Introduction to Epidemiology

Definition Epidemiology: Epi (upon) + demos (people) + logy (study of) The study of the distribution of a disease or condition in a population and the factors that influence that distribution. Epidemiology is a discipline that describes, quantifies, and postulates causal mechanisms for health phenomena in populations. Epidemiology is concerned with the distribution and determinants of health and diseases, morbidity, injuries, disability and mortality in populations. The Study of the distribution and determinants of health - related states or event in specific populations, and the application of this study to the control of health problems Determinants: ANY FACTOR, EVENT CHARACTERISTIC OR ANY OTHER DEFINABLE THING THAT BRINGS CHANGE IN A HEALTH CONDITION: BACTERIAS, CHEMICAL AGENTS STRESS ADVERSE LIFESTYLE DIET HIGH IN SATURATED FAT

Basic Research Terminology Retrospective: Refers to time of data collection Case Control Study: Persons w/ disease & those w/out are compared Cohort Study: Persons w/ and/or w/out disease are followed over time

Aims & Levels As the basic method of public health, epidemiology is concerned with efforts to: Describe the health status of the population Explain the etiology of disease Predict the occurrence of disease Control the distribution of disease ENUMARATE CASES OF d, OBTAIN Frecuencies of the D w/n subgroups, trends in the occurrence of the disease Discover causes of the disease and modes of transmission Estimates # of cases & identify distribution w/n Population -----------this is important for the planning, intervention and allocation of resources Prevent occurrence of new cases, eradicating existing cases, and to prolong the lives of those with the disease

Terminology (Cont.) Cross-sectional Study: Presence or absence of exposure to possible risk factor measured at one point in time. Prevalence obtained. Prevalence: The # of new cases and existing cases during specified time period. Incidence: The # of NEW cases per unit of a population at risk for disease occurring during stated time period.

Foundations Human diseases do not occur by chance Factors that cause or contribute to diseases and injuries can be identified by means of systematic investigation

Goals Improve the understanding of the natural history of disease and the factors that influence its distribution Intervention

Historical Antecedents Hippocrates: Environment – disease causation Graunt: Vital statistics in the mid 1600 Snow: Natural experiments to track cholera outbreak in London Koch: Human disease - micro-organism / a disease 1 Epidemiology began with the Greeks 400 BC Ancient epidemic ----- disease: environment Disease associated with the physical environment ….moved away from the supernatural explanations Bio –Statistics pioneer, compiled 4. His postulates advanced the theory of specific disease agent

Epidemiologic Approach Quantification Use of special vocabulary Interdisciplinary composition Quantification: enables the E to investigate the source of variation of a disease by Tiempo, Place and person It is the translation of qualitative impressions into numbers

Epidemiologic Approach: Special Vocabulary Epidemic – Pandemic – Endemic Epidemic: Excessive occurrence of a disease. Epidemicity: Relative to usual frequency of the disease in the same area, among the specified population, at the same season of the year. OCURRANCE OF more cases of disease than expected in a given area or among a specific group of people over a particular period of time

Pandemic: Epidemic on a worldwide scale Endemic: Characterize a disease that is habitatually present in a particular geographical region. (Malaria and Cholera) Surveillance: Systematic collection, analysis, interpretation, dissemination of data. PANDEMIC Epidemic occurring in several countries or continents, afecting a large proportion of the po ENDEMIC Constant present of a disease or infectious agent within a given geographical area or population group

Descriptive Variables for the Health of the Community Demographic & Social Variables Age and Sex Socioeconomic Status Family structure Racial, ethnic and religious composition Variables related to community infrastructure Availability of social and health services Quality of housing stock Social stability Family Structure includes marital status and number of single parent family Social Stability – residencial mobility

Health Related Variables Homicide and suicide rates Infant Mortality Rate Mortality from Specific Condition Magnitude of chronic and infectious diseases Alcoholism and drug abuse rates Teenage pregnancy rates Birth Rate

Population Dynamics and Epidemiology Three factors affect the size of populations: Birth Death Migration When is the Po in equilibrium?, What is a fixed Po?, What is a dynamic Po?, When a Po is in steady state? When B, D and M do not contribute to either increase or decrease the number of persons A fixed Po adds no new members, so decreases in size as a result of death only Adds new members through B and M and looses members through D and Emigration When the # of members exiting equals the # of members entering

