Dynamics of Disease Transmission (4-5).ppt

Nursing lecture notes epidemiology

1. Dynamics of Disease
Transmission
Muhammad Tahir,MPH,MSc

2. Objectives
•At the end of lecture students will be able
to :
–Understand iceberg phenomenon of disease
–Know definitions of important terms i.e.
epidemic, endemic and pandemic
–Explain herd immunity
–Understand steps of outbreak investigation

3. Spectrum of disease
•The idea that an exposure can lead to
varying signs, symptoms and severity of
the same disease in the population is the
spectrum of disease.
•Why do we have varying degrees of
severity?
•The outcome will depend on the
interactions of host, agent and
environmental factors.

4. Factors Affecting Disease Transmission
Host
Vector
Agent Environment
-- Susceptibility
-- Immune response
-- Resistance
--- Portal(s) of entry
-- Virulence Toxigenicity
-- Infectivity Resistance
-- Pathogenicity Antigenicity
VECTOR
-- Prevalence
-- Portal(s) of entry
-- Balance of immune to
susceptible individuals
--- Opportunity for
exposure (e.g. crowding)

5. Routes of transmission

6. Timeline for Infection
I
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f
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c
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Susceptible
Susceptible
Dynamics of
infectiousness
Dynamics of
disease
Incubation
period
Symptomatic
period
Non-diseased
Latent
period
Infectious
period
Non-infectious
I
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Time
Time

7. Transmission
Cases
• Index – the first case identified
• Primary – the case that brings the
infection into a population
• Secondary – infected by a primary case
• Tertiary – infected by a secondary case

8. Classification of diseases according to Classification of diseases according to
spectrumspectrum
Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS
(Low pathogenicity and low virulence)

9. Examples: Measles, Chickenpox
(High pathogenicity & low virulence)
Examples: Rabies, Hemorrhagic fevers caused by Ebola Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg and Murberg
viruses.viruses.
(High pathogenicity & high virulence)(High pathogenicity & high virulence)

10. Iceberg Phenomenon
•Cases of illness correctly diagnosed by clinicians in the community
often represent only the “tip of the iceberg.”
•Many additional cases may be too early to diagnose or may remain
asymptomatic.
•Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.
•The risk is that persons with in-apparent or undiagnosed infections
may be able to transmit infection to others.

11. Iceberg Phenomenon

12. Iceberg Phenomenon--application
•Persons with in-apparent or undiagnosed infections can
transmit infections to others.
•Control measures must not be directed solely for
clinically apparent cases.
•Control measures must be directed toward all infections
capable of being transmitted to others;
–both clinically apparent cases and
–those with in-apparent or undiagnosed infections.

13. Important terms
Reservoir
•A host that carries a pathogen without injury to
itself and serves as a source of infection for
other host organisms (asymptomatic infective
carriers)
Vectors
•A host that carries a pathogen without injury to
itself and spreads the pathogen to susceptible
organisms (asymptomatic carriers of pathogens)

14. Endemic
•The constant presence of a disease or infectious
agent within a given geographic area or
population group; may also refer to the usual
prevalence of a given disease within such area
or group.
Epidemic
•The occurrence of more cases of a disease than
expected in a given area or among a specific
group of people over a particular period of time.

15. Pandemic
•An epidemic occurring over a very wide
area (several countries or continents) at
the same time and usually affecting a
large proportion of the population.
e.g. Influenza, cholera
Outbreak
•A more or less localized epidemic affecting
large number of a group, in the community
e.g. outbreak of food poisoning

16. Sporadic
•Cases occur irregularly, haphazardly from
time to time and generally infrequently.
•Cases are few and separated widely in
space and time showing no connection to
each other.

17. Endemic-epidemic-pandemic
Endemic
Transmission occur, but the number of cases remains constant
Epidemic
The number of cases increases
Pandemic
When epidemics occur at several continents – global epidemic
Time
C
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s

18. Epidemic vs Endemic
Time
Endemic
Epidemic
N
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b
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o
f

C
a
s
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o
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a

D
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a
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19. Herd effect (Herd immunity)
Immunised individuals provide indirect protection to
susceptible (unvaccinated, partially vaccinated)
individuals:
–fewer people infected
–lower shedding by vaccinees












20. Herd effect (Herd immunity)
•Most mass vaccination provides herd
immunity
•Protection occurs even when vaccination
coverage is less than 100% of the population
•The greater the infectivity (reproductive rate)
of a disease, the higher the immunisation
rate needed to achieve herd immunity

21. What is an outbreak ?
•Occurrence of more cases of disease than
expected
–in a given area
–among a specific group of people
–over a particular period of time

22. Food-or waterborne outbreak
(WHO definition)
•two or more persons
•similar illness
•after ingestion of the same type of food
or water
•from the same source
•epidemiological evidence - the food or
the water - the source of the illness

23. Why investigate outbreaks?
•Stop the outbreak
–Find and neutralise the source (cause)
–Prevent additional cases
•Prevent future outbreaks
•Improve surveillance and outbreak detection
•Improve our knowledge
•Keep the public’s confidence
•Training

24. Specific demands
when investigating outbreaks
•Unexpected event
• Act quickly
• Rapid control
• Bias caused by media reports
• Legal and financial pressure
• Interdisciplinary coordination
• Work carried out in the field
Systematic approach

