Once a case definition has been developed, it will be used to identify and count cases in the
population at risk of developing the disease.
Case finding is undertaken for the following reasons:
- To ensure that the true scale of the outbreak has been recognised. This is very important
for determining the amount of resources that should be allocated to the investigation.
- To be able to start refining the initial case definitions
- To minimise bias that could potentially result from an investigation focusing only on cases
- To provide more statistical power to identify risk factors.
- Aids development of appropriate control measures by defining the population with the
exposure of interest.
Note: not every single case is needed to obtain useful information in an
outbreak. However, it is important to identify as many cases as possible and to avoid
identifying only a subgroup of the cases because they may not be representative of all cases.
Information to collect during the case finding process
The information collected depends on the outbreak, but it can be grouped into four categories:
Identifying information enables the investigator to obtain further details about
the case or to tell the case about the investigation.
Demographic information enables the investigator to describe at-risk group(s) of
Clinical information allows the investigator to verify that the case definition
has been met, to characterise the disease, and to create an epidemic curve.
Risk factor information allows the epidemiologist to focus the investigation.
The relevant risk factor information depends on the nature of the outbreak.
Active and passive case finding
Active case finding: case finding in which the investigator collects case information
Passive case finding: case finding that relies on providers/ institutions to report
Without this step, inappropriate control measures may be implemented. However, an investigation
should not be excessively slowed down while trying to find every last case.
For point source outbreaks, case finding can often be tightly focused on the event itself
by locating a list of individuals who attended the event associated with the
outbreak . If a list of attendees is unavailable, contacting the event
organiser(s) might be useful in obtaining further details. If neither approach is productive,
cases identified so far could be interviewed for names of other individuals attending the
event. This may be particularly useful for performing a cohort study. Other case finding
strategies depend on the type of the outbreak. It may be useful to:
Interview family contacts of cases
notifiable disease reports
Request hospitals and general practitioners to
report (retrospectively or prospectively) patients who meet the case
Request laboratories to report (retrospectively
or prospectively) patients who are reported as having legionellosis
Review accident and emergency department
1. DEN BOER J. W, et al (2002) A large outbreak of
Legionnaires' disease at a flower show, the Netherlands, 1999 Emerging Infectious Diseases
8(1), pp.37-43 http pdf