OutbreakI.ppt PowerPoint presentation ppt

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1. Introduction to
Outbreak Investigation
Epidemiology

2. What is Epidemiology?
•“Epidemiology is the study of the
distribution and determinants of
health-related states or events in
specified populations, and the
application of this study to the
control of health problems.” -CDC

3. What is Epidemiology?
•Science of Public Health that studies:
–Distribution of disease
–Determinants of health/disease
–Specific populations
•Look for patterns of disease
–Time, place, personal characteristics
•Interventions
–Prevention is key

4. What does Epidemiology study?
•Just about anything
•Health-related
•Infectious disease
•Chronic disease

5. How is Epidemiology Used?
•Population/community health
assessment
•Personal decision-making
•Complete clinical picture
•Evaluate interventions
•Search for cause
–Exposure and relationship to disease
–Outbreak investigation

6. Epidemiologic Information
•Case definition
•Person
•Place
•Time

7. Case Definition
•Standard set of criteria
•Clinical and lab
•Allows for comparison
•Sensitive vs. Specific

8. Case Definition
•Smallpox
–Clinical Description
•An illness with acute onset of fever >101 °F followed by a
rash characterized by vesicles or firm pustules in the same
stage of development without other apparent cause.
–Laboratory Criteria for Confirmation
•Isolation of smallpox (variola) virus from a clinical specimen,
or
•Polymerase chain reaction (PCR) identification of variola
DNA in a clinical specimen, or
•Negative stain electron microscopy (EM) identification of
variola virus in a clinical specimen (Level D laboratory or
approved Level C laboratory)

9. Suspected Case of Smallpox
•A case that meets the clinical case
definition but is not laboratory confirmed
and does not have an epidemiological link
to a confirmed or probable case of
smallpox, OR a case that has an atypical
presentation that is not laboratory
confirmed but has an epidemiological link
to a confirmed or probable case of
smallpox. Atypical presentations of
smallpox include a) hemorrhagic lesions OR
b) flat, velvety lesions not appearing as
typical vesicles nor progressing to pustules.

10. Probable Case of Smallpox
•A case that meets the clinical case
definition that is not laboratory
confirmed but has an epidemiological
link to another confirmed or probable
case.

11. Confirmed Case of Smallpox
•A case of smallpox that is laboratory
confirmed.

12. Case Definition Gradient
Suspected ProbableConfirmed
Low Specificity High Specificity

13. Working Case Definition
•Smallpox Outbreak
- Anyone who meets original case
definition
- Anyone with fever (>101 °F ) or rash who
was in a confirmed exposed area during
the BT event or came in contact with a
confirmed or probable case should be
considered a case.

14. Person
•Age
•Sex
•Race/Ethnicity
•Socio-Economic Status
•Behaviors

16. Place
•Geographic Distribution
–Natural
–Clustering vs. uniform
•Home
•Work
•School
•Hospital room

17. Geographic Distribution of LaCrosse

18. “Place” in LaCrosse Case Investigation
•Neighborhood
•Home visits
•Mosquito Breeding Sites
–Tires
–Pots
–Standing Water

19. -







-







Clustered
Scattered

20. Time
•Onset of symptoms
•Incubation Period
•Infectious Period
•Seasonality
•Baseline vs. epidemic
•Interval
•Long-term trends
•Shorter for environmental exposure

22. Measles Outbreak
•Baseline
•Normal occurrence
•Onset
•Incubation period
•Infectious period
•Mode of transmission

23. Time in Outbreak investigation
Visited health dept.Visited health dept.
Conference in NairobiConference in Nairobi
HomeHome
Rash onsetRash onset
ER VisitER Visit
Church activitiesChurch activities
Health dept. notifiedHealth dept. notified
IgM resultsIgM results
InterventionsInterventions
SeptemberSeptember
26261919
AugustAugust
22 99 16162828
JulyJuly

24. Measles
Southwestern VA, 1999
Measles
Southwestern VA, 1999
0
1
2
3
4
5
Date of Onset
C
a
s
e
s

September October
Second generation case
Index case
First generation case
Third generation case

25. Incidence
•Number of new events occurring in a defined
population during a specified period of time
•Incidence =
New cases/population at risk/time
•Used to measure current disease activity
•Allows comparison between areas with
different populations

26. Contrasts with Prevalence
•Prevalence is the number of new and
existing cases divided by the total
population (can be during a period of time
or at a given point)
•Prevalence =
(New cases + existing cases)/Total population
-Can be expressed as a percent
-Can give a picture of disease burden within a
population

27. Information for any outbreak
•Symptom onset date
•Symptoms present and agent if known
•Suspected exposure date if known
•Residence
•Age
•Gender
•Laboratory testing
•Outbreak Case Definition
•Organized case information in a line list

