Introduction To Epidemiology
Introduction To Epidemiology
Introduction To Epidemiology
Topics to be covered
Define epidemiology, basic concepts and application of epidemiology Describe measures of disease frequency
Describe measures of effect or associations Epidemiological study designs
Topics to be covered
Determine causal-effect relationships
Prevention levels
Screening
Coggon D, Rose G, Barker DJP. Epidemiology for the Uninitiated: 5th edition; BMJ Books
INTRODUCTION TO EPIDEMIOLOGY
Learning Objectives
At the end of this session, you should be able to:
Define epidemiology
Describe the basic epidemiologic methods (descriptive and analytical) List the uses of epidemiology in public health and medicine
Describe the history of epidemiology
What is epidemiology?
Epidemiology
The word epidemiology comes from the Greek words epi = meaning on or upon demos = meaning people logos = meaning the study of or doctrine
Thus the word epidemiology has its roots in the study of what befalls a population or study of what is happening to people
(Introduction to Modern Epidemiology, 1984)
Fundamental axioms
Diseases (or other health events) do not occur at random (disease is not randomly
distributed throughout a population, sub groups with differing disease frequency)
Diseases (or other health events) have causal and preventive factors that can be identified (Follow patterns that reflect
the operation of underlying factors, knowledge of uneven distribution help to investigate etiology & lay ground for prevention)
Definition of epidemiology
The study of the distribution and determinants of health related states or events in specified populations
Key words:
Study
Distribution
Basic science
Time, place, person Cause, risk factors
Distribution
Distribution
Frequency: refers to the number of health
events & proportion, rate or risk of disease e.g.
Number or proportion of pregnant women received syphilis screening
Number of cases of diabetes in a population Number of people with mental disorder in a population Number of children under one year of age vaccinated for measles
Number of cases should always be related to the size of population
Town
No of deaths
844
118
Town
844
167, 654
5.0
118
19, 133
9.8
Distribution 2
Distribution 2
Distribution 2
Determinants
Causes or other factors that influence the occurrence of disease and other health-related events
e.g. Smoking and lung cancer; intake of saturated fat and heart disease
However, a risk factor is not necessarily a direct cause of disease Risk: is the probability that an undesirable event will occur in a defined period of time (Not every smoker gets lung cancer, but the
probability is much higher than for non-smokers)
Non-communicable diseases Injuries, birth defects, maternal-child health, occupational health and environment health Health behaviors e.g. amount of exercise, diet, seat belt use Violence Lastly molecular epidemiology e.g. genetic markers of disease risk
Specified population
Epidemiologist and clinicians differ on how they view the patient. Concerned with collective health of people in the community
e.g. patient with diarrheal disease- source, other people infected/exposed, interventions to prevent additional cases
Application
Apply knowledge gained to community based practice.
Use both descriptive and analytical epidemiology in diagnosing the health of the community Propose appropriate, practical, and acceptable public health interventions to prevention and control of disease
Examples Below are key terms of definition of epidemiology. Match it with the activity a. Distribution b. Determinants c. Application
1. Graph the number of cases of congenital syphilis by year for the country
2.Mark on a map the residences of all children born with birth defects within 2 miles of waste hazards
3.Compare food histories between persons with mushroom poisoning and those without. 4.Recommend that close contacts of a child recently reported with meningococcal meningitis receive Rifampicin
Difference
What is a difference between descriptive epidemiological studies and analytical epidemiological studies?
Descriptive Epidemiology
Describe distribution of disease and healthrelated characteristics in the population.
Characterizing health events by time, place, and person are activities of descriptive epidemiology
Use: formation of hypothesis health planning & allocation of resources
(Case reports, case series, correlation studies, descriptive cross sectional study)
Male lung cancer death rate Female lung cancer death rate
60 50 40 30 20 10
0 0 1900 1908 19161924 1932 1940 1948 1956 1964 19721980 1988 1996
Year
Time trends: Incidence and mortality rates of breast cancer for white and black women in the United States
Incidence Mortality
White Black
Black White 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 Year
Figure 5
45
1.5-5 0-1.5
48
age, gender, race, SES, marital status, educational level, income, sexual preference Age: most fundamental when describe disease occurrence e.g. incidence of chronic disease with age, infectious disease most common in childhood etc
2.
