Infectious Disease Epidemiology, Module 3

Surveillance & Outbreaks

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Definition “Ongoing systematic collection, analysis,

and interpretation of health data essential to the planning, implementation, and evaluation of public health practice, as well as the timely dissemination to those who need to know”

• Centers for Disease Control. Comprehensive plan for epidemiologic surveillance. 1986

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Surveillance objectives

Monitor geographic and temporal trends Monitor occurrence

With or without intervention Planning Further study

Investigate abnormal events Prevent

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Public Health Law - Communicable Disease Control

Reporting and surveillance Investigation as to source Examination Treatment Isolation/quarantine

with or without judge’s permission order with or without a warning of an order

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Reporting Legislation requires the report

name, address, locating and demographics laboratory data clinical treatment source, contact data

Reportable by; Laboratory Health professionals

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Case definition Defined by government

As part of regulations or Act includes disease of unusually high numbers Or disease with unusual manifestations ( high

fatality) Laboratory test component Clinical Or both Ad hoc

Take an inclusive list of 80-90% of first 3-5 presenting cases at least

Be aware that agent may change, and hosts may become immune.

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Surveillance & Management Ensure the case, and contacts are properly

managed Monitor the number of cases for unusual changes

risk, manifestation, affected population, geographic area

Consists of : collection compilation analysis dissemination

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Surveillance tasks

1. Set objectives Monitor

2. Collect data Notifiable disease reports

3. Analyse data Preliminary associations by person or place

4. Disseminate To public health colleagues

5. Hypothesis formation To determine source eg, contaminated water

OUTBREAKS!!

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Outbreak steps

1. Confirm existing cases2. Verify that outbreak exists3. Describe outbreak in terms of;

Person Place Time

4. Hypothesize source, agent5. Identify populations at risk6. Test hypothesis (case control study)7. Disseminate analysis, and prevent

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Outbreak tasks

Repeat as often as necessary Compile information Draw conclusions from info (several) Form hypothesis and collect the relevant

information Test hypothesis Reformulate hypotheses

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Confirm diagnosis

False positive Normal flora?

N. meningitidis Streptococcus pyogenes

Carrier state Contamination during collection Lab. Error Multiple infections Data entry error

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If no diagnosis

Some indication of organism System affected; enteric, respiratory

Incubation period Frequency distribution of signs, symptoms

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Normal incidence?

H0 All months which can be reasonably compared with this one have similar numbers of cases

H1 All months are not similar, and some have significantly higher numbers of cases ( p<0.05)

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How many cases are normal?

Frequency distribution, monthly numbers of infectious syphilis cases, British Columbia, 1991-97

0

10

20

30

40

1 2 3 4 5 6 7

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Chart cases over time

Infectious syphilis, British Columbia Sept 1995 – 1997

0

2

4

6

8

Months

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Chart cases over time

Infectious syphilis, British Columbia, Sept 1995 – 1997

0

2

4

6

8

Months

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Advantages of threshold Defines clearly when outbreak starts

Helpful in justifying more funding Compare cases before and after start to

detect possible differences Defines when outbreak is over

Keep threshold constant during outbreak Compute thresholds for seasonal

infections Note; allow for 5% error in detecting an

outbreak will be indicated incorrectly

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Describing the epidemic

Time Period of outbreak Use diagnosis to estimate exposure

Place Geographic distribution of cases

Common venues among cases Spot maps

Person Attack rates by age Gender Race

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Identifying etiologic agent

Pathogen will narrow the range of exposure

Hypothesize (may be more than one) The people exposed to “x” will have a

higher attack rate than those not exposed, p<0.05

Χ2 tables for food

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Implementing control measures

Immunization passive, active

Destroy contaminated food Boil, chorinate, filter water Inspect Remedy Close Isolate, quarantine

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Evaluate intervention

Monitor cases Report investigation

Introduction Background Methods of studies conducted Analysis and Results Control measures Recommendations

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Investigation of a foodborne illness, Winnipeg, 1991

Bride became ill with diarrhoea Admitted to hospital June 9 Other reports of food borne illness too

Wedding June 1, 155 guests Public health inspector investigated chain of

events Obtain info on guests Menu Food preparation and transport Identify food handling practices which may

contribute

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Investigation, cont.

Medical Health Officer Requested guests to complete a

questionnaire Age, gender, address Symptoms, laboratory tests, time off work Food eaten June 1 and 2

Samples, bride + 8 others Leftover food tested

Salmonella, B. cereus, Campylobacter

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Results, chain of events

Roast turkey Dressing Mashed potatoes Corn Cabbage rolls Meatballs Coleslaw plain& creamy Mushroom gravy Rolls Cranberry sauce Black forest cake Tea, coffee, milk

Baker Boy catering cooked in Selkirk

Transported to venue 05:30 pm

Served 6:30 pm?? Turkey served later

Meal over at 8:30 Leftovers in containers for

family Only coleslaw refrigerated

due to space Late night meal 22h00 Leftovers served next day

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Onset Histogram, Winnipeg, 1991, food borne illness

0

2

4

6

8

10

12

14

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Age specific attack rate

01

2

3

45

6

7

89

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Gender

ILL (%) WELL TOTAL

FEMALES 12 (19.4)

50 62

MALE 15 (32.6)

31 46

TOTAL 27

(25.0)

81 108

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Symptoms

Cases, n=31 Percent

Diarrhoea 27 84.4

Weakness 21 65.6

Abdominal pain 20 62.5

Anorexia 20 62.5

Nausea 15 46.9

Fever 13 40.6

Headache 13 40.6

Vomiting 6 18.8

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Χ2 tables for menu items

Coleslaw dressed

ILL (%) WELL TOTAL

Yes 13

(13.2)

28 41

No 14

(14.3)

41 55

TOTAL 27

(28.0)

69 98

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Laboratory results

Salmonella hadar in individuals Leftover corn Salmonella hadar

Avian type Maximum 72 hour incubation period Why did the corn test positive? One of the 5 people who were ill before

the wedding was +ve for S. hadar

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Control measures

Restricting preparation of turkey to one area

Only certain staff to prepare meat All raw meat processed at one time Restricting sets of implements to only

meat processing Correct food storage

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Summary

Verify the diagnosis Compare numbers of cases with normal Describe the outbreak by;

Person Place Time

Develop hypotheses of etiologic agent, test Control Evaluate

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References

Teutsch, SM and Churchill, RE, eds. Principles and practice of public health surveillance. Oxford University Press, 1994

Morbidity and Mortality Weekly Report. Supplement, December 1992. Proceedings of the 1992 International Symposium on Public health surveillance. Vol 41.

Thacker SB, Berkelman RL. Public health surveillance in the United States. Epidemiologic Reviews 1988;120:164-188