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Indicators for Waterborne Pathogens 2 Health Effects Assessment INTRODUCTION The foremost goal of developing and using indicators for waterborne pathogens is public health protection. This chapter provides an overview of health effects assessment for waterborne pathogens and their indicators, and includes a brief review of surveillance and epidemiologic study designs, an historical review and current status of health effects assessment, and a detailed discussion of quantitative microbial risk assessment. Furthermore, health effects assessment is discussed throughout this chapter in the context of drinking water and of fresh and marine recreational waters. This chapter also includes a description of the national surveillance system for waterborne disease outbreaks and several related epidemiologic studies currently being conducted. The final section is a summary of the chapter and its conclusions and recommendations. This chapter is not intended to serve as a comprehensive review of epidemiology as a methodologic tool or waterborne disease in humans, both of which are beyond the scope of this report. Rather, it provides some substantive background information on epidemiology and health effects assessment within the overall context of indicators for waterborne pathogens as discussed throughout this report and especially in support of a phased approach to microbial water quality monitoring that is provided in Chapter 6. Approaches to Health Effects Assessments Health effects assessments for waterborne pathogens can be based on a number of approaches, all of which have been used to document and quantify the
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Indicators for Waterborne Pathogens health risks resulting from microorganisms in water. These approaches include (1) assessments of epidemiologic evidence for waterborne-based outbreaks; (2) human volunteer studies showing that a known or potential waterborne pathogen is infectious by the oral ingestion route and capable of causing infection and disease at particular doses (dose-response studies); (3) various types of retrospective and prospective epidemiologic studies for health effects assessments; (4) estimates of health risks by linking epidemiologic evidence for disease to measured concentrations of either pathogens or indicators in the water; (5) estimates of the ratios of pathogens to indicators in the exposure vehicle (e.g., feces, sewage, fecally contaminated water); and (6) quantitative microbial risk assessments that integrate human exposure and health effects data for quantitative risk estimations or characterizations. Health Effects Concerns and Early Studies of Microbial Water Quality Outbreak Investigations and Risk Estimates from Pathogen-to-Indicator Ratios in Water As discussed in Chapter 1, concerns about the sanitary quality of drinking water and the risks of enteric infectious diseases in the United States go back to at least the late 1800s, when enteric disease outbreaks were first recognized and linked at least tentatively to these exposure routes. Similar concerns for U.S. recreational and shellfish waters started in the 1920s. The initial recognition of and concern about infectious disease risks from these sources of exposure focused on enteric bacterial diseases, and early health effects assessments of enteric bacterial pathogens and waterborne outbreaks date back to the early 1920s. Human health risks from enteric viruses and parasites in water were first recognized and addressed during and shortly after World War II. However, civilian risks from these waterborne pathogens were not widely documented and appreciated until studies of waterborne outbreaks and waterborne pathogen occurrence were first reported in the 1960s. The recognized viral and protozoan pathogens of initial concern were infectious hepatitis viruses, polio, and other enteroviruses, and Entamoeba histolytica and Giardia lamblia, respectively. Perhaps the first attempts at linking health effects assessments of waterborne pathogens to microbial water quality were based on ratios of Salmonella typhi to fecal indicator (coliform) bacteria in feces and sewage and the allowable limits of coliforms in drinking water and, later, in recreational bathing and shellfish waters (Kehr and Butterfield, 1943; Prescott et al., 1945). Early Health Effects Assessments of Enteric Pathogens from Human Dose-Response Studies The first human health effects dose-response studies appear to be with the
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Indicators for Waterborne Pathogens protozoan parasites Entamoeba histolytica and other Entamoeba species conducted using enemy prisoners in the Philippines by U.S. Army medical officers early in the twentieth century (Walker and Sellards, 1913). This study also showed for the first time that E. histolytica caused dysentery, that the cysts and trophozoites (see also Chapter 3) were different forms of the same microorganism, and that other Entamoeba species (notably Entamoeba coli) did not cause dysentery or other enteric disease. It was not until the 1950s that a researcher attempted to determine the number of enteric parasites necessary to cause infection in human dose-response studies with Entamoeba coli (Rentdorff, 1954a) and Giardia lamblia (Rentdorff, 1954b). Dose-response data on bacterial infectivity from human volunteer studies date back to at least the 1940s when different doses of Shigella paradysenteriae (now S. flexneri) were administered in vaccine trials (Shaughnessy et al., 1946). For enteric viruses, the first human volunteer studies were with infectious hepatitis viruses during and after World War II (Cameron, 1943; Lainer, 1940; MacCallum and Bradley, 1944; Voegt, 1940). Studies by MacCallum and Bradley’s group are considered the first to distinguish infectious from serum hepatitis. Some of these early studies provided the first dose-response data for infectious hepatitis, but lack of knowledge about actual virus concentrations in the inocula has hampered the use of these data and subsequent infectious hepatitis human volunteer dose-response data for health risk assessments. Studies to estimate dose-response for virus infectivity were conducted using candidate live oral poliovirus vaccines in the 1950s (Koprowski, 1956; Sabin, 1955) and 1960s (Katz and Plotkin, 1967), and infectivity for humans could be related to virus concentrations as measured by other methods. Mathematical modeling of the data sets, taking into account the number of subjects used at each dose and the sensitivity of the dose-response study, was not undertaken until the 1980s (Haas, 1983b). However, many data sets, along with the best-fit models for bacteria, protozoa, and viruses, have since been compiled (Haas et al., 1999b). Prospective Epidemiologic Studies of Microbial Water Quality and Health Effects Prospective epidemiologic studies have attempted to link health effects in exposed individuals to the microbial quality of water. This approach has been used primarily for recreational waters and dates back to studies by Stevenson (1953) on Lake Michigan and the Ohio River. Those studies reported epidemiologically detectable health effects in bathers from waters containing about 2,300-2,400 total coliforms per 100 mL. Based on several lines of evidence—including outbreak data, the ratios of Salmonella typhi to fecal indicator (coliform) bacteria in feces and sewage, and epidemiologic studies of enteric illness in bathers at beaches having different levels of fecal contamination—the U.S. Public Health Service (USPHS) and later the Federal Water Pollution Control Association
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Indicators for Waterborne Pathogens (FWPCA; predecessor to the U.S. Environmental Protection Agency) developed bacteriological quality guidelines for recreational waters, as noted in Chapter 1 (Cox, 1951; NTAC, 1968; Scott, 1951). Public Health Risk Assessment Framework The ultimate objective of determining the microbiological quality of water is to identify and then minimize the public health risk from consuming water intended for drinking and from exposure to recreational water. Data are used to develop approaches to remediate or control this public health risk by reducing the potential exposure to levels that are considered acceptable (e.g., by controlling contamination sources) or developing communication strategies to prevent exposure (e.g., by closing a beach). Indicators are measured for many purposes (see Chapter 4 for a detailed discussion of indicator applications). In terms of public health protection, indicators for pathogens in water intended for drinking are measured to determine the level of microbial contamination of source water (see also Chapter 6), whether existing water treatment processes are adequate, and whether the integrity of the distribution system has been breached. In addition, indicators can be used to measure the quality of the water in unregulated private wells. The measurement of indicators in the recreational water setting is typically conducted to determine if the level of contamination of surface waters such as oceans, lakes, and rivers is sufficiently elevated to pose a human risk and, therefore, to determine whether warnings should be issued or recreational waters should be closed to the public. In drinking water and food, philosophically a zero-tolerance approach has been taken for indicators. Thus, it is presumed that if a measured indicator concentration is zero through water protection and treatment, the health risk is also zero. However, this traditional strategy does not provide an effective framework for decision making in the context of what is currently known about indicators. All ambient waters (including groundwater) will be subject to some level of microbial indicators and contamination whether associated with fecal sources (both human and nonhuman) or with naturally occurring microorganisms. Thus, the regulatory question remains, What measurable microorganisms in water best represent a risk to human health and at what levels would they be of concern? Such criteria and standards can be established by determining two relationships: (1) between the density of the indicators and the occurrence of adverse health outcomes, and (2) between the density of the indicators and the presence of pathogenic organisms in the water. Although the association between the occurrence of a pathogen (or its indicator) in water and a human health effect is a difficult one to determine, epidemiologic studies, surveillance, and risk assessment are useful tools to help establish this association. Risk assessment is a process that allows for the integration of scientific data