Epidemiologic Triad: Traditional Model of Infectious Disease Causation The epidemiologic triangle recognized three factors in the pathogenesis of disease: Host Susceptible host External agent The E brings the H and A together Agent Environment

The Agent Agent: must be present for an infection to occur: Microbial agents Characteristics of Infectious disease agents: Infectivity Pathogenicity Virulance Toxigenicity Resistance Antigenicity Microbial Agents include: bacteria, viruses, fungis, intestinal parasites, insect vectors that carry a disease agent from its reservoir to humans Infectivity: Capacity of the A to enter and X in a susceptible host and produce infection or disease Pathogeneticity: Capacity of the A to cause disease in a infected host Virulence: refers to the severity of the disease Toxigenicity: refers to the K of the agent to produce a toxin or poison Resistance: refers to the ability of the agent to survive adverse environmental conditions Antigenicity: refers to the ability of the agent to produce antibodies in the host RESERVOIR habitat where As lives (humans, animals, environmental) AGENT: A factor such as a micro organism, chemical substance, from of radiation…whose presence, excessive presence or relative absent is essential for the ocurrance of a disease

Agent Host After exposure: from sub-clinical infection (inapparent) to active case of the disease. End Result: Recovery, disability, disfigurement, death. Ability to fight infections, comprises 2 broad categories: Non-specific defense mechanisms Disease specific defense mechanisms HOST: person or living organism that can be infected by an infectious agent saliva, skin, tears immunizations

The Environment Domain in which the disease-causing agent may exist, survive or originate. Acts as a reservoir or niche that fosters the survival of infectious disease agents. The reservoir may be a part of the physical environment or may reside in animals or insects (vectors) or other human beings (human reservoir – host) External Environment: physical, biologic, social, economic components Human: case or carrier Case – person identified w/ disease Carrier – asymptomatic, lack of infection Physical: weather, temperature, humidity, geological foundation Social: behavior, personality, culture Enhance or diminish survival of disease agents and may serve to bring agent and host into contact

Means of Transmission – Directly or Indirectly from reservoir Direct Transmission: Spread of infection through person to person contact Portals of exit: sites where infectious agents may leave the body (respiratory passages, the alimentary canal, the openings in the genitourinary system, and skin lesions. Also through insect bites, the drawing of blood, surgical procedures and accidents) Portal of entry: respiratory system (influenza, cold), the mouth & digestive system (hepatits A), mucous membranes or wounds in the skin.

Indirect Transmission: through an intermediary source Vehicle: contaminated H2O, infected blood on used hypodermic needles. Fomites: inanimated objects: doorknob or clothing. Vectors: animate, living insect or animal that is involved with transmission of the disease agent. Vehicle: an inanimated intermediary in the indirect transmission of an agent that carriers the A from a reservoir to a susceptible H Vector: an animate intermediary Human: case (person infected with the D) / carrier (asymptomatic, lack of infection)

Some Measures Used in Epidemiology Counts: Number of individuals with a certain disease/exposure Proportions: (P) One’s status in relation to group Ratio: A fraction in which there is not necessarily any specified relationship between the numerator & denominator. Rate: A ratio in which time forms part of the denominator Prevalence: # of existing cases of a disease / health condition in a Po at some designated time (specified or not). Proportions: 10 UC ss with Hepatitis How large of a problem this is: 20 Ss (50%) vs 500 Ss (2%)

Infant Mortality Rate: (IMR) risk of dying during the first year of life among infants born alive (aged 0-365 days) Perinatal MR: measures risk of dying among late fetal death + infant deaths within the 7 days of birth. (28 weeks gestation to 7 days of life) Noenatal MR: measures risk of dying among infants younger than 28 days Postneonatal MR: measures risk of dying among older infants (28 days to 12 months)

Incidence Rate: (IR) rate of development of new cases Crude Mortality Rate: (CMR) # of annual death Po Maternal Mortality Rate: MMR number of maternal death ascribed to childbirth Age-Specific Mortality Rate: ASMR # deaths in specific group per total number of people in that age group Proportional Mortality Rate: PMR # of death within a Po due to a specific disease

Specialized Epidemiological Measures Years of Potential Life Lost (YPLL) Measures # of years lost due to a cause It is based on projected life expectancy Dependency Ratio (DR) Proportion of dependents in a society to the number of adults (19-63) DR= # under 18 and over 64 # between 18 and 65