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Epidemiology: a tool for the assessment of risk - WHO

2001 World Health Organization (WHO). Water Quality: Guidelines, Standards and Health. Edited byLorna Fewtrell and Jamie Bartram. Published by IWA Publishing, London, UK. ISBN: 1 900222 28 07 epidemiology : a tool for theassessment of riskUrsula J. Blumenthal, Jay M. Fleisher,Steve A. Esrey and Anne PeaseyThe purpose of this chapter is to introduce and demonstrate the use of a key toolfor the assessment of risk. The word epidemiology is derived from Greek and itsliteral interpretation is studies upon people . A more usual definition, however,is the scientific study of disease patterns among populations in time and chapter introduces some of the techniques used in epidemiological studiesand illustrates their uses in the evaluation or setting of microbiologicalguidelines for recreational water, wastewater reuse and drinking INTRODUCTIONM odern epidemiological techniques developed largely as a result of outbreakinvestigations of infectious disease during the nineteenth Quality: Guidelines, Sta

Epidemiology: a tool for the assessment of risk 137 1987; Rothman and Greenland 1998). The case studies include examples of the elements described here.

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Transcription of Epidemiology: a tool for the assessment of risk - WHO

1 2001 World Health Organization (WHO). Water Quality: Guidelines, Standards and Health. Edited byLorna Fewtrell and Jamie Bartram. Published by IWA Publishing, London, UK. ISBN: 1 900222 28 07 epidemiology : a tool for theassessment of riskUrsula J. Blumenthal, Jay M. Fleisher,Steve A. Esrey and Anne PeaseyThe purpose of this chapter is to introduce and demonstrate the use of a key toolfor the assessment of risk. The word epidemiology is derived from Greek and itsliteral interpretation is studies upon people . A more usual definition, however,is the scientific study of disease patterns among populations in time and chapter introduces some of the techniques used in epidemiological studiesand illustrates their uses in the evaluation or setting of microbiologicalguidelines for recreational water, wastewater reuse and drinking INTRODUCTIONM odern epidemiological techniques developed largely as a result of outbreakinvestigations of infectious disease during the nineteenth Quality.

2 Guidelines, Standards and HealthEnvironmental epidemiology , however, has a long history dating back to Romanand Greek times when early physicians perceived links between certainenvironmental features and ill Snow s study of cholera in London and its relationship to water supply(Snow 1855) is widely considered to be the first epidemiological study (Baker etal. 1999). Mapping cases of cholera, Snow was able to establish that cases ofillness were clustered in the streets close to the Broad Street pump, withcomparatively few cases occurring in the vicinity of other local investigations can provide strong evidence linking exposureto the incidence of infection or disease in a population.

3 They can provideestimates of the magnitude of risk related to a particular level of exposure ordose and so can be used in the evaluation of appropriate microbiological qualityguideline levels or standards. Epidemiological methods can quantify theprobability that observed relationships occurred by chance factors and they alsohave the potential to control for other risk factors and/or confounders of theoutcome illness being studied. Epidemiological studies used for the evaluationor setting of guidelines must be of high quality, so that there is confidence in thevalidity of the following sections outline the basic elements of epidemiological studies(including comments on features that are important for high quality studies), thedifferent types of epidemiological study, and the use of epidemiology inguideline setting, with case studies of the use of epidemiology in recreationalwater, drinking water and wastewater reuse BASIC ELEMENTS OF EPIDEMIOLOGICALSTUDIESThe basic elements of an epidemiological study can be characterised as follows.

4 Formulation of the study question or hypothesis selection of study populations and study samples selection of indicators of exposure measurement of exposure and disease analysis of the relationship between exposure and disease evaluation of the role of bias evaluation of the role of elements will be considered here in a simplified format. Readers arereferred to epidemiology textbooks for consideration of the factors in moredetail (Beaglehole et al. 1993; Friis and Sellers 1996; Hennekens and BuringEpidemiology: a tool for the assessment of risk1371987; Rothman and Greenland 1998). The case studies include examples of theelements described Formulation of the study question or hypothesisThe study question must be formulated so that it can be tested using statisticalmethods.

5 For example: Exposure to wastewater (meeting the WHO guidelines) comparedwith no exposure to wastewater does not increase the rate ofAscaris null hypothesis (which implies there is no relationship betweenpostulated cause and effect) states that observed differences are due to samplingerrors ( to chance). Stated in the null form, the propositions are refutable andcan be assessed using statistical tests (see section ). Selection of study populationsA study population exposed (to the factor of interest) and a control population(not exposed to the factor of interest) need to be selected (except in aprospective cohort study where a single cohort is studied and analysis is onexposure status).

6 A sample from the exposed and control populations needs tobe selected to be as similar as possible in all factors other than the factor ofinterest socio-economic status, and other risk factors for the diseaseoutcome of interest. Since samples are never totally similar, we need to recordpossible confounding factors and control for them in the analysis (see below).For enteric infections arising from exposure to contaminated water, such factorswould include sanitation, personal hygiene, drinking-water supply, foodhygiene, and travel. It is important that both exposure and disease can bemeasured as accurately as possible in the chosen populations.

7 For example, instudies on drinking water, the drinking water source (and therefore the quality)for each household needs to be known accurately. In most studies, a sample willbe selected from a larger population exposed to the factor of interest, using asampling frame. This needs to be done so that it is representative of the largerpopulation difficulties here can arise due to selection bias and inadequatesample size (see also sections and ). The choices of study populationwill depend on the type of epidemiological study selected (see section ).138 Water Quality: Guidelines, Standards and Selection of indicators of exposureThe quality of the water to which the population is exposed needs to bemeasured.

8 The use of indicators of contamination are preferred tomeasurements of pathogenic organisms in the water due to the low numbersof pathogenic organisms present, the difficulties in detecting them and theexpense involved (see Chapter 13). Indicators should be selected that areappropriate to the water being studied thermotolerant coliforms or used in assessing the quality of drinking water whereas these are lesssuitable for assessing the quality of coastal recreational waters whereenterococci and faecal streptococci are generally preferred. Where thedensity of an indicator does not accurately reflect the relative density of theunderlying pathogenic organism, then it is not a valid indicator is a particular concern when bacterial indicators are used to indicate thepresence of both bacterial and viral pathogens, as treatment methods areoften less effective against viruses.

9 This has led to concern about theadequacy of the zero faecal coliform guideline for drinking water quality(Payment et al. 1991). Measurements of exposure and disease statusIn the study population measurements of exposure and disease status need to bemade while minimising the various types of error that can occur. Where errorsoccur, this is called information bias and results in misclassification (see below).For exposure to occur, an individual must have contact with water of a givenquality. It is preferable to measure exposure at an individual level, but in manystudies, exposure status is measured at a group level, which can give rise tomisclassification of exposure for the individual.

10 For example, in studies of theeffects of aerosol exposure from wastewater irrigation in Israel, exposure statuswas assigned at the kibbutz level and no differences in individual exposurestatus were measured. However, the effect of exposure was assessed separatelyfor children and agricultural workers and for the general population, so allowingfor some differences in exposure between sub-groups (Fattal et al. 1986; Shuvalet al. 1989). Where the misclassification does not depend on disease status, thenthis is called non-differential misclassification, and the bias would be towardsthe null, making it more difficult to detect true associations between exposureand disease.


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