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Indicators for Waterborne Pathogens regarding an environmental hazard into a framework that addresses the risk of exposure and its potential health impacts (NRC, 1983). The process is quantitative in nature and attempts to address both the nature and the magnitude of the risk. This process has proven invaluable to the regulatory community, industry, and risk managers and has direct application to public health risk from water. The value of such a framework is that many different types of information—various indicator data, epidemiologic data, and data specific to the nature of the exposure (e.g., recreational or irrigation waters versus source of potable water supply)—can be used to define public health safety goals. In ambient recreational waters, there is a need to understand the nature and level of the risk and, therefore, to take a risk assessment approach. In the risk assessment process, hazard identification has traditionally been separated from exposure analysis. In this case, the nature of the microbial hazard and its identification are closely tied to the sources and fate of the pathogenic microorganisms and, thus, the exposure. For example, enteric viruses detected at beaches can be tied to human fecal inputs and the ability of the virus to survive and cause illness at low doses and concentrations. Many attributes of indicators (see Chapter 4 for further information) and indicator methods that may lend themselves to the risk assessment process are currently available (e.g., identifying sources of microbial contamination), whereas other attributes will be difficult to determine and may not prove feasible (e.g., establishing a quantitative relationship between concentrations of indicators and the degree of public health risk). As in most science-based evaluations, uncertainty will have to be described, and quantifying uncertainty is most problematic in the exposure portion of the analysis where indicators are used to estimate the potential for exposure to actual pathogens. A microbial risk framework can be developed and used to understand the basic principles and data gaps in the study of public health risks associated with the characterization of recreational water quality using a variety of methodologies. Such an approach will lead to a decision support system for data gathering (types of data and methods) and for response and mitigation efforts. SURVEILLANCE AND EPIDEMIOLOGIC STUDY DESIGNS As noted previously, health outcomes can be linked to exposure data by various epidemiologic methods. A brief overview of these methods follows. Various introductory and advanced textbooks, as well as review articles on epidemiologic methods, can be consulted for more comprehensive coverage of this topic and for detailed definitions of various epidemiologic terms used in this chapter and report (e.g., Gordis, 2000; Last, 2001; Lavori and Kelsey, 2002; Matthews, 2000; Meinert, 1986; Rothman and Greenland, 1998; Rothman, 2002; Schlesselman, 1982).
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Indicators for Waterborne Pathogens Surveillance Modern public health surveillance of disease was defined by Langmuir (1963) as “the continued watchfulness over the distribution and trends of incidence through the systematic collection, consolidation and evaluation of morbidity and mortality reports and other relevant data.” It is now standard practice to add to this definition the concept of applying these data to prevention and control of disease. The steps in surveillance include the systematic collection of data, analyses to produce statistics, interpretation to provide information in a timely manner, actions taken as a result of the data, and continued surveillance to evaluate the success of the actions taken. Guidelines for evaluating surveillance systems have been proposed by the Centers for Disease Control and Prevention (CDC, 2001). Epidemiologic Study Designs Epidemiologic studies fall into two general categories: (1) experimental studies (e.g., randomized controlled intervention or clinical trials) in which investigators control the conditions of exposure in the study and (2) observational studies (e.g., cohort, case-control, cross-sectional, and ecologic studies) in which investigators do not control the exposure or most other aspects of the process being studied. Of the epidemiologic studies, randomized controlled trials provide the strongest epidemiologic evidence of an etiologic association between exposure and outcome, followed in decreasing order by cohort studies, case-control studies, cross-sectional studies, and ecologic studies. Randomized Controlled Trial This epidemiologic experimental design is regarded as the most scientifically rigorous method of hypothesis testing available. Subjects are randomly allocated into two groups, one that will receive an experimental treatment or intervention and the other that will not. Randomization tends to produce comparability between the two groups with respect to factors that might affect the health outcome being studied and, thus, to minimize the potential for confounding variables.1 Additional objectivity is provided when subjects, investigators, and statisticians analyzing the data are unaware of the subject’s allocation to a particular treatment or intervention (known as randomized triple-blinded trials). The scien- 1 Confounding variables (“confounders”) are variables that can alternatively cause or prevent the outcome of interest in an epidemiologic study and are associated with the factor under investigation. As such, confounding variables may be due to chance or bias, and unless adjusted for, their effects cannot be distinguished from those factors being studied.
