Legionnaires' disease outbreak investigation toolbox

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Examples of case definitions used in previous outbreak investigations

(a) Outbreak at a flower show (den Boer et al., 2002) [1]

Confirmed case: radiologically-confirmed pneumonia in a visitor to the exhibition or a member of the exhibition staff, with onset no earlier than February 19, 1999, and no later than March 21, 1999, as well as laboratory evidence of Legionella pneumophila infection. Laboratory evidence included isolation of L. pneumophila from respiratory secretions, detection of L. pneumophila antigens in urine, or a fourfold or higher rise in antibody titers to L. pneumophila in paired acute- and convalescent-phase sera, as reported by clinicians.

Probable case: radiologically-confirmed pneumonia with onset no earlier than February 19, 1999, and no later than March 21, 1999, in an exhibition visitor or a member of the exhibition staff who did not meet laboratory criteria for a confirmed case, but who had no laboratory evidence of infection by other microorganisms.

(b) Community outbreak linked to cooling towers (Nguyen et al , 2006) [2]

Confirmed case: a person who (1) had radiologically-confirmed pneumonia and laboratory evidence of infection with Legionella pneumophila serogroup 1 (Lp-1) (i.e. , isolation of Lp-1 from respiratory secretions, detection of Lp-1 antigens in urine, or a minimum of a 4-fold increase [to _128] in antibody titers to Lp-1), (2) became ill between 1 November 2003 and 31 January 2004, and (3) lived in or visited Harnes or its neighbouring communes during the 10 days before the illness. Persons who had been hospitalised or travelling continuously outside of the community during the 10 days before the illness were excluded.

(c) Cooling tower as source of outbreak (Kirrage et al., 2007) [3]

Confirmed case clinical diagnosis of pneumonia, with date of onset after 1st October 2003 and lived in, worked in or had visited Hereford within 2 weeks of the date of onset of their disease. Isolation of Legionella species from clinical specimens; or a four-fold or greater increase in the titre of serum antibodies against L. pneumophila serogroup by indirect immunofluorescent antibody test (IFAT); or the detection of Legionella antigens in urine.

Most outbreak investigators describe both "confirmed" and "probable" cases within their case definition. It may not be feasible to use confirmed cases only - if there is a delay in identifying the outbreak and necessary samples have not been collected, the numbers of laboratory-confirmed cases may be low.

  1. DEN BOER J. W, YZERMAN P. F.E., SCHELLEKENS J., LETTINGA K. D, BOSHUIZEN H. C, VAN STEENBERGEN J. E, BOSMAN A., VAN DEN HOF S., VAN VLIET H. A, PEETERS M. F., VAN KETEL R. J., SPEELMAN P., KOOL J.L., & CONYN VAN SPAENDONCK M. A. E. (2002) A large outbreak of Legionnaires' disease at a flower show, the Netherlands, 1999 Emerging Infectious Diseases 8(1), pp.37-43 http pdf
  2. NGUYEN T. M. N., ILEF D., JARRAUD S., ROUIL L., CAMPESE C., CHE D., HAEGHEBAERT S., GANIAYRE F., MARCEL F., ETIENNE J. & DESENCLOS J.C. (2006) A community-wide outbreak of Legionnaires' disease linked to industrial cooling towers: How far can contaminated aerosols spread? Journal of Infectious Diseases 193, pp.102-111 http
  3. KIRRAGE D., REYNOLDS G., SMITH G.E. & OLOWOKURE B; HEREFORD LEGIONNAIRES' OUTBREAK CONTROL TEAM (2007) Investigation of an outbreak of Legionnaires' disease: Hereford, UK 2003 Respiratory Medicine 101(8), pp.1639-1644 http