3.
4.
Guiding the analysis of the analytic study (when the descriptive analysis is a component).
Does adolescent pregnancy increase the risk of maternal death? Was infant vaccination status associated with measle outbreak in Mwanga district?
Analytical
Analytical
Only quantitative techniques can be used
Primary concern: testing of hypothesis & proving causality Examples: case-control, cohort, analytical crosssectional & experimental studies
Uses of epidemiology
Epidemiology helps to
Describe the distribution of diseases and burden in a population (descriptive epidemiology)
Identification of etiological factors in pathogenesis (analytical epidemiology) Provide and analyze information for planning, implementation and evaluation of health status (descriptive and analytical)
Study the natural history of disease (cohort)
Epidemiology helps to
Determine the trends of disease (repeated cross
sectional, descriptive over time, surveillance)
Evaluate the impact of the control measures Make individual decisions e.g. quit smoking,
climbing stairs vs. elevator, eat salad vs. chips or using condoms in casual sex
Epidemiology helps to
Used in disease surveillance Used in screening (x-sectional, case-control, cohort, RCT) Used in outbreak investigation (case-control)
Selection of therapy is based on the results of large treatment studies such as clinical trials (experimental)
Clinical Practice
Clinical practice and health policy can not be based on clinical experience alone - need to be based on scientific evidence
- understanding epidemiology and methods used to study health and disease is pre-requisite
Success of epidemiology
Snow: control of cholera before discovery of causating
agent Scurvy: fruits reduce the problem 200 years before discovery of cause (Goldenberg et al) Cigarette smoking: and lung cancer lead to strict
legislation in advertising and smoking in public places Secondary smoking and cancers Smallpox: WHO elimination in 1978
In summary
Public health: primarily concerned with prevention of
Need to address individual, population, and contextual factors in looking for disease/ health events risk factors (Causal web is complex)
Objectives met?
Can you now:
Define epidemiology Describe the branches of epidemiological studies List the uses of epidemiology
History of epidemiology
Read
.. Natural history and spectrum of disease .. Different modes of transmission of communicable disease in a population
Levels of Disease
Sporadic
Epidemiologic Triad
Agent
Host
Environment
Host factors
Physiologic
Genetic Immunologic
Behavioral
Influence the chance for disease or its severity
Agent Factors
Infectious
Toxic Nutritional
Diseases with no agent
Necessary for disease to occur
Environmental Factors
Physical
Biologic Socioeconomic
Agent
Biologic Microorganisms
Environment
Disease vectors Population density Substances in Chemical surroundings and Toxins, tobacco, workplace alcohol, drugs Air quality Weather Physical Noise Trauma, radiation, Food and water sources fire Special environments: Nutrition Lack of, excess
Hospitals, day-care, institutions, bath houses, crack houses, refugee camps
Epidemics occur when host, agent and environmental factors are not in balance
New agent Change in existing agent (infectivity, pathogenicity, virulence) Change in number of susceptibles in population Environmental changes affecting transmission of agent or growth of agent
Reservoir
Habitat in which the agent normally lives and multiplies People Symptomatic - Smallpox Asymptomatic - HIV Animals (zoonoses) Brucellosis Plague Environmental Histoplasmosis Legionnaires bacillus
Mode of Transmission
Direct
Contact - Cutaneous Anthrax, hookworm Droplet Smallpox
Indirect
Airborne Histoplasmosis, Inhalation Anthrax Vehicleborne food or water - Salmonella Vectorborne
Mechanical Shigella by fly appendages Biological Malaria (maturation)
Chain of Infection
Mode of Transmission Reservoir Mode of Transmission Susceptible Host