Legionnaires' disease outbreak investigation toolbox

Download Page

Case finding

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 identified early.
  • 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 individuals.
  • 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 from providers/institutions.

Passive case finding: case finding that relies on providers/ institutions to report cases.

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 [1]. 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

  • Review notifiable disease reports

  • Request hospitals and general practitioners to report (retrospectively or prospectively) patients who meet the case definition

  • Request laboratories to report (retrospectively or prospectively) patients who are reported as having legionellosis

  • Review accident and emergency department records

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