28. Exposure
•Possible Cause of illness
•Know your agent/disease
–Clinical picture
–Pathogenesis
–Mode of transmission
–Natural Reservoir
–Common Vehicle or Vector

29. Chain of Infection

30. Transmission
•Direct
–Contact
–Droplet
•Portal of Exit
•Portal of Entry
•Indirect
–Airborne
–Vehicle
–Vector
•Mechanical vs.
biologic

31. Types of Outbreaks
•Propagated
–Indicative of person to person
transmission
•Point-source
–Indicative of a common exposure to a
contaminated vehicle or reservoir

32. Measles
Southwestern VA, 1999
Measles
Southwestern VA, 1999
0
1
2
3
4
5
Date of Onset
C
a
s
e
s

September October
Second generation case
Index case
First generation case
Third generation case

33. Figure 3. Onset of Illness in Wilsonburg Grade School
0
2
4
6
8
10
12
14
16
18
20
12pm
5/1
6pm 12am
5/2
6am 12pm 6pm 12am
5/3
6am 12pm 6pm
Date and Time of Onset in 6 Hour increments.
N
u
m
b
e
r

o
f

C
a
s
e
s
Sick

34. Analytic Epi Studies
•Associations between exposure and
disease
•Experimental
•Observational
–Cohort
–Case Control
–Cross-sectional

35. Cohort Study
•Objective: In a population of
individuals initially free of disease,
determine the risk factors associated
with development of disease.
–Select Population
–Categorize individuals based on presence
or absence of select risk factors
–Follow individuals to determine which
develop disease
•Good for rare exposures

36. Case-Control Study
•Objective: Compare risk factors in diseased
and non-diseased individuals to determine
possible associations.
–Select population
–Select cases that meet the disease case definition
–Select non-diseased individuals from the
population to act as controls
–Gather previous exposure/risk factor histories
from both groups
•Many outbreak investigations use this study
design
•Good for rare diseases

37. John Snow and Cholera
•“Broad Street Pump”
–1854 cholera epidemic in London
–Looks at cholera deaths
–Geographical mapping
•Where do cases live?
•Where do cases work?
–Water source

39. Use of Two-By-Two Tables
•Calculate association between
disease and exposure
dc
ba
NTotal
Unexposed
Exposed
TotalWellIll
ad/bc = Odd Ratio  Relative Risk

40. Interpreting Measures of Association
•Odds ratios and risk ratios measure the
degree of relatedness of an exposure
and a health event (an outcome)
•The farther away the OR/RR is from 1,
the more we would say the exposure
and outcome are associated
•Confidence intervals and p-values help
to determine if association is due to
chance

41. OR = 12 x 17/2 x 5
= 204/10
OR = 20.4
People who ate from menu A were about 20 times more
likely to have diarrhea than those that did not eat from
menu A.

42. More OR interpretation
•OR = odds of exposure among ill
odds of exposure among well
•If OR(RR) > 1, then the exposure is a risk
factor for being ill
•If OR < 1, then the exposure is
protective of illness
•If OR = 1, then there is no association
between exposure and illness
•OR can NEVER be negative!

43. P-value
•Used in LOTS of statistical tests
•A guide to tell us that a result is
“significant”
•Generally at the 95% level
•p < 0.05 = Significant at the 95% level =
there is a 95% probability that the result
is accurate (not by chance)
•Example: OR=4, p-value=0.01
•Example: OR=15, p-value=0.36

44. 95% Confidence Intervals
•Used with risk ratios and odds ratios
•Tell us about both precision and accuracy
•With an OR or RR we have estimated the
magnitude of the association – 95%
confidence intervals tell us that we can be
95% sure that the true association is
somewhere in that interval
•Example: OR = 7 95%CI= (5.2, 8.8)
•Example: OR = 7 95%CI= (0.4, 18.7)

45. Strength of Association
•Magnitude of Odd Ratio
•95% Confidence Interval or P-value
•Be careful
–Plausibility
–Confounding

46. Attack Rate
AR = # of people who became illx10
n
# of people at risk
–People at risk could be those at a party,
in a class, on a cruise ship…
–Usually express AR as a %, so n = 2

47. Secondary Attack Rate
•Measure of frequency of new cases
among contacts of known cases
# cases among contacts of primary
cases SAR = during the period
total # of contacts

49. The calculations
•# of children in day care center = 70
•# of ill children at day care center = 7
•# of contacts (of those 7) at home = 25
•# of ill contacts = 5
•AR = 7/70 = 0.1 x 100 = 10% AR
•SAR = 5/25 = 0.2 x100 = 20% SAR

50. Summary
•Case definition, person, time, place
•Know disease/agent
•Recognize Point-Source vs.
Propagated
•Set up analytic epi study
•Measures of association