. 2020; Wang et al. 2022) data were utilized to evaluate human exposure to fecal contamination in Dhaka city. This assessment was conducted by the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR,B) in collaboration with Emory University, Georgia, USA, in 2017. The objective was to quantify the ingestion of fecal contaminants by populations – both adults and children – living in urban environments through various exposure pathways. Understanding these pathways is critical for comprehending pathogen transmission within Dhaka's urban setting, which faces significant sanitation and water quality challenges. Considering the availability of measured pathogen concentration data in Dhaka city, the exposure pathways included in this study are surface water, open drain water, and flood water. Surface water encompasses rivers, canals, and ponds where residents engage in activities such as fishing, bathing, and washing clothes or utensils. Open drain water refers to water from open channels that carry sewage, rainwater, or flood water. Flood water is defined as stagnant water remaining in the neighborhood for at least 1 h.

The SaniPath tool provided data on monthly exposure levels for both children and adults by calculating exposure based on behavioral patterns. Respondents were asked about the frequency of direct contact with open drain or flood water and their visits to surface water bodies for bathing or other household activities over the previous month. The study assessed microbial infection risk based on both the volume of water ingested and direct contact (or via an object). Specifically, the average volumes of water ingested per individual during recreational swimming or bathing, or through contact with open drains or flood water, were measured. The ingestion rates used were 3.7 mL per bathing event for adults (≥18 years, 62% adults of the total population) and 20.4 mL for children (5–17 years, 24%), while, for open drain and flood water, they were 0.06 mL for adults and 1.0 mL for children according to the SaniPath assessment data. Children aged below 5 years are excluded, assuming that they are usually not exposed to the selected pathways.

To assess the annual disease burden, the annual risk of illness was calculated by multiplying the annual risk of infection (P_annual) by the probability of illness. For V. cholerae, NoV-GII, and Giardia, the probabilities of illness were 0.53, 0.55, and 0.40, respectively, as reported by Foster et al. (2021). The number of illness cases was then determined by multiplying the incidence rate by the total population, including both adults and children.

The overall disease burden was quantified in terms of disability-adjusted life years (DALYs). To convert illness cases into DALYs, the analysis involved calculating both the years of life lost and the years lived with disability associated with each disease. For cholera, norovirus disease, and giardiasis, the DALY per case values used were 0.05, 0.02, and 0.08, respectively. These values are representative of developing country contexts and were derived from previous studies (Hotez & Bundy 2017; Vos et al. 2017; Reyes 2022).

To evaluate how varying assumptions and parameters affect the risk of illness and overall disease burden, a sensitivity analysis was conducted. This analysis explored the impact of several key variables, including the volume of water ingested per exposure event, pathogen concentrations in different water sources, and exposure frequencies. Specifically, the sensitivity tests involved increasing and decreasing the volume of water ingested by 25 and 50% for both children and adults. The resultant changes in the risk of infection per exposure event were then calculated for each water source and pathogen. The variations in pathogen concentrations were tested by adjusting the levels up and down by 50 and 100%. The analysis also explored the impact of different exposure frequencies on the annual risk of illness by increasing and decreasing exposure frequencies by 25 and 50%. The impact of these variations on the annual risk of illness and total annual cases was then calculated for each pathogen and water source.

The estimated total annual DALYs are approximately 2,441.4 among the 4,792 exposed people. This total includes cholera, 813.8 cases (33% of the total), norovirus gastroenteritis 325.5 cases (13.3%), and giardiasis 1,302.1 cases (53.3%). The estimated DALYs per person per year (pppy) for this population are approximately 0.169, 0.068, and 0.272 for cholera, norovirus gastroenteritis, and giardiasis, respectively.

The analysis also explored how different exposure frequencies impact the annual risk of illness. By adjusting the frequency of exposure events throughout the year, we assessed how changes in this parameter influenced the total annual cases of illness. The results demonstrated that increasing the frequency of exposure events led to a proportional increase in the annual risk of illness for each water source. This finding reinforces the observation that more frequent exposures contribute significantly to the overall disease burden.

A strong positive correlation of 0.83 was observed between the risk of illness and the volume of water ingested. Additionally, there was a notable positive correlation of 0.78 between the risk of illness and the dose–response parameters. The correlation between the risk of illness and the frequency of exposure events was also positive at 0.74.

The results from this study underscore the differential risks associated with exposure to various water sources and highlight significant variations in infection risk between children and adults. Our findings indicate that the risk of infection per exposure event is notably higher in children compared to adults. This increased risk is primarily due to the larger volume of water ingested by children during exposure events, which aligns with previous research highlighting children's greater susceptibility to waterborne diseases due to higher water ingestion rates (Fuhrimann et al. 2016; Islam & Islam 2020). The increased ingestion volume amplifies the likelihood of pathogen exposure and infection, particularly in settings with poor water quality.

Importantly, children are generally more vulnerable to the impacts of waterborne diseases compared to adults due to physiological factors such as lower immunity and underdeveloped immune systems. This makes them more susceptible to severe health outcomes from infections, including dehydration and malnutrition, which can have long-lasting effects on their health and development. While the study primarily focuses on exposure levels, these additional health risks should be considered when interpreting the results. Children's smaller body size and higher water ingestion rates further contribute to the elevated risk of illness, underscoring the critical need for interventions that protect this vulnerable population.

