Quantifying public health risks from exposure to waterborne pathogens during river bathing as a basis for reduction of disease burden M. M. Majedul Islam and Md. Atikul Islam

A Quantitative Microbial Risk Assessment (QMRA) technique was applied to assess the public health risk from exposure to infectious microorganisms at bathing areas of three rivers in Bangladesh. The QMRA assessed the probability of illness due to the accidental ingestion of river water impacted by untreated sewage. The simplified QMRA was based on average concentrations of four reference pathogens Escherichia coli (E. coli) O157:H7, Cryptosporidium spp, norovirus and rotavirus relative to indicator bacterium E. coli. Public health risk was estimated as the probability of infection and illness from a single exposure of bathers. The risks of illness were ranged from 7 to 10% for E. coli O157:H7, 13 to 19% for Cryptosporidium, 7 to 10% for norovirus and 12 to 17% for rotavirus. The overall risk of illness at the rivers was slightly higher in children (9–19%) compared to adults (7–16%). The risks of illness in individuals exposed to the river bathing were unacceptably high, exceeding the USEPA acceptable risk of 3–6 illnesses per hundred bathing events. This study gives a basis for reducing the burden of disease in the population by applying appropriate risk management. Findings and methods of this study will be helpful for other countries with similar socio-economic and geographic settings.

. This contaminated river water is widely used, which increase the people's vulnerability to waterborne diseases. In Bangladesh, for example, diarrheal disease is very common and every year it causes around a hundred thousand deaths (Faruque ). QMRA is a tool that has been developed for assessing the risk of disease from a specific pathogen. For QMRA, the main factors required are pathogen concentration in surface water, a pathogen-specific dose-response analysis and risk characterization. A QMRA for every pathogen would be time-consuming and the necessary information is currently not available for many pathogens. To overcome this difficulty, WHO () recommended using 'reference pathogens', with similar nature.
The scarcity of pathogen data in developing countries requires that a QMRA can be based on faecal indicator bacteria (Dias et al. ). Despite the weaknesses of using indicator organisms, many initial QMRAs will have to be performed using data on indicator organisms due to inadequate data for the occurrence of pathogens. The use of indicator organisms does, however, require assumptions to be made about the relationship between pathogens and indicators that introduces an additional level of uncertainty into the risk assessment (Howard et al. ).
There are challenges to applying the method particularly in developing countries where data and resources are very limited. In the 3rd edition of its Guidelines for Drinking-Water Quality (), WHO promotes the use of risk assessment coupled with risk management for the control of water safety in water sources. QMRA is a technique to estimate the disease burden from waterborne pathogens using the level of pathogen or an appropriate surrogate. This information may then be used to formulate appropriate management strategies of the water supply system. Although QMRA has been applied in developed countries, the technique has not been evaluated in developing regions where relevant data are very limited. So far, one study focused on modelling Dhaka city flooding and cholera risk assessment (Mark et al. ), but no such QMRA study has been performed for surface waters in Bangladesh.
In this study, we aim to apply a simplified risk assessment procedure to calculate the probability of illness from four reference pathogens (pathogenic Escherichia coli (E. where waterborne diseases are endemic (Islam et al. b). The data on pathogens were collected from literature, because the laboratory facilities to measure a variety of viruses are not available in Bangladesh. If QMRA is to gain acceptance and be used in developing countries, it must be offered in a workable and simple form. This is the first QMRA study in Bangladesh that provides useful information for river water quality management in the country.
The study will inform policy makers and help to identify interventions to reduce the burden of disease on the population. The study demonstrates the potential of a simplified QMRA as a tool in the context of a developing country with limited data to evaluate public health risk and regulate river bathing.

Study area
The study sites are located in the Rupsha and Bhairab rivers near Khulna city and the Betna river near Satkhira town.
Water samples were obtained from several sewer drains and the selected three rivers to measure concentrations of E. coli. Sampling was done bi-weekly during a wet season After incubation at room temperature on the urea substrate, yellow, yellow-green, or yellow-brown colonies were counted as E. coli.