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Indicators for Waterborne Pathogens tific rigor of this study design is its chief advantage, while its cost, often in the millions of dollars, is its greatest disadvantage. Therefore, randomized controlled trials are generally used only when a well-defined hypothesis is being tested. Randomized controlled trials have additional benefits. They provide a temporal association between the exposure and the health outcome, which is one measure of causality, because the exposure precedes the outcome. They also allow for the calculation of incidence rates of disease in each group (i.e., the occurrence of a certain disease or health outcome in a group or population over a specified period of time) and their relative risk rather than being limited to calculations of odds ratios. As defined in A Dictionary of Epidemiology, edited by Last (2001), a relative risk (RR) or risk ratio is “the ratio of two risks, usually exposed/ not exposed.” The odds ratio (OR) is defined differently according to the situation (e.g., calculation of odds of exposure or odds of disease). As defined in Last (2001), “The exposure-odds ratio for a set of case-control data is the ratio of the odds in favor of exposure among the cases to the odds in favor of exposure among non-cases” and “the disease-odds ratio for a cohort or cross-sectional study is the ratio of the odds in favor of disease among the exposed to the odds in favor of disease among the unexposed.” Problems associated with randomized control trials include noncompliance, participant dropout, and generalizability of results. A related type of experimental design is a community intervention trial in which the exposure is assigned to groups of people rather than singly. This type of experiment is often used to study environmental exposures. Most community intervention trials do not employ random assignment for the experimental treatment; rather, they use a cross-over design (i.e., before and after treatment) where the community serves as its own control. Cohort Study A cohort is defined as a group of persons who are followed over a period of time and usually includes individuals with a common exposure. A cohort study involves measuring the occurrence of disease within one or more cohorts that have differing exposures during a certain period of follow-up. John Snow’s study of the cholera outbreak in 1854 (see Chapter 1 for a brief description; Snow 1854) is an elegant example of a cohort study. Cohort studies can be prospective (exposure information is recorded at the beginning of the follow-up and the period of time at risk is forward in time) or retrospective (cohorts are identified from recorded information and the follow-up time occurred before the beginning of the study). Cohort studies have several advantages. They allow for the association of multiple health outcomes or diseases, or multiple endpoints within the progression of one disease, along with the exposure of interest. Like randomized controlled trials, cohort studies provide a temporal association between exposure and health outcome. Finally, incidence rates of disease in the cohorts being assessed and their relative risk can also be calculated.
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Indicators for Waterborne Pathogens Cohort studies are subject to several types of potential bias, including bias in the selection of the cohorts’ exposure, bias in assessment of the health outcome, and bias if the two cohorts have differing response rates. A cohort design is generally selected when there is good evidence of an association of a health outcome with a certain exposure and when the exposure is relatively rare but the incidence of disease among the exposed group is high. Attrition of the study population is minimized when the time between exposure and disease is short. Although not as costly as a randomized experiment, cohort studies are generally more expensive that other types of epidemiologic designs. As with most epidemiologic studies, cohort studies are subject to confounding. Known confounding factors can be controlled for in the analysis of the data, but unknown confounders are by their nature impossible to adjust for in the analysis. Case-Control Study This type of study aims to achieve the same goals as the cohort study while minimizing the need to obtain information on exposure and outcome from large populations. Samples are taken from the source population to reduce the number of study participants. Properly designed and conducted, case-control studies provide information that is similar to what could be collected from a cohort study but at considerably less cost and time. In this study design, the investigator selects individuals with the health outcome of interest (cases) and appropriate individuals without the health outcome (controls), collects information regarding their past exposure, and then compares the rates of exposure of the two groups. Issues to be considered when using this study design include the ascertainment of cases of disease (e.g., diagnostic criteria, population source, incident or prevalent cases) and the selection of appropriate controls (i.e., should controls be comparable to cases in all respects other than having the disease, and how many variables between cases and controls should be matched?). A case-control study design is often used when investigators want to determine the association of a health outcome, especially a rare one, with multiple rather than single exposure factors. Case-control studies are subject to several biases, including recall bias (i.e., cases might be more likely to remember their past exposure than controls), selection bias, and nonresponse bias. Case-control studies suffer from the same problems with confounding factors as cohort studies. Another disadvantage of case-control studies is the inability to calculate incidence rates and their differences and ratios (e.g., relative risks); investigators can calculate only the ratio of incidence or prevalence rates or risks (e.g., odds ratios; see previous discussion of relative risk and odds ratio). Cross-Sectional Study This type of study provides a snapshot of the status of a target population
