Week 8- Communicable diseases: Outbreaks Flashcards
What is an outbreak?
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.
Why do outbreaks occur?
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)
Why do an outbreak investigation?
PRIMARY AND SECONDARY GOALS
Primary goals: –IDENTIFY pathogen or source –CONTROL (i.e. stop new cases) –PREVENT future outbreaks Secondary goals: –Learn more about a disease –Training opportunity
What Factors that impact our process and decision-making?
case numbers
disease severity
risk to others
public / political / legal concerns
What are the Steps of the outbreak investigation?
10 steps
- Prepare for field work
- Verify the existence of the outbreak
- Confirm the diagnosis
- Define and identify cases
- Describe / orient data (Person, Place, Time)
- Develop hypotheses
- Evaluate hypotheses
- Refine hypothesis / perform additional studies
- Implement control and prevention measures
- Communicate findings
Step 1. Prepare for field work
What does it include?
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
Step 2. Verify existence of outbreak
Defined as “more cases than usual”
How to find out What is “usual”?
– Must have access to “baseline” data
– Specific to region
– Specific to current time
How to find out Where are the cases?
– Surveillance: monitoring of disease in populations
– Different types of surveillance: passive, active,
sentinel (targeted) surveillance
What are the source of information for verifying the existence of outbreak?
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.
What are Some causes of false alarms?
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
Step 3. Confirm the diagnosis
How does this happen?
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
Step 4. Define and identify cases
Case definition is important and can change
Sensitive vs. Specific
What are the components of case definition?
– 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
Cases divides into
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
How does Case finding occur?
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
What are some Barriers to case detection?
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
Step 5. Describe & orient the data
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
Epi curves:
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
Steps 6-8: Develop / test / refine hypothesis
What is the source of the infection?
Case-control or cohort study?
Other investigations?
Case-control study
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)
Cohort study
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
Step 9: Control and prevention
What to do ?
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
Step 10. Communicate findings
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
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
The usual disease patterns and reporting practices
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
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
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
The incubation period
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
A disease outbreak
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
Define a case
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
Implicate the food most eaten by those with symptoms
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%
5%
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
1
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?
5
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
81
