Week 8- Communicable diseases: Outbreaks

Question 1

What is an outbreak?

Answer 1

An outbreak or epidemic exists when there are
more cases than expected of a particular disease among a specific group of people and/or in a
given area, over a particular period of time.

Question 2

Why do outbreaks occur?

Answer 2

 New appearance of an infectious agent (or toxic
material) from the environment or from an
infected source
 Arrival of susceptible people to an environment
with an endemic pathogen
 Introduction of effective route of transmission
from source to susceptible population (e.g.
climate change bringing new mosquito species)

Question 3

Why do an outbreak investigation?

PRIMARY AND SECONDARY GOALS

Answer 3

Primary goals:
 –IDENTIFY pathogen or source
 –CONTROL (i.e. stop new cases)
 –PREVENT future outbreaks
Secondary goals:
 –Learn more about a disease
 –Training opportunity

Question 4

What Factors that impact our process and decision-making?

Answer 4

 case numbers
 disease severity
 risk to others
 public / political / legal concerns

Question 5

What are the Steps of the outbreak investigation?

10 steps

Answer 5

1. Prepare for field work
2. Verify the existence of the outbreak
3. Confirm the diagnosis
4. Define and identify cases
5. Describe / orient data (Person, Place, Time)
6. Develop hypotheses
7. Evaluate hypotheses
8. Refine hypothesis / perform additional studies
9. Implement control and prevention measures
10. Communicate findings

Question 6

Step 1. Prepare for field work

What does it include?

Answer 6

Epidemiological knowledge & homework
 What are the usual risk factors? Previous outbreaks?Supplies & equipment
 Competent laboratory
 Portable computer, phone, supplies
Team composition
 Who should be in team? Who is in charge?
 Statistical advisors, public relations, local information
Administrative
 Transport and communication
 Travel, cash, credit cards
 Immunization, prophylaxis
Personal

Question 7

Step 2. Verify existence of outbreak

Answer 7

 Defined as “more cases than usual”

Question 8

How to find out What is “usual”?

Answer 8

– Must have access to “baseline” data
– Specific to region
– Specific to current time

Question 9

How to find out Where are the cases?

Answer 9

– Surveillance: monitoring of disease in populations
– Different types of surveillance: passive, active,
sentinel (targeted) surveillance

Question 10

What are the source of information for verifying the existence of outbreak?

Answer 10

 Sources of information
 Surveillance records
 Local / state health departments
 Other registries
– hospital discharge records
– death records
– cancer or birth defect registries
 Clinicians
 Laboratories
 Child-care centers / schools / clubs etc. etc.

Question 11

What are Some causes of false alarms?

Answer 11

 Change in surveillance/policy of reporting
 Change in case definition
 Improved diagnosis
– New test, increased GP awareness, new doctor in town
 False positives
– All diagnostic tests have some errors
 Increased public awareness
– Media coverage leading to demand for tests
 Increased reporting
– New staff in department, “performance” incentives

Question 12

Step 3. Confirm the diagnosis

How does this happen?

Answer 12

 Review clinical findings
 Review laboratory results of cases
 Be as specific as possible about the causative
agent
– “Outbreak strain” of bacteria / virus
– May need specialised laboratory input
 Talk to cases

Question 13

Step 4. Define and identify cases

Answer 13

 Case definition is important and can change

Sensitive vs. Specific

Question 14

What are the components of case definition?

Answer 14

– Components include
 Clinical features (e.g. gastroenteritis / bloody diarrhoea)
 Person – who do you want to include / exclude
 Place – place of residence, dined in a particular restaurant
 Time period of interest
 Laboratory results

Question 15

Cases divides into

Answer 15

Divide into
 Confirmed- usually laboratory confirmed
 Probable - laboratory results not available-Suspected case evaluated by a clinician
 ± Possible – often less stringent clinical criteria
Suspected
Non-case Suspected or probable case with negative lab result

Question 16

How does Case finding occur?

Answer 16

 Use as many sources as you can
 Health care facilities and laboratories
 Alert the public and ask cases to identify
themselves
 Identify people in place of interest at time of
potential exposure and directly contact them

Question 17

What are some Barriers to case detection?

Answer 17

 Lack of public understanding
 Lack of communication with the public
– On existence of outbreak
– On role of imported health care workers
 Community distrust
 Lack of inclusive planning for outbreak
resolution

Question 18

Step 5. Describe & orient the data

Answer 18

 TIME: Draw an epidemic curve
– Number of cases on y-axis
– Time on x-axis
– May provide hints about source of outbreak
 PLACE: Map the data
 PERSON: Explore data according to personal
characteristics

Question 19

Epi curves:

Answer 19

Common point source outbreak: Steep upslope,more gradual downslope
Continuous extended source epidemics: Sharp up-swing followed by a plateau that corresponds to the
duration of exposure, then a right tail
Propagated or progressive outbreak: Transmitted
person to person, leading to serial transmission
 A series of progressively taller peaks one incubation period apart

Question 20

Steps 6-8: Develop / test / refine hypothesis

Answer 20

 What is the source of the infection?
 Case-control or cohort study?
 Other investigations?