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Indicators for Waterborne Pathogens with regard to exposure status, health outcome, or both at a specific point in time. Cross-sectional studies attempt to enumerate the population and assess the prevalence of various characteristics. This design is characterized by the fact that only one set of observations is taken from each person. Although it cannot measure disease incidence because information across time is not available, disease prevalence can be assessed. Cross-sectional studies often are the first type of epidemiologic study conducted to determine the association between a health outcome and several possible exposure variables. As the hypothesis to be tested is refined, investigators typically progress to one of the other study designs. In some instances, however, cross-sectional surveys are conducted repeatedly over long periods as a form of disease surveillance system. Ecologic Study An ecologic study, also known as an aggregate study, compares groups rather than individuals. This design is most often used when individual-level data are missing. Although ecologic studies are relatively easy and inexpensive to conduct, their results are often difficult to interpret. Ecologic studies are used to study environmental exposures because it is difficult to accurately measure relevant exposures or doses at the individual level for large numbers of persons. In addition, exposure levels may vary little within a study area. The major limitation of ecologic analyses is the so-called ecologic bias in which the expected ecologic effect fails to accurately represent the biologic effect at the individual level. Robinson (1950) was the first to describe mathematically how ecologic associations could differ from the corresponding associations at the individual level. This phenomenon has become widely known as the ecologic fallacy. Other Types of Studies Some epidemiologic studies can be considered a specific type of the aforementioned study designs or use methods that incorporate multiple elements of these basic study designs. Longitudinal time series studies and seroprevalence studies are two such examples. Longitudinal time series are cohort-type epidemiologic studies that correlate exposure variables of interest (e.g., an environmental indicator such as water turbidity or the presence of a waterborne pathogen like Cryptosporidium parvum in water) with health outcomes (e.g., a clinical measurement such as an immunologic marker of exposure in a serologic specimen or a direct measurement such as occurrence of diarrhea) over a specified period of time. These studies incorporate temporal factors in their analyses with exposure occurring before the health outcome. The need to include the (most) appropriate time lag between measurement of the exposure and health outcome make these analyses complex and difficult to successfully accomplish as well as making interpretation of the results difficult at times. However, these studies can
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Indicators for Waterborne Pathogens provide good epidemiologic associations between exposures and adverse health outcomes. Seroprevalence studies are a specific type of cross-sectional study design. These studies measure the prevalence of a serologic marker in study participants as the health outcome of interest. Serologic markers can be difficult to interpret, however, because their measurements represent historical exposure and it is not always clear when and for how long the selected marker is present after exposure to a pathogen of concern. In addition, there may be multiple markers to choose from (e.g., circulating antibodies to several antigens), which further complicates interpretation of results. Nevertheless, seroprevalence studies are useful in determining population exposure to a pathogen even when the pathogen itself cannot be detected. Lastly, measuring seroprevalence is especially useful in investigations of waterborne disease outbreaks because it can establish that individuals were previously exposed and infected. HISTORICAL REVIEW AND CURRENT STATUS OF HEALTH EFFECTS ASSESSMENT The following section provides an overview (historical and current) of the most salient epidemiologic assessments of health effects associated with drinking water and recreational water exposure. Please refer to the previous section for a review of the surveillance and epidemiologic terms and methods used in the health assessments described below. U.S. National Waterborne Diseases Outbreak Surveillance System Surveillance for outbreaks associated with drinking water and recreational water has been going on since 1920 (Craun, 1986). The CDC, the U.S. Environmental Protection Agency (EPA), and the Council of State and Territorial Epidemiologists (CSTE) have maintained a collaborative surveillance system of waterborne disease outbreaks (WBDOs) since 1971 (see also Chapter 1). The National Waterborne Diseases Outbreak Surveillance System (WBDOSS), located at CDC, collects data regarding outbreaks associated with drinking water and recreational water. Moreover, in recent years (1999-2000), the WBDO surveillance system has also provided data on outbreaks that occurred as a result of occupational exposure to water. The primary objective of collecting outbreak data is ultimately to reduce the occurrence of WBDOs by characterizing the epidemiology of the outbreaks, identifying the etiologic agents, and determining the reasons for the occurrence. Results from these efforts provide the opportunity to issue public health prevention and control messages.
