Transcript lect2(091703)

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
Not to be confused
with person, place,
time
VECTOR
Environment
Agent
Characteristics of
host/agent/environment
Host
Agents
Environment
Age
Sex
Biologic
Chemical
Temperature
Humidity
Religion
Culture
Occupation
Physical
Nutritional
Altitude
Crowding
Housing
Genetic profile
Previous diseases
Immune Status
Water
Air pollution
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
Host Response
DIED
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
Agent
enters
healthy
body
First
symptoms
of disease.
Characteristic
symptoms
(peak)
First signs of
Return to full
recovery.
health.
Disease ends,
(Recovery)
becomes latent
or
intermittently
reoccurs
Highly
communicable
Latent
period
Convalescent
stage
Period in which illness is apparent
Period of communicability
Time (total period of disease)
Carrier
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 onset
y-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
15
# of
Cases
Date of onset for 63 cases living in
Ogemaw County
10
5
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
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 investigation
1. 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 investigation
3. Look for combinations (interactions) of
relevant variables
4. Develop hypotheses based on the following:
a. Existing knowledge of the disease
b. Analogy to diseases of known etiology
5. Test Hypotheses
a. Further analyze existing data
b. Collect additional data
6. Recommend control measures
a. Control of present outbreak
b. Prevention of further similar outbreaks