Descriptive Epidemiology Lecture 4.3 PDF

Descriptive Epidemiology Dr Muhammad Ahmed Alshyyab Lecture 3 Chapter 4 Lecture objectives To identify the Epidemiologic variables in terms of, place, time To discuss the ecological correlations in terms of aggregate-level data, the ecological fallacy Pl...

Descriptive Epidemiology Dr Muhammad Ahmed Alshyyab Lecture 3 Chapter 4 Lecture objectives To identify the Epidemiologic variables in terms of, place, time To discuss the ecological correlations in terms of aggregate-level data, the ecological fallacy Place variables Place variables are characteristics of the locale in which people live, work, and visit. Place variables may be defined in terms of geographic boundaries (e.g., street, city, state, region, country) or environmental characteristics (e.g., rural/urban, domestic/foreign, institutional/noninstitutional).  International An example of an infectious disease that shows international variations and decreasing incidence is polio, which at one time occurred worldwide. National (Within Country) Many countries, especially large ones, demonstrate with in country variations in disease frequency. Regional differences in factors such as climate, latitude, and environmental pollution affect the prevalence and incidence of diseases. Urban-Rural Differences Urban and rural sections of the United States show variations in morbidity and mortality related to environmental and lifestyle issues. Urban diseases and causes of mortality are more likely to be those spread by person-to-person contact, crowding, and inner-city poverty or associated with urban pollution. Agriculture is a major category of employment for the residents of rural areas. Farm workers often are exposed to hazards such as toxic pesticides and unintentional injuries caused by farm equipment. Localized Patterns of Disease Localized patterns of disease are those associated with specific environmental conditions that may exist in a particular geographic area. Illustrations include cancer associated with radon gas found in some geographic areas and arsenic poisoning linked to high levels of naturally occurring arsenic in the water. Local environmental conditions also may support disease vectors that may not survive in other areas. (Vectors are intermediaries insects or animals—involved in the transmission of disease agents.) An example of a localized pattern of disease is provided by dengue fever, a viral disease transmitted by a species of mosquito (a vector) that is present along the border that separates Texas from Mexico near the Gulf of Mexico. Localized populations of the mosquitoes are thought to have contributed to an outbreak of dengue fever in 2005. Place Variables Where people live and work Illustration: Age- adjusted breast cancer mortality in 23 countries, 1958–59 Example 1 Victorian water pipes Time variables Time variables address disease occurrence in relation to various time parameters such as time since exposure, calendar time, and seasonality. Examples on time variables are Secular Trends, Cyclic (seasonal) Trends, Point Epidemics. Secular Trends Secular trends refer to gradual changes in the frequency of diseases over long time periods. Hypertension; Effective regimens and medications; despite this fact, nearly 1/3 of the U.S. population has hypertension. Among all adults, this level did not change very much over the seven years period, which tracks the age-adjusted prevalence of hypertension. Comparisons by gender, age, or race/ethnicity demonstrate that there has been no secular change. Cyclic (seasonal) Trend Cyclic trends are increases and decreases in the frequency of a disease or other phenomenon over a period of several years or within a year. Mortality from pneumonia and influenza peaks during February, decreases during March and April, and reaches its lowest level during the early summer. Enteroviruses are common viruses that affect human beings globally and are linked to a spectrum of illnesses that range from minor to severe; detections of enterovirus infections have increased in frequency during the summer months within the past two decades. Point Epidemics A point epidemic may indicate the response of a group of people circumscribed in place to a common source of infection, contamination, or other etiologic factor to which they were exposed almost simultaneously. An example was demonstrated by an outbreak of Vibrio infections that followed Hurricane Katrina in 2005. A Vibrio is a bacterium that can affect the intestines (producing enteric diseases) and can cause wound infections. One of the illnesses caused by Vibrio is cholera (agent: Vibrio).  Epidemic curve A common way to explore the distribution of cases over time is in the form of an epidemic curve. The Y axis of an epidemic curve represents the number or percent of cases that occurred during the time-interval indicated on the X axis. Epidemic curves provide insight into the induction period of disease and the temporal course of disease occurrence. The temporal patterns: Sporadic (occurring rarely and without regularity) Endemic (occurring predictably with only minor or predictable variation) Point epidemic (occurring in clear excess over a period of time and then rapidly returning to normal) Propagating epidemic (occurring in clear excess with continuing increases over Time Variables Example: Epidemic curves (A) Sporadic (B) Endemic (C) Point epidemic (D) Propagating epidemic Ecological studies Epidemiologic studies based on aggregate-level variables are called ecological studies. Ecological data are often incomplete and lacking information on the multiple factors that contribute to disease occurrence. This is one of the reasons why ecological studies fall toward the descriptive end of the descriptive-analytic spectrum of epidemiologic study designs. Illustrative Example Dietary fat and cardiovascular disease (ecological correlation) The ecological fallacy The ecological fallacy (aggregation bias) consists in thinking that an association seen in the aggregate holds true for individuals when in fact it does not (Thorndike, 1939; Selvin, 1958). The ecological fallacy can thus be viewed as a form of confounding. Illustrative Example Farr’s faux pas (ecological fallacy) William Farr’s 1852 study on cholera c and geographic altitude in 19th century London provides an opportunity to illustrate an ecological fallacy. At the time, Farr accorded only a small role for contagion as a cause of cholera, placing much greater emphasis on social and environmental conditions Water Source (Eyler, 1980). In 1852 Farr wrote: ‘‘Notwithstanding the disturbance produced by the operation of other causes, the mortality from cholera in London bore a certain constant relation to the elevation of the soil, as is evident when the districts are arranged by groups in the order of Elevation Cholera their altitude.’’