Descriptive epidemiology

Initial information will be obtained by interviewing the cases using a trawling questionnaire. The cases should be described by time, place and person.

Describing cases by time involves plotting an epidemic curve and clarifying whether the outbreak is due to a point source release (single release over a small time period) or a continued exposure to an ongoing source. The time unit is usually one day, but as more cases are identified the control team might wish to use a unit of 2 or 3 days (aggregating to larger numbers might mean that contextual information or patterns relating cases together might be lost). Consider using one small square to represent each case, initially and whilst the outbreak under investigation is not too large, perhaps writing the number of each case from the line listing of cases in the square. This makes it easy to go back and forth between the list and the curve.

Describing cases by person includes demographic details, clinical features, and pre-existing co-morbidities.

Describing cases by place includes home/work address, travel history and places where exposure may have occurred. The most critical information aiding identification of the source of infection is a clear history of exposure for the two week period prior to the onset of illness, from the patient, relatives or friends (using a trawling questionnaire). The full address and postcode of place of residence, place of work and details of travel (with overnight stays) should be obtained. In addition, details of visits to, or overnight stays in, hospital should be ascertained, as well as information on other potential common sites and exposures to Legionella. These include exposure to industrial or commercial wet cooling systems, whirlpool spas in domestic, leisure, retail or commercial settings, and showers and respiratory equipment in hospital or domestic settings.

A detailed epidemiological description of the cases may provide investigators with a hypothesis regarding the source of exposure to Legionella; review of a line listing extract of the data will facilitate this. Further microbiological or environmental investigations and/or an analytical epidemiological study, may add weight to the evidence supporting the association between the source of exposure and infection.

Critical information therefore is:

• 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.