Question 21

Case-control study

Answer 21

 Compute an odds ratios of exposure (estimate
strength of association between source and
disease)
 Case control good if:
 Clearly defined single outcome, rare disease
 Large population at risk of disease
 Multiple exposures to be examined
 “Fishing expedition” – need to be wide-ranging
 Potential problems with case-control:
 Choice of appropriate controls
 Recall bias for cases (incentive to remember)

Question 22

Cohort study

Answer 22

 Follow up a group, exposed & unexposed
 Can be used if source is known or strongly
suspected so that all exposed individuals can be
identified
 Can calculate attack rates for exposed and
unexposed (to various exposures if necessary),
and compute relative risks of getting disease
in exposed vs. unexposed

Question 23

Step 9: Control and prevention

What to do ?

Answer 23

 Once source of outbreak has been identified,
aim to break links in chain of infection
 Do as soon as possible
 Examples
Remove contaminated food
Sterilise contaminated water
Spray mosquitoes / destroy breeding sites
Isolation / treatment of cases
Immunise contacts / susceptibles
Prophylactic medication
Behaviour modification

Question 24

Step 10. Communicate findings

Answer 24

Must occur throughout outbreak investigation
 Investigation team / Partners / Press / Public
Prepare timely reports
 Initial oral briefing (at time leaving the field)
 Final written report
 ±Publications
Clearly state facts
Beware of legal implications
Aim to improve practice

Question 25

Cases of the ‘flesh eating’ group A streptococcal disease are reported in a defined population. Which of the following types of information would be most helpful for determining whether these cases represent a disease outbreak?

Select one:

a. The clinical features and methods of diagnosis
b. The disease vector and reservoir
c. The exact location and timing of disease onset
d. The incubation period and pattern of disease transmission
e. The usual disease patterns and reporting practices

Answer 25

The usual disease patterns and reporting practices

Question 26

An outbreak of disease should be reported to the local health department if:

Select one or more:

a. If the outbreak involves more than 10 people
b. If the diagnosis is uncertain
c. If the disease is infectious
d. If the disease is serious

Answer 26

a. If the outbreak involves more than 10 people
b. If the diagnosis is uncertain
c. If the disease is infectious
d. If the disease is serious

Question 27

The time interval between entry of an infectious agent into a host and the onset of symptoms is called:

Select one:

a. The noncontagious period
b. The preinfectious period
c. The communicable period
d. The decubation period
e. The incubation period

Answer 27

The incubation period

Question 28

The following paragraph refers to questions 4-6.

A politician invites 16 friends to a business dinner. Within 24 hours, 11 of the 17 diners experience abdominal pain, vomiting and diarrhoea. The host feels fine. Of the 11 symptomatic guests, 4 have fever and 7 do not; 5 have an elevated white blood cell count and 6 do not; 6 ate prawns and 5 did not; 9 ate salmon mousse and 2 did not; and 1 goes on to have surgery for acute cholecystitis resulting from an impacted bile duct. Of the 11 symptomatic guests, 10 went on to recover fully within 3 days (apart from the guest who ended up having surgery and recovered over a longer period). The guests had not shared any other meals together recently.

Select one:

a. A disease outbreak
b. Should be investigated by the Victorian Department of Human Services
c. Not an outbreak, because the usual pattern of disease is unknown
d. A coincidence until proved otherwise
e. Attributable to bacterial infection

Answer 28

A disease outbreak

Question 29

An early priority for the investigator would be to:

Select one:

a. Perform a case-control study
b. Define a case
c. Perform stool samples
d. Close the kitchen indefinitely
e. Submit food samples to the laboratory

Answer 29

Define a case

Question 30

The investigators were unable to identify a single food that was eaten by every symptomatic guest. Therefore, they should:

Select one:

a. Implicate the food least eaten by those without symptoms
b. Abandon the investigation, since the disease is not very serious
c. Conclude the investigation but without identifying the source
d. Conclude that the disease was not food-borne
e. Implicate the food most eaten by those with symptoms

Answer 30

Implicate the food most eaten by those with symptoms

Question 31

During an 8-hour shift at a corporate office building, 30 employees (20 females and 10 males) visited the company’s physician with complaints of nausea, vomiting, headaches and dizziness. All affected individuals responded to supportive treatment and were sent home.

If 600 persons worked in the building, what was the attack rate?

Select one:

a. 15%
b. 12%
c. 20%
d. 5%
e. 30%

Answer 31

5%

Question 32

An outbreak of Ebola haemorrhagic fever occurred in Uganda in late 2000:

The distribution of dates of onset was from September 18 through to October 28, with the following numbers of cases in successive weeks of the outbreak: 3, 3, 4, 4, 9, 3, 1. The sentinel cases for this outbreak most likely occurred in which week?

Select one:

a. 3
b. 7
c. 5
d. 9
e. 1

Answer 32

1

Question 33

The distribution of dates of onset was from September 18 through to October 28, with the following numbers of cases in successive weeks of the outbreak: 3, 3, 4, 4, 9, 3, 1. The sentinel cases for this outbreak most likely occurred in which week?

The incidence rate was probably highest in which week?

Answer 33

5

Question 34

The distribution of dates of onset was from September 18 through to October 28, with the following numbers of cases in successive weeks of the outbreak: 3, 3, 4, 4, 9, 3, 1. The sentinel cases for this outbreak most likely occurred in which week?

A total of 22 patients died. The case-fatality rate (in %) was

Answer 34

81