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Indicators for Waterborne Pathogens Characteristics of the Surveillance System State, territorial, and local public health agencies are responsible for detecting and investigating WBDOs. As a federal agency, CDC participates in outbreak investigations only by invitation from a state or territorial epidemiologist or if an outbreak involves multiple states. Reporting is voluntary and passive, and varies by state. States and territories report their outbreaks annually on a standard (hard copy) form (CDC Form 52.12), and CDC compiles, analyzes, and publishes the data. Since 1989, when responsibility for the surveillance system was moved to CDC’s Division of Parasitic Diseases, the data have been published every two years as a Morbidity and Mortality Weekly Report (MMWR) Surveillance Summary (Barwick et al., 2000; Herwaldt et al., 1991; Kramer et al., 1996; Lee et al., 2002; Levy et al., 1998; Moore et al., 1993). Both the surveillance system’s submitted hard copy report forms and the electronic database reside at CDC. Two major categories of data are reported on the forms: (1) epidemiologic data such as type of exposure; number of persons exposed, ill, and hospitalized; number of fatalities; symptoms; etiologic agent; and results from clinical laboratory data; and (2) environmental data such as the type of water system involved, results from water testing, and factors that contributed to contamination of the water. CDC contacts the state’s environmental agency if additional information regarding source water, treatment, or supply is needed to flesh out the investigation and to work through the issues that led to the contamination. Completion rates for the report forms vary tremendously by outbreak investigation, as well as by the intensity and scope of the investigation. Unlike most surveillance systems, the unit of analysis in WBDOSS is an outbreak rather than an individual case of a specific disease. Two major criteria must be met for an event to be classified as an outbreak: (1) at least two persons must have experienced a similar illness after consumption of a common source of drinking water or after exposure to water used for recreational purposes and (2) epidemiologic data must implicate water as the probable source of the illness. However, the stipulation that at least two persons be ill is waived for single cases of laboratory-confirmed primary amoebic meningoencephalitis (PAM; see also Chapter 3) and for single cases of chemical poisoning if water quality data indicate contamination by the chemical. An outbreak that meets both criteria will be included in the surveillance system whether the etiologic agent is infectious, chemical, or unidentified. WBDOs are classified (Class I-IV) according to the strength of the evidence implicating water. Epidemiologic data are weighted more heavily than water quality data, and outbreaks that are reported without supporting epidemiologic data are excluded from the surveillance system (see Table 2-1). In addition, each drinking water system associated with an outbreak is classified by the following types of problems: untreated surface water, untreated groundwater, treatment deficiency (e.g., inadequate disinfection), distribution
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Indicators for Waterborne Pathogens tailed knowledge such as flow and microbial quality about the sources of the pathogens as well as the incorporation of various decay processes (perhaps as influenced by temperature, turbidity, and sunlight) that affect the indicators and the pathogens during transport. Assessing other factors including dilution and accumulation in sediments may also be necessary. Future Directions for QMRA To date, most applications of QMRA have focused on the prediction of primary infections or illnesses resulting from exposure to a contaminated medium (water, food, etc.). However, it is clear that for at least some illnesses, significant impact results from secondary transmission (Kappus et al., 1982; Mac Kenzie et al., 1994; Morens et al., 1979). In recent years, mathematical models have been increasingly applied to understanding of disease transmission process—including the processes of secondary transmission and immunity (Eisenberg et al., 1996, 1998). However, such approaches require a variety of data that are not readily available, including intensity and duration of contagion, duration and strength of immunity, and so on. The increased use of such dynamic mathematical frameworks in a sensitivity analysis to help determine the potentially most influential parameters for which there are data gaps, and to use such knowledge in focused epidemiologic investigations to fill these gaps, appears to have merit. The models must be used to inform data gathering, as well as be informed by data. To fully understand long-term and endemic risks associated with certain waterborne illnesses, it will also be necessary to develop models that account for pathogen dynamics in nonhuman reservoirs and survival in water bodies. Feedback between modelers and experimentalists will also be needed to develop data necessary for better quantitative understanding of