25. Steps of an outbreak
investigation
•Confirm outbreak and diagnosis
•Define a case
•Identify cases & obtain information
•Describe data collected and analyse
•Develop hypothesis
•Test hypothesis: analytical studies
•Special studies
•Communicate results,
–including outbreak report
•Implement control measure
C
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m
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26. Detection
Routine surveillance
Clinical / Laboratory
General public
Media

27. Confirm outbreak and diagnosis
Is this an outbreak?
•More cases than expected?
•Surveillance data
•Surveys: hospitals, labs, physicians
Caution!
•Seasonal variations
•Notification artefacts
•Diagnostic bias (new technique)
•Diagnostic errors (pseudo-outbreaks)

28. Cases of legionellosis by week of notification
France, January 1996 - August 1997
0
1
2
3
4
5
6
7
8
147101316192225283134374043464952
1996 Week of notification 1997
3

29. Cases of legionellosis by week of notification
France, January 1996 - August 1997
0
1
2
3
4
5
6
7
8
147101316192225283134374043464952
National meeting:
legionellosis diagnosis and
reporting

30. Confirm outbreak and diagnosis
•Laboratory confirmation
–serology
–isolates, typing of isolates
–toxic agents
•Contact (visit) the laboratories
•Meet attending physicians
•Examine some cases
Not always necessary to confirm all the
cases
but confirm a proportion
throughout the outbreak

31. Outbreak confirmed 
Further
investigation?
Immediate control
measures?
- aetiological agent
- mode of transmission
- vehicle of transmission
- source of contamination
- population at risk
- exposure causing illness
- prophylaxis
- exclusion / isolation
- public warning
- hygienic measures
- others

32. Outbreak confirmed,
further investigations warranted
Form Outbreak
Control Team?
Team coordinates
field investigation
Epidemiologist
Microbiologist
Clinician
Environmentali
st
Engineers
Veterinarians
Others

33. Descriptive epidemiology
- Who are the cases? (person)
- Where do they live? (place)
- When did they become ill? (time)

34. Case definition
•Simple, practical, objective
•Sensitive?
•Specific?
•Multiple case definitions
–confirmed
–probable
–possible

35. Identify &
count cases
notifications
laboratories
hospitals, GPs
schools
workplace,
cases,
media, etc

36. Identify &
count cases
Obtain
information
Identifying information
Demographic information
Clinical details
Exposures and known
risk factors

37. Identify &
count cases
Obtain
information
Analysis of
descriptive data
Describe in
- time
- place
- person

38. Time
Epi Curve
•Histogram
•Distribution of cases by time of onset of symptoms, diagnosis or
identification
–time interval depends on incubation period
Cases
0
1
2
3
4
5
6
7
8
9
10
1 2 3 4 5 6 7 8 9101112
Days

39. Epicurves
0
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4
5
6
1 2 3 4 5 6 7 8
0
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9
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1 2 3 4 5 6 7 8 9101112
0
2
4
6
8
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12
12345678910111213
Common point source Common persistent source
Propagated source
cases
cases
cases
hours
days
weeks

40. Epi curve
•Describe
–start, end, duration
–peak
–importance
–atypical cases
•Helps to develop hypotheses
–incubation period
–etiological agent
–type of source
–type of transmission
–time of exposure

41. Place
•Place of residence
•Place of possible exposure
–work
–meals
–travel routes
–day-care
–leisure activities
•Maps
–identify an area at risk

42. Person
•Distribution of cases by age, sex, occupation,etc (numerator)
–60 female
–50 male
•Distribution of these variables in population (denominator)
–600 females
–350 males
•Attack rates
–female: 60/600
–Males: 50/350

43. Develop hypotheses
- Who is at risk of becoming ill?
- What is the disease?
- What is the source and the vehicle?
-What is the mode of transmission?

44. Compare hypotheses
with facts
Test specific hypotheses
Analytical studies
- cohort studies
- case-control studies

45. Testing hypothesis
•Cohort
–attack rate exposed group
–attack rate unexposed group
•Case control
–% of cases exposed
–% of controls exposed

46. Verify hypothesis
Special investigations/studies
• Microbiological investigation
• Environmental investigation
• Veterinarian investigation
• Trace back investigations (origin of foods)
• Meteorological data
• Entomological investigations

47. Implement control measures
1) Control the source of pathogen
2) Interrupt transmission
3) Modify host response
At first, general measures
According to findings,
more specific measures
May (must) occur at any
time during the outbreak!!

48. Outbreak report
•Regular updates during the
investigation
•Detailed report at the end
–communicate public health messages
–influence public health policy
–evaluate performance
–training tool
–legal proceedings

49. Steps of an outbreak
investigation
•Confirm outbreak and diagnosis
•Define case
•Identify cases and obtain information
•Descriptive data collection and analysis
•Develop hypothesis
•Analytical studies to test hypotheses
•Special studies
•Communication,
including outbreak report
•Implement control measures
c
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50. References
•http://www.dorak.info
•Tayal,S. Assistant professor King Saud
University
•Leon Gordis. Epidemiology (3
rd
ed.)