Epidemiology
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5 Defining the current situation – epidemiology Paul R Hunter and Helen Risebro The first step in any economic appraisal or evaluation is to understand the underlying problem being addressed (see Chapter 1). Clearly, such an analysis of drinking-water interventions will have a strong public health element. This chapter discusses the role of epidemiology in identifying the burden of disease1 in a community that may be attributable to lack of access to safe drinking-water or adequate sanitation. In order to determine the scale of the problem, there are three questions to be asked: • What is the burden of disease in the target group? 1 WHO measures the burden of disease in disability-adjusted life years (DALYs). This time-bound measurement combines years of life lost as a result of premature death and years of healthy life lost because of time lived in a status of less than full health. Mortality and morbidity are linked to other indicators such as financial costs. © 2011 World Health Organization (WHO). Valuing Water, Valuing Livelihoods. Edited by John Cameron, Paul Hunter, Paul Jagals and Katherine Pond. Published by IWA Publishing, London, UK. 76 Valuing Water, Valuing Livelihoods • What proportion of the burden of disease is caused by deficiencies in access to drinking-water that are to be remedied by the intervention? • Are there any spin-off livelihood effects that would result from the outcomes of the intervention? This chapter focuses on the first two questions. Specific data challenges related to livelihood analysis are raised in Chapter 6. This chapter aims to assist decision-makers in gathering evidence to enable them to make an informed decision about whether or not there is a public health need for an intervention. A decision about the existence of a public health need is a prerequisite to undertaking a full economic assessment. The chapter outlines some of the methods of epidemiology, as a basis for better understanding epidemiological papers and reports. The chapter then goes on to describe how existing analyses may be used to estimate disease burden. MEASURES OF DISEASE OCCURRENCE The two predominant measures of disease occurrence are prevalence and incidence. Prevalence measures the amount of disease in a population at a given time and can be expressed as a percentage or shown as cases per population: Number of existing cases in a defined population at a given point in time Number of people in the defined population at the same point in time The point prevalence is a single assessment at a fixed point in time, whereas the period prevalence is the percentage of a population who have the disease at any time within a stated period. Period prevalence is preferred in infectious disease epidemiology because it can be used when there are repeated or continual assessments of the same individuals over a period of time (such as multiple episodes of diarrhoea). Longitudinal prevalence can be calculated using the following formula (Morris et al., 1996): Number of days with diarrhoea Number of days under observation Defining the current situation – epidemiology 77 Incidence measures the number of new cases of disease in a population over a specific time. When the population is constant, the incidence risk is measured as: Number of people who develop disease over a defined period of time Number of disease-free people in that population at the start of the time period When the population is not constant, for example, through deaths, migration, births, or through additional participant recruitment, the incidence rate should be calculated as: Number of new events in a defined population over a defined period of time Total person−time at risk during the defined period of time When studying illnesses that last a short time (days or a few weeks), such as acute diarrhoea, then incidence would usually be the most appropriate measure. For more protracted diseases, such as the health effects of arsenic poisoning, prevalence would be the more appropriate measure. ESTIMATING DISEASE OCCURRENCE There are different approaches to estimating disease occurrence in a population. The choice of approach will depend on many different factors, such as the amount of resources available and the accuracy of result required. Whatever approach is used, one of the most important starting points is to develop a case definition. Case definition The case definition is essential for both the epidemiological studies and any subsequent cost–benefit analyses. The case definition will enable the researcher to know whether or not a particular health event should be included in the analysis and will enable the cost–benefit analyst to determine the cost of the disease outcome. A case definition may be based on symptoms (such as the presence of diarrhoea or clinical features of arsenic poisoning) or the results of laboratory investigations (such as whether or not a stool sample is positive for Cryptosporidium). For example, WHO defines diarrhoea as three or more loose or fluid stools (which take the shape of a container) in a 24-hour period (WHO, 1993). Case definitions may also include age ranges, geographical location or dates of onset. Whatever case definition is used, it should be clear and standardized to minimize disease misclassification bias. Standardizing case definitions is 78 Valuing Water, Valuing Livelihoods especially important when there is more than one field researcher or interviewer or clinician, or when the study is carried out in more than one community. This is because definitions of diarrhoea can be culture- or person-specific. For example, a study conducted in a rural municipality in Nicaragua in Central America identified a classification encompassing nine different types of diarrhoea (Davey-Smith et al., 1993). The classifications used in Nicaragua were influenced by the place and the person consulted for treatment. The source of any existing data on the use of health care should therefore always be carefully considered. Primary surveys Where prior information is not available from local health care facilities or is suspected to be unreliable it may be most appropriate to collect data directly from the population concerned. Such primary data are especially valuable for estimating the burden of disease for illnesses that are unlikely to cause people to visit their local health care provider. In particular, such data collecting is valuable for self-reported diarrhoea. Data collecting can also be especially valuable in poor or remote communities, with limited access to health care. In these circumstances, even people with severe and chronic disease may not come to the attention of the health services. Such population surveys are, however, poor at identifying uncommon illnesses. These surveys usually involve a questionnaire and this may solely be concerned with determining whether the respondent reports various symptoms, to enable a diagnosis on whether or not the symptoms satisfy the case definition. Sometimes a physical examination, or even a laboratory or radiological examination may be included. For example, stool samples may be collected in a study of gastrointestinal disease. Examination of the teeth and radiological examination of the skeleton may be necessary for exposure to fluoride at toxic levels. There are two forms of population survey: the cross-sectional survey and the cohort study. Cross-sectional studies are a relatively quick way of getting an estimate of disease incidence or prevalence in a community. Cross-sectional studies look at the disease status of all or a sample of a population at a particular moment in time. In general, each individual would be contacted only once. For diseases with seasonal variation in their incidence, the results of the survey would clearly depend on what time of year the study was carried out. Conducting repeat studies or lengthening the duration of the data collection period may improve the results. Cross-sectional studies can be conducted using various ways of contacting the participants. The choice of approach will depend on resources available, costs and existing communications. Researchers may contact people Defining the current situation – epidemiology 79 by visiting them in their homes, or by post or telephone. Response rates are generally poorest for postal surveys (typically around 50%), slightly better for telephone surveys and best for direct visits to the home. Clearly, if a physical examination is part of the survey, face-to-face contact is essential. A cohort study follows a group of individuals over a period of time. During this period, researchers monitor participants for the appearance of the disease outcome of interest. Usually the initial contact includes an expanded baseline questionnaire. There are several ways of recording the presence or absence of illness. Probably the simplest method is for the researcher to visit or contact the participant at regular intervals. Sometimes people are asked to keep a daily diary of symptoms, which is then collected by the researcher. An example of a pictorial diary used for recording frequency and consistency of stool is shown in Figure 5.1. Figure 5.1 Example of a symptom diary. Source: Wright et al. (2006). Some of the advantages and disadvantages of cross-sectional and cohort studies are listed in Table 5.1. In general, cross-sectional surveys are quicker and less costly than cohort studies but they can suffer from recall bias in that people may overreport very recent or current diarrhoea (Boerma et al., 1991). Cohort studies are generally more expensive and take