Legionnaires' disease outbreak investigation toolbox

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Creating a trawling questionnaire for a Legionnaires' disease outbreak

A template trawling questionnaire is available here and an EpiInfo version here. The rest of this page considers why and how one might conduct a trawling questionnaire.

Hypothesis generation is a critical step in an outbreak situation. By administering an open-ended hypothesis-generating questionnaire to some of the first case-patients the investigator(s) will attempt to learn about potential exposures to known risk factors. This information can be viewed as a line listing and from this, along with results from a literature review if appropriate, specific hypotheses about the cause of the outbreak can be formulated. Often these hypotheses will become the basis of a second structured hypothesis-testing questionnaire which itself can be used to test these hypotheses in an analytical study. Please note the trawling nature of hypothesis generating questionnaires will usually be much more exhaustive in their questions than those associated with more standard surveillance operations (See version of surveillance questionnaire in EpiInfo format).

Objectives of a trawling interview (hypothesis-generating) questionnaire

  • to gather preliminary data to:
    • generate hypotheses as to the potential source/cause of the outbreak so that control measures can be applied to prevent further cases.
    • highlight aspects of the investigation that demand collection of additional or more detailed data.

Conducting a trawling interview questionnaire

  • Language

The age, education level and other characteristics of the case-patients will guide the language used in the interview to ensure that questions are approached in a culturally-sensitive and appropriate way particularly if some areas of questioning are uncomfortable. Having memory prompts available, such as calendars, may help case-patients remember details.

  • Questions
Use a combination of closed-ended (e.g. yes/no) and open-ended questions in a trawling questionnaire. Closed-ended questions are easier to standardise, code and analyse, but they may not provide complete or exhaustive detail. Open-ended questions are less constrained, which allows interviewees to express themselves freely, but they are more difficult to code and analyse.

  • The interviewer

If feasible, it is preferable for one person, or an organised and trained group of people, to conduct all of the interviews as this makes it easier to identify any commonalities or peculiar findings. The interviewer should have sufficient background knowledge of Legionnaires' disease, as well as have good interviewing and people skills. Additionally, the interviewer must be professional but friendly, and should show concern for the well-being of the case-patient. Additional interview guidance is available here.

  • The interview
Explain the purpose of the interview, being careful not to make comments that could bias responses, and tell the case how long the interview will take. A trawling questionnaire is, by nature, often exhaustive and may take a relatively long time. If the length of time is too demanding for the patient or the interview is impossible for other reasons suitable proxies for the information can be indentified (family/friends, clinicians/nurses etc) though such alternatives should be clearly stated to enable future analysis/interpretation.

Questioning should be systematic but flexible enough to allow the case-patient to provide comments that may give additional insights into the cause of the outbreak. Once the interview is completed, provide contact information, thank the interviewee for participating, and acknowledge his or her contribution to the investigation.

Sources for template trawling questionnaire

a) http://www.invs.sante.fr/publications/2007/legionellose_alpes_maritimes/legionellose_alpes_maritimes.pdf France.

b) Vertaalde versie van Legionellose bijlage 1 December 2007.doc. Holland.

c) Surrey and Sussex Health Protection Unit, United Kingdom

  1. FOCUS ON FIELD EPIDEMIOLOGY Hypothesis generation during outbreaks UNC School of Public Health 1(6) http pdf