GIS and Health Geography. What is epidemiology?

Transcription

1 GIS and Health Geography { What is epidemiology?

2 TOC GIS and health geography Major applications for GIS Epidemiology What is health (and how location matters) What is a disease (and how to identify one) Quantifying disease occurrence Incidence vs prevalence Identifying the population Working with small area data

3 GIS and health Geography A GIS can be a useful tool for health researchers and planners because, as expressed by Scholten and Lepper (1991): Health and ill-health are affected by a variety of lifestyle and environmental factors, including where people live. Characteristics of these locations (including socio-demographic and environmental exposure) offer a valuable source for epidemiological research studies on health and the environment. Health and ill-health always have a spatial dimension, therefore. More than a century ago, epidemiologists and other medical scientists began to explore the potential of maps for understanding the spatial dynamics of disease.

4 1. Spatial epidemiology 2. Environmental hazards 3. Modeling Health Services 4. Identifying health inequalities Major applications for GIS

5 Spatial epidemiology Spatial epidemiology is concerned with describing and understanding spatial variation in disease risk. Individual level data Counts for small areas Recent developments owe much to: Geo-referenced health and population data Computing advances Development of GIS Statistical methodology

6 Framework for analysis Population is unevenly distributed geographically. People move around (day-to-day movements; longer term movements including migration). People possess relevant individual characteristics (age, sex, genetic make-up, lifestyle, etc). People live in communities (small areas).

7 Why small area analyses? Provides a qualitative answer about the existence of an association (e.g. between environmental variable and health outcome). May provide evidence that can be followed up in other ways.

8 Geographical correlation studies These studies typically involve examining geographical variations in exposure to environmental variables (air, water, soil, etc.) and their association with health outcomes while controlling for other relevant factors using regression.

9 Issues: Spatial misalignment

10 Issues: Uncertainty Frequency and quality of population data (e.g. Census every 5/10 years). Spatial compatibility of different data sets. Availability of data on population movements. Measuring population exposure to the environmental variable. Environmental impacts are often likely to be quite small (relative to, for example, lifestyle effects) and there may be serious confounding effects. Cannot estimate strength of an association. Ecological (or aggregation) bias.

11 Allow for heterogeneity of exposure. Use well defined population groups. Use survey data to help obtain good exposure data. Allow for latency times. Allow for population movement effects. Issues: Best practices (Richardson 1992)

12 Spatial epidemiology Dr. John Snow s Map of Cholera Deaths in the SOHO District of London, 1854

13 1. Spatial epidemiology 2. Environmental hazards 3. Modeling Health Services 4. Identifying health inequalities Major applications for GIS

14 Environmental hazards Hazard Surveillance Hazardous agent present in the environment Route of exposure exists Exposure Surveillance Host exposed to agent Agent reaches target tissue Agent produces adverse effect Outcome Surveillance Effect clinically apparent

15 GIS: Identify causal and mitigating factors Environmental hazards

16 1. Spatial epidemiology 2. Environmental hazards 3. Modeling Health Services 4. Identifying health inequalities Major applications for GIS

17 ARIA (Accessibility/Remoteness Index of Australia) A generic index of accessibility/ remoteness for all populated places in non-metropolitan Australia. A model which allows accessibility to any type of service to be calculated from all populated places in Australia.

18 AIRA

19 Where do infants and children die in WA? Jane Freemantle, PhD. November 2004 Geographical location Remote Rural Metro. non-aboriginal Aboriginal Mortality Rate / 1000 live births Mortality rate of infants

20 SES and Heart disease Identifying health inequalities: Well-known relationship 25% 50% of observed gradient due to risk factors like smoking, hypertension and diabetes in lower socio-economic groups (Marmot et al.,1997) Access to healthcare (Bosma et al., 2005) Imbalance between workplace demands and economic reward (Lynch et al.,1997) Poor education, lower levels of health literacy, low birth weight (Marmot, 2000) Relationship may vary with gender, with the association thought to be stronger in males (Thurston, 2005) (should we question this?)

21 The Data Number of daily hospital discharges (Y) with Ischemic Heart Disease (IHD) where admission had been via emergency room for 591 postcodes in NSW Every day from July 1, 1996 to June 30, 2001 Males and females 5-year age increments Denominator (N) obtained from census Social disadvantage measured at postal area level using the census-derived SEIFA (Socio-Economic Indexes for Areas) index

22 SEIFA distribution in NSW High values indicate social advantage

23 NSW IHD rates

24 TOC GIS and health geography Major applications for GIS Epidemiology What is health (and how location matters) What is a disease (and how to identify one) Quantifying disease occurrence Incidence vs prevalence Identifying the population Working with small area data

25 What is epidemiology? The study of the distribution and determinants of health and disease-related states in populations, and the application of this study to control health problems. the product of [epidemiology] is research and information and not public health action and implementation (Atwood et al. 1997) epidemiology s full value is achieved only when its contributions are placed in the context of public health action, resulting in a healthier populace. (Koplan et al. 1999) (Source: Rutgers)

26 DESCRIPTIVE Health and disease in the community What? Who? When? Where? What are the health problems of the community? What are the attributes of these illnesses? ANALYTIC Why? What are the causal agents? How many people are affected? What are the attributes of affected persons? How? Over what period of time? Where do the affected people live, work or spend leisure time? Etiology, prognosis and program evaluation By what mechanism do they operate? What factors affect outcome? Epidemiologic approaches

27 What are health and disease? Dorland's Illustrated Medical Dictionary (28th ed.): Health "a state of optimal physical, mental, and social well-being, and not merely the absence of disease and infirmity. Disease "any deviation from or interruption of the normal structure or function of any part, organ, or system (or combination thereof) of the body that is manifested by a characteristic set of symptoms and signs...".