microbial risk. SUMMARY: CONCLUSIONS AND RECOMMENDATIONS Health effects assessments for waterborne pathogens can be based on a number of approaches. Each approach has strengths and weaknesses, and all have been or are being used to document and quantify the health risks of microbes in water. Epidemiologic methods are a well-established and essential tool for determining linkage between the presence of identified waterborne pathogens and their indicators and human disease. However, the significant cost and methodological difficulty of designing, conducting, and interpreting such studies have limited their use. The comprehensiveness of investigations of waterborne disease outbreak in the United States varies by the type of outbreak and by state, and results are compiled in CDC’s surveillance system. However, this system has low sensitivity and does not consistently provide information that links indicator and patho-
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Indicators for Waterborne Pathogens gen data with adverse health outcomes. This gap occurs because most outbreak investigations include primarily the epidemiologic component, which concentrates on linking illness to water and might include determination of the agent in clinical specimens, but tends to neglect the environmental component, which would include the determination of water quality through measurement of indicator and pathogen occurrence in water. This gap occurs more frequently with outbreaks associated with drinking water than with those associated with recreational water. In addition, 40-50 percent of identified outbreaks are of unknown etiology. Under the SDWA Amendments of 1996, recently completed (though largely unpublished at the time this report was finalized) epidemiologic studies of drinking water and endemic disease have focused on establishing associations between water consumption and gastrointestinal illness. Thus far, they have not established a good correlation between indicators of waterborne pathogens, the pathogens themselves, and adverse human health effects, although some earlier studies have shown an association between tap water and endemic gastrointestinal illness with attributable fractions ranging between 14-40 percent. To have adequate statistical power to address the epidemiologic association of health outcomes with specific indicators and specific waterborne pathogens, the study sample needs to be large, leading to significant costs. In addition, methodologic complexities as well as difficulty in interpretation of results have limited the use of some of the studies. In contrast, epidemiologic studies involving recreational bathing waters have shown predictive associations between several swimming-associated health effects and various microbial indicators or pathogens. A systematic review and meta-analysis of recreational waterborne studies (both freshwater and marine) confirmed that indicators can provide reliable estimates of water quality that are predictive of human health risks under some, but not all, water quality conditions, and the committee supports several conclusions provided in that study as related to this report. Under the BEACH Act, the recently initiated EPA study of midwestern and eastern freshwater beaches is commendable, but limited in scope to the study of point-source contamination and acute disease; it does not yet include western regions or ocean beaches. Knowledge is lacking about the epidemiology of chronic or recurrent illness (i.e., gastrointestinal, respiratory, dermatologic illnesses) associated with habitual users of recreational waters subject to point and nonpoint source microbial contamination, and knowledge of the epidemiology of disease outbreaks associated with use of tropical and subtropical recreational waters and ocean beaches is fragmentary. Quantitative microbiological risk assessment follows the traditional framework proposed for chemical risk assessment with several substantive differences. QMRA is a useful tool for identifying data gaps, especially models that include infectious disease parameters such as immunity. However, some of the key needs for QMRA are dose-response and exposure information (e.g., intensity and dura-
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Indicators for Waterborne Pathogens tion of contagion), which are often lacking. In some cases, impacts from such population level phenomena may dramatically alter projected estimates of human risk. Building on its conclusions, the committee makes several recommendations regarding future directions for epidemiologic and microbiological research as related to health effects assessment of waterborne pathogens and their indicators. The committee first recommends that EPA and CDC take a greater leadership role in such efforts, and fund and work with stakeholders and academic researchers in the following areas: CDC should actively work with state and local health departments to encourage testing for pathogens (especially viruses and parasites) in clinical specimens during waterborne outbreak investigations. Standardized laboratory methods for clinical specimens as well as water samples which are both sensitive and specific must