Introduction to Epidemiology

Sia E Msuya (MD, PhD)

Community Health Department November 2012

Bsc Lab 3

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

Interpret epidemiological results (chance, bias, confounding)

Topics to be covered
Determine causal-effect relationships

Prevention levels
Screening

Use of routine data & sources

Group presentations of research papers

Reference books or materials

Hennekens CH, Buring JE & Mayrent S. Epidemiology in Medicine. 1st edition; 1987. Beaglehole R, Bonita R & Kjellstrm T. Basic epidemiology. WHO; 2004.(In the internet)

Coggon D, Rose G, Barker DJP. Epidemiology for the Uninitiated: 5th edition; BMJ Books

Learning process & Examination

60% lectures 40% self study of reference books and papers Self study (questions)

Group work- questions or review of research papers End of semester examination

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

and the application of this study to the control of health problems.

Key words:
Study
Distribution

Basic science
Time, place, person Cause, risk factors

Determinants Event / Health status Population Application

Community Information for action

Distribution

Distribution is concerned with the

frequency and pattern of health events in a population

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

No of deaths Total population

Deaths per 1,000 population

844

167, 654

5.0

118

19, 133

9.8

Distribution 2

Pattern of disease refers to the

occurrence of health-related events or disease by time, place, and person.

Distribution 2 Time patterns may be;

seasonal, annual, monthly, weekly, daily, hourly, weekday versus weekend,

decades, secular trends

Distribution 2

Place patterns include:

residence (urban/rural differences), geographic variation, country variation,
location of work sites or schools.

Distribution 2

Personal characteristics related to risk of illness

age, gender, marital status, religion, education level, SES, life style, behaviors etc.

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)

Try to answer why and how to disease

Health related events 1

Anything affects the well being of a population

E.g. lack of physical activity,

smoking, secondary smoking, alcohol intake NCDs

HIV, violence, accidents

Health related state or events 2

Originally focus was on communicable disease

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

Clinic vs. Epidemiology

Clinician Epidemiologist Individual Population/ community Medical history, physical examination Surveillance Differential diagnosis descriptive epidemiology Diagnostic test Analytical Treatment epidemiology Intervention (prevention/control)

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

Basic principles of epidemiology

Quantitative description of distribution of disease (how much, distribution in time, place, person)
Identification of determinants of disease (within populations, between populations)

Intervention to control disease and formulation of effective public health policy

Examples Below are key terms of definition of epidemiology. Match it with the activity a. Distribution b. Determinants c. Application

Examples :Is it distribution, determinant or 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

Examples :Is it distribution, determinant or application?

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

Key features of descriptive and analytical of epidemiology

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)

Examples of descriptive epidemiology

Child mortality globally (< 5 yrs)

(8.8 million child deaths/year)

Where are they occuring Who is more affected When do they occur What are the causes and progress?

Maternal mortality globally (pregnancy & childbirth)

(358,000 deaths per 100,000) Where are they most occuring? Who is more affected? When do they occur?

Childrens death per year

Description by Time (when)

- Monitor disease occurrence in community and assess whether public health made a difference over time - How does disease frequency change according to time-factors, and what does this mean? Time: plotted x-axis (broad as years or decades, brief
as days or hours )

Y-axis: number, proportion or rate of cases

Example of Secular trends

5000 4500 4000 3500 3000 2500 2000 1500 1000 500 100 90 80 70

Per captia cigarette consumption

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

*Age-adjusted to 2000 U.S.

Year

Lung cancer death rate per 100,000*

Per capita cigarette consumption

Time trends: Incidence and mortality rates of breast cancer for white and black women in the United States

160 140 120 100 80 60 40 20 0

Rate per 100,000

Incidence Mortality

White Black

Black White 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002 Year

Description by place (where)

Country, region, residence (urban vs. rural), workplace, location of the room, topography Provide insight to geographical extent of disease and geographic variation

Figure 5

U5 mortality rates by country; WHO 2010

Source: The Lancet 2010; 375:1988-2008 (DOI:10.1016/S0140-6736(10)60703-9) Terms and Conditions

Anaemia among pregnant women

Anaemia in preschool children (< 5 yr)

45

Heterogeneity of HIV in Tanzania

15+ 10-15 5-10

1.5-5 0-1.5

Sentinel surveillance preg women 4.8%-15.3%

48

Description by person (who gets a

disease)
Personal characteristics of the population that may affect illness or related to risk of illness

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

Likely reasons for the importance of age:

Differing susceptibility with different stages of human development

Increasing immunity with age

Slowly accumulating damage (e.g., atherosclerosis) Lifestyle (exposure) variation with age

Descriptive data analysis is useful for:

1. Providing clues about disease causation and prevention (i.e., generating hypotheses) that are often investigated further in targeted analytic studies. Assessing trends in the health status of a population
Help in efficient allocating of resources and targeting populations for educational or preventive programs

2.
3.

4.

Guiding the analysis of the analytic study (when the descriptive analysis is a component).

Analytic Epidemiology (answer why & how)

Try to see if there is a relationship (association) between a risk factor (exposure) and an outcome (disease) Identify and quantify the relationship between an exposure and a health outcome (Determinants). The hallmark of such a study is the presence of a comparison group (e.g. an outbreak of cholera).

Hypothesis testing is the primary tool of inference

Analytical studies cont.

Are persons with a particular character more likely than those without characteristics to contract disease character is said to be associated with the disease
e.g. Did the women who delivered at home died more compared to those who delivered at health facilities?

Factors identified target public health prevention and control activities

Concept of exposure and outcome

Exposure (risk factor)--------------- Outcome (disease/ negative health event)

Does smoking increase the risk of lung cancer?

Exposure? Outcome?

Does adolescent pregnancy increase the risk of maternal death? Was infant vaccination status associated with measle outbreak in Mwanga district?

Example: Breastfed children had higher or lower occurrence

of diarrhoea compared to non-BF children?

Analytical

Use of analytical studies

(observe or experiment and compare outcomes)

Test the hypothesis

Determinants or risk factors of disease (explain and quantify)

Help in establishing causal-effect relationships (quantify)

Knowledge of risk factors and natural history help in introduction of preventive measures

Descriptive vs analytical studies

Descriptive vs. Analytical studies

Descriptive
Qualitative & quantitative techniques may be used
Primary concern: description

Analytical
Only quantitative techniques can be used
Primary concern: testing of hypothesis & proving causality Examples: case-control, cohort, analytical crosssectional & experimental studies

Examples: case series, descriptive cross sectional, ecological descriptive 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)

Modeling Public health policy

The relationship between Epidemiology and Clinical practice

Clinical practice uses population data:

Diagnoses are defined and determined from large group of patients (observation & experimental) Prognosis is based on experience of large groups of patients with the same disease, stage of disease, and treatments (cohort & experimental)

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

For the ability to appraise critically the evidence in scientific literature

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

disease in human populations

Epidemiology: is the branch of public health which attempts to
discover the causes of disease in order to make disease prevention possible

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

Concepts of Disease Occurrence

..The epidemiologic triad

.. Natural history and spectrum of disease .. Different modes of transmission of communicable disease in a population

Epidemiology and Disease

Levels of Disease

Increasing amount of disease Pandemic Epidemic

Endemic

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

Host, Agent, Environment

Host
Age Sex Race/Ethnicity Religion SES Marital status Lifestyle Exercise Behavior Co-morbidity Genetic makeup

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