28 What is health Health, as defined in the World Health Organization's Constitution, is "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity." Health is seen as more than just the absence of disease, and depends upon a complex suite of factors, with location taking the lead. A location is more than just a position within a spatial frame (e.g., on the surface of the Earth or within the human body). Different locations on Earth are usually associated with different profiles: physical, biological, environmental, economic, social, cultural and possibly even spiritual profiles, that do affect and are affected by health, disease and healthcare. (Source: Dr. M. N. Kamel Boulos)

29 Location and health An example of how location matters and carries with it other factors into play: The body weight of infants at birth is one readily available piece of data, and the relationship between low birth-weight and maternal and child health is a continuing line of research. In New York City, Sara McLafferty and Barbara Tempalski have studied the spatial distribution of low birth-weight infants and identified areas in which the number of low birth-weight infants increased sharply during the 1980s. Their results indicated that the rise in low birth-weight was closely linked to women's declining economic status, inadequate insurance coverage and prenatal care, as well as the spread of crack/cocaine.

30 Location and health

31 Location and health

32 TOC GIS and health geography Major applications for GIS Epidemiology What is health (and how location matters) What is a disease (and how to identify one) Quantifying disease occurrence Incidence vs prevalence Identifying the population Working with small area data

33 What is disease Manifestional criteria: refer to symptoms, signs, and other manifestations of the condition. Defining a disease in terms of manifestational criteria relies on the proposition that diseases have a characteristic set of manifestations. This defines disease in terms of labeling symptoms. Causal criteria: refer to the etiology (cause) of the condition, which must have been identified in order to be employed. This defines disease in terms of underlying pathological etiology.

34 Manifestational Criteria How do you identify a disease? The Acquired Immunodeficiency Syndrome (AIDS) was initially defined by the CDC in terms of manifestational criteria as a basis for instituting surveillance. The operational definition grouped diverse manifestations Kaposi's sarcoma outside its usual subpopulation, PCP and other opportunistic infections in people with no known basis for immunodeficiency. This was based on similar epidemiologic observations (similar population affected, similar geographical distribution) and a shared type immunity deficit (elevated ratio of T-suppressor to T-helper lymphocytes).

35 Causal Criteria Around 1984 Human Immunodeficiency Virus (HIV, previously called human lymphotrophic virus type III) was discovered and demonstrated to be the causal agent for AIDS. AIDS could then be defined by causal criteria.

36 Challenges with Disease Classifications A single causal agent may have multiple clinical effects. Multiple etiologic pathways may lead to apparently identical manifestations, so that a manifestationally-defined disease entity may include subgroups with differing etiologies (equifinality). Not all persons with the causal agent develop the disease (e.g., Typhoid Mary).

37 Onset of disease Diagnosis of disease Cause-specific mortality Physiologic Underlying Abnormalities Genetic Susceptibility Sub-clinical disease Clinical disease X Environmental & Behavioral Factors (Spatial dependence) The natural history of disease

38 TOC GIS and health geography Major applications for GIS Epidemiology What is health (and how location matters) What is a disease (and how to identify one) Quantifying disease occurrence Incidence versus prevalence Identifying the population Working with small area data

39 To study disease, we need measures of its occurrence. Some measures of disease occurrence Counts Prevalence Incidence Mortality Measures of disease occurrence

40 DESCRIPTIVE Health and disease in the community What? Who? When? Where? What are the health problems of the community? What are the attributes of these illnesses? How many people are affected? What are the attributes of affected persons? Over what period of time? Where do the affected people live, work or spend leisure time? Each of the measures can be calculated for different combinations of What? Who? When? and Where? Each of the W s needs to be defined carefully to get comparable measures across a province or state, a nation, the world. Epidemiologic approaches

41 Incidence and Prevalence Incidence and prevalence measure different aspects of disease occurrence Prevalence Incidence Numerator: Denominator: Measures: All cases, no matter how long diseased All persons in pop Presence of disease Only NEW cases Only persons at risk of disease Risk of disease Most useful: Resource allocation Risk, etiology Etiology: the study of a disease s causes.

42 Numerator Number of deaths Denominator Number of individuals in population (how defined?) Time interval 1-year: Annual Mortality Rate Specifier (typical to use an annual rate) age, sex, race, etc. Mortality Rate Incidence of death

43 Mortality rates

44 Prevalence numerator case definition AIDS cases, United States Result of new definition 1 st Quarter of 1993: Expansion of surveillance case definition

45 The demi in Epidemiology Understanding population dynamics is crucial to epidemiology. Demography = the study of population dynamics including fertility, mortality and migration Γρεεκ epi demos logy English among people study

46 TOC GIS and health geography Major applications for GIS Epidemiology What is health (and how location matters) What is a disease (and how to identify one) Quantifying disease occurrence Incidence vs prevalence Identifying the population Working with small area data

47 Data considerations Developing multi-level models for spatiallycorrelated data requires confidence in the dependent data. Data for disease mapping often consists of disease counts and exposure levels in small adjacent geographical areas. The analysis of disease rates or counts for small areas often involves a trade-off between statistical stability of the estimates and geographic precision.

48 Methodological toolboxes Spatial Analytic Techniques for Medical Geographers (Albert et al., 2000)

49 Summary GIS and health geography Major applications for GIS Epidemiology What is health (and how location matters) What is a disease (and how to identify one) Quantifying disease occurrence Incidence versus prevalence Identifying the population Working with small area data