EPIDEMIOLOGY Introduction and Disease Transmission

EPIDEMIOLOGYIntroduction and Disease

Transmission

Sue Lindsay, Ph.D., MSW, MPH

Division of Epidemiology and Biostatistics

Institute for Public Health

San Diego State University

Epidemiology

The study of patterns of health, disease, and injury in

human populations and the application of this

study to the control of health problems

ASPH Ten Epidemiology Competencies

Upon Graduation, a student with an MPH should be able to:

1. Identify key sources of data for epidemiologic purposes

2. Identify the principles and limitations of public health screening programs

3. Describe a public health problem in terms of magnitude, person, time, and place

4. Explain the importance of epidemiology for informing scientific, ethical, economic, and political discussions of health issues



5. Comprehend basic ethical and legal principles pertaining to the collection, maintenance, use, and dissemination of epidemiologic data.

6. Apply the basic terminology and definitions of epidemiology.

7. Calculate basic epidemiology measures.

8. Communicate epidemiologic information to lay and professional audiences.



9. Draw appropriate inferences from epidemiologic data.

10. Evaluate the strengths and limitations of epidemiologic reports.

Underlying Assumptions

• Illness and disease are not randomly distributed

in human populations

• Each human being has characteristics that either

predispose toward illness, or protect from illness.

• These characteristics are identifiable and modifiable

• Communities and neighborhoods also have characteristics

that either predispose toward, or protect from illness.

As Medical Detectives Epidemiologists Must Find:

• Who?

• What?

• Where?

• When?

• Why?

• How?

The Five Objectives of Epidemiology

1. To identify the cause(s) of a disease and the risk factors for that disease.• How is the disease or condition transmitted or acquired? Are there

sub-groups of the population at high risk for the disease?

2. To determine the extent of the disease found in a community or population - surveillance

• What is the burden of the disease or condition?


3. To study the natural history and prognosis of the disease

• Severity, lethality, duration, survivorship

4. To evaluate existing and new preventive and therapeutic measures as well as modes of health care delivery

• Does screening for disease impact outcome?


5. To provide the foundation for developing public health policy and regulatory decisions

• How do environmental problems impact human health?

Epidemiologic Areas of Study

• Observational Epidemiology

• Natural Experiments

• Experimental Epidemiology

Historical Examples of Epidemiology in Practice

The Story of Smallpox

• Major worldwide epidemic in the late 1700’s

• Known immunity from re-infection among survivors

• “Variolation”: early attempts at control were done by using infected smallpox pus and tissue to “variolate” healthy people


• Dairy Maids - young women who milked cows got mild disease known as “Cowpox”

• During smallpox outbreaks, dairy maids did not develop smallpox

• Edward Jenner (born 1749), physician practicing in England believed cowpox could protect against smallpox


• 1778: Jenner decides to test his hypothesis

• Innoculates an 8 year old “volunteer” James Phipps with cowpox material from a dairy maid

• Six weeks later Jenner exposes the boy to a smallpox infection

• Smallpox did not infect the boy

The First Vaccination

The Story of Cholera

• Cholera was a major public health problem in England in the mid-19th century

• First week of September 1854: 600 Deaths among people living near Broad Street in London


• John Farr, Registrar General

• John Farr believed the disease was transmitted by a cloud or “miasma” clinging low to the earth

• He hypothesized that greater altitude would be protective against cholera

Deaths from Cholera in 10,000 Inhabitants by Elevation Above Sea

Level, London 1848-1849

0

20

40

60

80

100

120

<20 20-40

40-60

60-80 80-100

100-120

340-360

Feet above sea level


• John Snow, physician to Queen Victoria

• Believed cholera was transmitted by contaminated water

• Public water companies transported water supply from polluted parts of the Thames River.

• The Lambeth Water Company moved their water intake upriver towards non-polluted water


• John Snow hypothesized that death rates would be lower in households buying water from the Lambeth Company

• 1854: Conducted a house to house survey

• Number of deaths/household

• Water company that supplied the household

Deaths From Cholera Per 10,000 Houses By Source of

Water Supply

WaterSupply

No ofHouses

DeathsFrom

Cholera

Deaths per10,000

Houses

Southwark 40,046 1,263 315

Lambeth 26,107 98 38

Other 256,423 1,422 56

Deaths From Cholera Per 10,000 Houses By Source of Water Supply

0

50

100

150

200

250

300

350 Southwark LambethOther

1952

Overweight and Obesity1960-2000

The Epidemiologic Approach

• How does the epidemiologist identify public health problems and design interventions?