Quantitative microbial risk assessment
To estimate the health impact of river water utilization, a simplified QMRA was conducted using the example of the Rupsha, Bhairab and Betna rivers in the southwest of Bangladesh. The probability of illness was estimated as a person contracting illness from the direct contact of contaminated river water during bathing or fishing. The probability of illness was estimated separately for sewage sources (for the Rupsha and the Bhairab rivers) and mixed sources (for the Betna river). For QMRA, the main factors required are pathogen concentration in surface water, a pathogen-specific dose-response relationship and exposure assessment (the consumed amount of water) to forecast the probability of infection due to exposure to waterborne pathogens in river surface water. This QMRA approach assumes that this indicator bacteria density has the probability to result in illness rates of 0.01-0.02 (1-2 illnesses per hundred bathing events). For QMRA, the following steps are described briefly: (i) identifying reference pathogens (hazard identification); (ii) exposure assessment; (iii) dose-response relationships; and (iv) risk characterization.

Reference pathogens
The QMRA has been estimated as the disease burden from four reference pathogens: E. coli O157:H7, Cryptosporidium

Dose-response equation
The reference pathogen (rp) doses were derived from the concentration of E. coli in the water from a specific source as described by Schoen & Ashbolt (). The calculation of the pathogen dose is based on the observed ranges of pathogen and faecal indicator densities in faecal waste.
The dose μ s rp of each reference pathogen in the units of cfu, genomes or (oo)cysts, from each source (S) was calculated as: where S is the faecal contamination source; C EC is the average concentrations of E. coli (cfu/100 mL) in a river water; F S EC is the density of E. coli in sewage (cfu/L); R S rp is the density of pathogen species in sewage (number of pathogens or genomes L À1 ); p S rp is the fraction of infectious pathogenic strains from source S; and V is the volume of water ingested (mL).
A detailed literature search was conducted to find appropriate values for each of the model parameters shown in Equation (1).

Risk characterization
Risk  (1). The total probability of illness from a specific source was calculated using the probability of illness from each source-specific reference pathogen as P S ill ¼ 1 À Q rp (1 À P S illrp ).

Quantifying probabilities of GI illness
The probability of microbial infection and illness from river bathing was calculated using the dose input presented in for Cryptosporidium as shown in Figure 2. The estimated daily risks of infection were varied with the pathogens ingested and exposure scenarios.
The resulting probabilities of illness varied between 7% and 17% for viruses, between 7% and 10% for E. coli O157: H7 and between 13% and 19% for Cryptosporidium ( Figure 3). The highest risk of illness was observed due to Cryptosporidium (Figure 3). Whereas E. coli O157:H7 was found to cause a higher rate of infection in all the three rivers ( Figure 2). The overall risk of illness for a single exposure of pathogens during bathing was substantially higher for children (9-19%), compared to adult (7-16%).
The risk of infection and illness (both in the adults and children) was slightly higher (1-3%) during the wet season, compared to the dry season. No substantial differences in the probabilities of gastrointestinal infection and illness were observed among the rivers.

Uncertainty and sensitivity analysis
Uncertainty analysis is an important part of the risk charac-

DISCUSSION
The risk of GI to bathers from waterborne pathogens at different rivers was estimated using a QMRA technique.
According to the European Commission () and USEPA (), the acceptable risk of illness is 3-5% and 3-6%, respectively. The present study results show that the   we found that E. coli concentrations became very high after a heavy rainfall event and decreased after three to four days.
Therefore, we suggest that swimming in the rivers should be avoided for at least four days after a heavy rainfall event until adequate wastewater treatments are not ensured. Many people swim/bathe and fish in the selected rivers.
In this study, the health risks have been assessed for the utilization of water for swimming or bathing only. However, the rivers are also extensively utilized for domestic purposes (e.g. washing cloths and utensils), fish farming and irrigation.  Since the water of these rivers is not treated at all, people use water from the studied sites may be exposed to unaccep-