Lecture 2: Dynamics of Disease Transmission Reading: Gordis - Chapter 2 Lilienfeld and Stolley -...
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Transcript of Lecture 2: Dynamics of Disease Transmission Reading: Gordis - Chapter 2 Lilienfeld and Stolley -...
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