• Frequency of health and disease

• Patterns of disease by age

• Patterns of disease by geography

• Patterns of disease by race/ethnicity

• Patterns of disease by gender

Frequency of Health and DiseaseTen Leading Causes of Death in the United States,

1900 and 1997

Patterns of Disease by AgeLife Expectancy At Birth and Age 65, By Race and Sex,

United States, 1900, 1950, 1996

Patterns of Disease by Geography

6.1

17.7

1.3

2.52.8

7.7

13.1

7.6 7.7

1.23.1

2.4

3.6

3.9

10.1

6.7

6.5

3.01.9

5.5

3.3

3.5

8.6

7.0 5.5

8.7

3.7

6.93.6

5.8

3.0

7.0

5.8

7.9

6.3

13.1

7.0

21.1

<55-9.9

10+

US rate =9.0

Rate per 100,000

N=14,813

6.220.6

MARICTNJDEMDDC

NH15.47.1

10.38.9

11.77.3

70.9

3.5

13.1

19.6<5 cases*

Includes cases with unknown state of residence†

*†

Adult/Adolescent AIDS Rates per 100,000 White Population

Reported in 1999

Adult/Adolescent AIDS Rates per 100,000 Black Population

Reported in 1999

**180.3 MA

RICTNJDEMDDC

173.0 81.2 82.4

147.6107.4114.7266.3

*45.6

70.3

*

*

*

**

*

*37.9

54.389.5 36.9

20.8

18.5

56.0

25.1

39.6

56.4

55.529.7

29.3

36.8

23.6

61.1

36.5

84.0

56.732.136.9

29.1

63.0

41.0

77.7

131.1

27.1

74.5

67.4

182.1


45.4

<5050-99

100+<5 cases

Rate per 100,000

US rate =84.2N=21,730

*

Adult/Adolescent AIDS Rates per 100,000 Hispanic Population

Reported in 1999 15.3

21.9

15.7

24.732.3 16.3

17.6

6.6

20.6

23.4

31.814.9

12.2

15.9

20.0

32.5

25.857.9

32.3

25.6

22.616.2

19.3

29.2

19.3

108.6

124.5

7.1

7.825.4

MARICTNJDEMDDC

NH127.360.089.243.642.723.5

112.2

60.7

17.2

P.R. 41.943.2

* **

*

*

*

*

**

*


14.8

<2020-49.9

50+

Rate per 100,000

<5 casesUS rate =34.6N=8,967

*†

Year of Report

Perc

ent o

f C a

s es

Proportion of AIDS Cases, by Race/Ethnicity and Year of Report,1985-1999, United States

1985 1987 1989 1991 1993 1995 1997 1999

0

10

20

30

40

50

60

70

American Indian/Alaska Native

Black, not Hispanic

Hispanic

Asian/Pacific Islander

White, not Hispanic

AIDS Cases Reported in 1999 and Estimated 1999 Population, by Race/Ethnicity, United States

White, not HispanicBlack, not HispanicHispanic

Asian/Pacific IslanderAmerican Indian/ Alaska Native

*Includes 120 persons with unknown race/ethnicity

4%1%

71%

12%13%

AIDS Cases N=46,400*

PopulationN=277,200,000

<1%1%

32%

47%19%

Noteworthy Examples of Epidemiologic Investigations

• Tampons and Toxic Shock Syndrome

• Legonnaire’s Disease

• Low Level Ionizing Radiation and Leukemia

• Hormone replacement therapy and heart attack, stroke, blood clots, breast cancer, reduced risk of colorectal cancer

Noteworthy Examples of Epidemiologic Investigations

• Passive Smoking

• Agent Orange

• Acquired Immune Deficiency Syndrome (AIDS)

• The Effect of DES on Off-Spring

• Severe Acute Respiratory Syndrome (SARS)