The comparative analysis of infection risk across different water sources reveals that individuals exposed to surface water experience a higher risk of infection than those exposed to open drains or flood water. This is primarily due to the higher frequency of exposure events and the greater volume of water ingested from surface water sources, which correlates with higher pathogen concentrations. Interestingly, when equal volumes of water are ingested from open drains and floodwater, the infection risk between these pathways becomes comparable. This finding suggests that, while open drains and floodwater may carry pathogens, the relatively lower infection risk associated with these sources, compared to surface water, can be explained by variations in the initial concentrations of pathogens present in these water sources. The study by Payment & Franco (2015) have similarly reported variations in pathogen loads across different water sources, highlighting the influence of initial contamination levels on infection risks.

The analysis reveals that the annual risk of illness, once infected, is higher in children compared to adults. This aligns with the increased frequency of exposure events and the cumulative health impact on younger populations, as noted in other research (Fuhrimann et al. 2016; Girardi et al. 2019; Islam & Islam 2020). Our study also found that the annual risk of illness increased with the frequency of exposure events for each water source. This finding underscores the cumulative effect of repeated exposure on the overall risk of illness, consistent with findings by Fewtrell et al. (2005), who demonstrated that frequent exposure to contaminated water significantly heightens disease risk.

The estimated total annual cases of cholera, norovirus gastroenteritis, and giardiasis in the study area – totaling 16,276 cases – highlight the substantial burden of waterborne diseases. Surface water was identified as the primary contributor to this burden, accounting for 37% of the total cases, followed by open drain water (34%) and floodwater (29%). These proportions reflect the predominant role of surface water in the observed health impact, aligning with other studies that have identified surface water as a critical factor in the transmission of waterborne pathogens (Girardi et al. 2019; Islam & Islam 2020; Kozak et al. 2020).

The QMRA estimated a burden of 2,441.4 DALYs per year due to exposure to wastewater among 4,792 exposed individuals in a low-income neighborhood in urban Dhaka. The estimated DALYs pppy for this population are notably high, particularly for cholera (0.169) and giardiasis (0.272). While these figures illustrate a significant health burden, it is important to recognize that the specific thresholds for DALYs pppy due to cholera and giardiasis can vary depending on regional health goals and available resources. Generally, public health objectives aim to minimize the DALY burden to levels as low as possible. For instance, a commonly recommended target is to achieve and maintain DALY levels below 0.1 per person per year, reflecting a comprehensive public health effort to reduce disease impact (WHO 2012; IHME 2021). Efforts to reduce these DALY values should focus on improving water, sanitation, and hygiene (WASH) infrastructure, enhancing disease surveillance, and implementing effective public health interventions to mitigate the risks associated with wastewater exposure.

The sensitivity analysis reveals critical insights into the factors influencing the risk of illness and disease burden. Variations in key parameters, such as the volume of water ingested and pathogen concentrations, significantly impact the estimated risks, which is in agreement with previous studies (Eregno et al. 2016; Islam & Islam 2020). Increasing the assumed volume of ingested water per exposure event led to a higher risk of illness, especially for surface water, highlighting the sensitivity of risk estimates to ingestion assumptions. Conversely, reduced ingestion volumes resulted in lower risks, underscoring the necessity for precise estimates in risk assessments. Additionally, higher pathogen concentrations in surface water were associated with an increased risk of illness, reinforcing the finding that surface water poses a higher risk compared to open drains and floodwater. The analysis also demonstrated that more frequent exposure events directly correlate with a higher annual risk of illness, emphasizing the substantial role of exposure frequency in determining the overall disease burden. These findings highlight the importance of accurately accounting for ingestion volumes, pathogen loads, and exposure frequencies in assessing and managing waterborne disease risks.

The study findings emphasize the need for targeted public health interventions to reduce the health burden associated with waterborne diseases. Improving water quality and sanitation infrastructure, particularly in areas with high exposure to surface water, should be a priority. Educational campaigns aimed at reducing water ingestion among children and promoting safe water practices can also mitigate risks. Additionally, interventions should be tailored to address the higher vulnerability of children, who are at greater risk due to their higher exposure volumes.

This study underscores the critical link between exposure to untreated surface water, open drain water, and floodwater with the heightened risk of waterborne diseases in low-income urban areas. The QMRA reveals a significant public health burden, primarily affecting children, who are particularly at risk due to low immunity and increased vulnerability to severe health outcomes. The findings highlight that untreated surface water, open drains, and floodwater contribute substantially to the incidence of gastrointestinal illnesses, with surface water accounting for the highest proportion of disease cases. The analysis estimates 16,276 disease episodes and a disease burden of 2,441.4 DALYs annually. Notably, giardiasis, cholera, and norovirus gastroenteritis contribute significantly to this burden, with giardiasis being the most severe. The results call for urgent and targeted public health interventions. The findings also emphasize the need for increased focus on sanitation interventions tailored to urban settings. Given the limited research on the health impacts of such interventions in similar contexts, future studies should aim to assess the effectiveness of different sanitation measures in mitigating waterborne disease risks. By implementing comprehensive water and sanitation strategies, we can significantly reduce the burden of waterborne diseases and improve overall public health outcomes.

This study does not involve personal data or information requiring ethical approval.

This research was conducted without external funding or financial support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.