Lecture 2:Dynamics of Disease

Transmission

Reading:

Gordis - Chapter 2

Lilienfeld and Stolley - Chapter 1, pp. 13-19; Chapter 3

The epidemiologic triad

Host – Agent – Environment for infectious disease

Host

AgentEnvironment

VECTOR

Not to be confused with person, place,

time

Characteristics of host/agent/environment

Host Agents Environment

Age Biologic Temperature

Sex Chemical Humidity

Religion Physical Altitude

Culture Nutritional Crowding

Occupation Housing

Genetic profile Water

Previous diseases Air pollution

Immune Status Noise

The infectious disease process

• Agents– what is causing the illness

• Reservoirs– where the agents live

• Transmission– how they get in

• Host immunity– what factors affect disease progression

Agents

• Biologic:– worms, fungi and yeast, protozoa, bacteria,

viruses, prions• Physical:

– noise, repetitive motion, violence• Chemical:

– tobacco, air pollutants, water pollutants• Nutritional:

– obesity, nutritional deficiencies

Characteristics of host

• An agent must be present for an infectious disease to develop

• But this is not a sufficient cause• Infection depends on agent factors and host

factors• Host immunity –

the capacity of a person when exposed to an infectious agent to remain free of infection or clinical illness

Characteristics of environment

• Physical – habitat, climate

• Biological– population density– flora – fauna

• Socioeconomical– occupation,urbanization , culture

Modes of transmission• Mode of transmission – any mechanism by

which an agent is spread to the host• Disease can be transmitted by

– Horizontal transmission Direct contact: person to person contact Indirect contact

1.Vectors (animate objects)2.Vehicles (inanimate objects)

– Vertical transmission (genetic transmission, mother-to-child during pregnancy or birth)

Modes of transmission

• Common vehicle spread– Air, water, food, blood, …

• Serial transfer– Human to human, – human to animal to human, – human to environment to human in

sequence

Modes of transmission

• Airborne – respiratory disease

• Food/Waterborne (fecal-oral ) – enteric disease, polio, hepatitis A.

• Bloodborne (parental, perinatal) – hepatitis, HIV

Reservoirs

• Reservoirs – the normal habitat in which the agent lives, multiplies, and grows.– Symptomatic cases– Carriers– Animals (zoonoses)– Inanimate objects: water, food, soil, air

Iceberg concept of infection

• Iceberg concept of Infection– Tip of the iceberg

• active clinical disease– Most people are subclinical– Substantial number of exposures may not

lead to infection

DIED

Host Response

Hospitalized

Classical disease

Moderate - Mild disease

Infection without clinical illness

Asymptomatic disease

(Self-reported)

(Found only on Population Screening)

Exposure without infection

Incubation Stage

Prodrome Stage

Clinical Stage

Decline Stage Convalescent stage

Agent enters healthy body

First symptoms of disease.

Highly com-municable

Character-istic symptoms (peak)

First signs of recovery. Disease ends, becomes latent or intermittently reoccurs

Return to full health. (Recovery)

Latent period

Period in which illness is apparent

Period of communicability Carrier

Time (total period of disease)

Endemic, Epidemic, Pandemic

• Endemic: – habitual presence of disease within a

given geographic area• Epidemic:

– occurrence of disease clearly in excess of normal expectancy

• Pandemic– Worldwide epidemic

Determinants of disease outbreaks

• Amount of disease in a population– depends on:

• number of infected• number not susceptible, or immune

Herd immunity

• The resistance of a group to an attack by a disease to which a large proportion of the members of the group are immune.

• Because disease spreads from person to person - the probability of reaching a susceptible person decreases as the proportion of immune increases.

Herd immunity

• We do not have to immunize 100% of the population to be successful

• For herd immunity to work– the disease agent must be restricted to a

single host species – transmission must be relatively direct from

one member to the other

Herd immunity

• If there is an outside reservoir– herd immunity will not operate because

other means of transmission are available– infections must induce solid immunity

• Herd immunity will only work if an infected person is random

Incubation period

• The period between exposure to the agent and onset of infection (with symptoms and signs of infection).

• In the incubation, infected persons feel well and show no signs, but can transmit the disease to others.

Epidemic curve

• A graphic with x-axis: the time of onsety-axis: the number of cases developing at certain time point

• In a single-exposure common-vehicle epidemic, the epidemic curve represents the distribution of the incubation period, and the median point on the curve represents the median incubation period.

• One can estimate the time of exposure by using the median incubation period.

Epidemic curve

# ofCases

5

10

15

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 1 3 5

April May June

Date of onset for 63 cases living inOgemaw County

Attack rate (incidence rate)

• Attack rate:Number of people at risk who develop a certain illness

Total number of people at risk

• Primary case: the first person in a family (or defined group) who acquires the disease from that exposure.

• Secondary attack rate:Number of exposed people developing the disease Total number of people exposed to the primary case

• Secondary attack rate can measure the degree of spread of a disease after the disease has been introduced to a population.

Outbreak investigation1. Define the epidemic

a. Define the “numerator” (cases):1) Clinical features – is the disease

known?2) What are its serological or cultural

aspects?3) Are the causes partially understood?

b. Define the “denominator” – What is the population at risk of developing disease?

c. Calculate the attack rates

Outbreak investigation

2. Examine the distribution of cases by the following:

a. Time

b. Place – look for time-place interactions.

c. Person – examine the risk in subgroups of the affected population according to persona characteristics: sex, age, residence, occupation, social group, etc.

Outbreak investigation3. Look for combinations (interactions) of

relevant variables4. Develop hypotheses based on the following:

a. Existing knowledge of the diseaseb. Analogy to diseases of known etiology

5. Test Hypothesesa. Further analyze existing datab. Collect additional data

6. Recommend control measuresa. Control of present outbreakb. Prevention of further similar outbreaks