be developed for many viruses. CDC and EPA should actively work with state and local health departments to encourage collection and testing of environmental data (i.e., water quality data for source, finished, and distribution system waters that include indicators and pathogens) during waterborne outbreak investigations. Standardized protocols and definitions are needed for outbreak investigations and epidemiologic studies, especially to help ensure a comprehensive investigation or study that includes the collection of clinical, laboratory, and environmental data (including co-occurrence of pathogens and indicators). Epidemiologic studies should be conducted to (1) assess the effectiveness and validity of newly developed indicators or indicator approaches for determining poor microbial water quality and (2) assess the effectiveness of the indicators or indicator approaches at preventing and reducing human disease. Fewer but more comprehensive epidemiologic studies should be conducted rather than multiple small-scale studies that do not adequately address multiple risk factors and health outcomes when working within a fixed or constrained budget. More specifically, the link between pathogens and their potential indicators, and among pathogens, indicators, and adverse health outcomes, would be strengthened by including in comprehensive and adequately funded studies, epidemiologic measurements of health outcomes, measurements of pathogens in clinical specimens, as well as measurements of pathogens and their potential indicators in relevant water samples. Additional epidemiologic studies are needed to look at the association between water consumption and gastrointestinal illness in groundwater systems, and to correlate water quality data (pathogens and indicators) with health outcomes. Furthermore, these studies should include the collection of epidemiologic, clinical, laboratory, and environmental data whenever feasible. Health outcomes studied in association with drinking water exposure
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Indicators for Waterborne Pathogens should not be limited to gastrointestinal illness (e.g., should consider respiratory and dermatological illnesses). The national estimate of waterborne disease should be expanded. Specifically, data have to be incorporated from sources other than randomized intervention trials and community trials (e.g., outbreaks, systematic reviews and meta-analyses, Cryptosporidium serologic data from NHANES, data from models derived from risk assessment). Additional epidemiologic studies should be conducted to determine the occurrence of chronic/recurrent disease attributable to waterborne pathogens in habitual users of recreational waters (e.g., surfers) from point and nonpoint sources of contamination. Studies of recreational waters should be carried out on a broader range of geographical and ecological sites, including tropical and subtropical waters and ocean beaches. Indicators being studied as part of the BEACH Act should not be limited to those than can yield results in two hours, as has been suggested. Since epidemiologic investigations are mandated as part of the BEACH Act, consistent scientific approaches should be used to monitor for various types of indicators as well as pathogens to establish dose-response relationships. Alternative sources to human volunteer data should be pursued to provide dose-response and exposure information for QMRA. Risk assessment with sensitivity analyses should be used to identify data gaps and help drive epidemiologic studies. REFERENCES Aragon, T.J., S. Novotny, W. Enanoria, D. Vugia, A. Khalakdina, and M. Katz. 2003. Endemic cryptosporidiosis and exposure to municipal tap water in persons with acquired immunodeficiency syndrome (AIDS): A case-control study. BioMed Central Public Health 2(3): 1-28. AWWA (American Water Works Association). 1999. Two-City Cryptosporidium Study. American Water Works Association Report. Bandaranayake, D.R., S.J. Turner, G.B. McBride, G.D. Lewis, and D.G. Till. 1995. Health Effects of Bathing at Selected New Zealand Marine Beaches. New Zealand. Bartram, J., and G. Rees. 1999. Monitoring Bathing Waters: A Practical Guide to the Design and Implementation of Assessments and Monitoring Programmes. London: E. and F.N. Spon. Bartram, J., L. Fewtrell, and T.A. Stenstrom. 2001. Harmonised assessment of risk and risk management for water-related infectious disease: an overview. Pp. 1-16 in Water Quality. Guidelines, Standards and Health: Assessment of Risk and Risk Management for Water-Releated Infectious Disease, L. Fewtrell and J. Bartram, eds. London: World Health Organization and IWA Publishing. Barwick, R.S., D.A. Levy, G.F. Craun, M.J. Beach, and R.L. Calderon. 2000. Surveillance for waterborne-disease outbreaks - United States, 1997-1998. MMWR 49 (No. SS-4): 1-35. Buchanan, R.L., and R.C. Whiting. 1996. Risk assessment and predictive microbiology. Journal of Food Protection Supplement: 31-36. Cabelli, V.J., A.P. Dufour, L.J. McCabe, and M.A. Levin. 1982. Swimming-associated gastroenteritis and water quality. American Journal of Epidemiology 115(4): 606-616.
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