Asia 2003, 12+ countries, 8,098 sick, 774 died

• Avian Influenza (Bird Flu)

Disease Transmission

The Epidemiologic Triad

Host

Agent Environment

Vector

An Example of The Epidemiologic Triad

Host (Person)

Agent

(Bacterium)

Environment

(Contaminated Water)Vector

(Mosquito)

Factors Which Influence Health and Disease in Humans

• Biological

• Physical

• Chemical and Environmental

• Genetics

• Nutrition

• Immunology

Host Characteristics

• Age

• Sex

• Race

• Occupation

• Religion

• Customs

• Family Background

• Previous Diseases

• Immune Status

• Genetic Profile

• Marital Status

Types of Agents

• Biologic

Bacteria, Virus

• Chemical

Poison, Alcohol,Smoke

• Physical

Trauma, Radiation, Fire

• Nutrition

Diet, Low / Excess Caloric Intake

Environmental Factors

• Temperature

• Humidity

• Altitude

• Crowding

• Housing

• Neighborhoods

• Violence

• Water

• Milk

• Food

• Radiation

• Air Pollution

• Noise

• Public health infrastructure

Modes of Disease Transmission

• Direct: Person to Person Contact

• Indirect: • Vehicle borne• Vector borne • Single exposure•Multiple exposures•Continuous exposure

Body Surfaces as Sites of Infection

• Mouth

• Respiratory Tract

• Alimentary Tract

Skin •

•Urinogenital Tract

The Iceberg Concept of Infectious Diseases

Asymptomatic Infection

Exposure Without Infection

Viral/Bacterial Transformation

Exposure Without Cell Entry

Cell transformation/ dysfunction Moderate/Severe Illness

Patterns of Disease

• Epidemic disease

• Endemic disease

• Pandemic disease

Epidemic Disease

• The occurrence of disease in a community or region, clearly in excess of normal expectations and derived from a common or propagated source

Tuberculosis: Frequency Distribution of Cases by Age in Minorities, United

States

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60 70 80

Age

Nu

mb

er o

f C

ases

Endemic Disease

• The habitual presence of a disease within a given geographic area

Malaria

• WHO estimates 300-500 million existing cases per year worldwide

• >90% of all cases occur in Sub-Saharan Africa

• Malaria is endemic in 101 countries reporting to the WHO

Pandemic: Worldwide Epidemic

• Three essential conditions for pandemics:

• A new infectious agent such that humans have no natural immunity

• Agent must evolve to be capable of infecting and killing humans efficiently

• Agent must succeed in being transferred from human to human

World Health Organization (WHO)Six Influenza Pandemic Phases

• Phase 1

• No viruses circulating among animals have infected humans

• An animal influenza virus has caused infection in humans

• Small clusters of disease in people. No human-to-human transmission sufficient to sustain community-level outbreaks.

• Phase 2

• Phase 3

World Health Organization (WHO)Six Influenza Pandemic Phases

• Phase 4

• Human-to-human transmission able to cause sustained community-level outbreaks.

• Human-to-human transmission into at least two countries in one WHO region. A global pandemic is imminent.

• Community level outbreaks in one other country in a different WHO region. A global pandemic is underway.

• Phase 5

• Phase 6

WHO Pandemic Phases

http://www.who.int/csr/disease/avian_influenza/phase/en/index.html

HIV/AIDS

• Worldwide epidemic

• No country is exempt

• WHO Estimates >42 million infected since the pandemic onset

• Each day, >16,000 newly infected cases worldwide

Herd Immunity

• The resistance of a group of people

to an attack by a disease because a

large proportionof the group members

are immune

Why Does Herd Immunity Work?

• A large proportion of immune persons in a population decreases the likelihood that any one person with disease will come into contact with susceptible individuals

Characteristics of Herd Immunity

• If a large percent of the population is immune, the entire population is protected

• Infected persons are unlikely to contact susceptible persons

Herd Immunity in Public Health

• The success of immunization programs depends on both immunization rates and achieving herd immunity

• It may not be necessary to achieve 100% immunization rates

Conditions for Herd Immunity

• Disease agent must be restricted to single host species

• Transmission must be direct from one member of host species to another

Documents

EPIDEMIOLOGY Introduction and Disease Transmission