Incubation period

The incubation period is the time interval between initial infection and the appearance of the first symptom or sign of disease. The range of incubation periods for Legionnaires' disease found in outbreaks is typically cited as being between 2 -10 days [1], athough it is recognised that it can extend to longer than 10 days.

Only three published peer-reviewed reports have been identified that describe useful data for understanding the incubation period distribution in outbreaks in more detail. The epidemiological study of the Legionnaires' disease outbreak associated with a flower show in the Netherlands [2] described the incubation time as ranging from 2 to 19 days, with a median of 7 days and with 16% of cases exceeding 10 days. A similar distribution of incubation periods was found in the Melbourne aquarium outbreak [3] which showed a range of 1 - 16 days and a median of 6 days, with 7% of cases exceeding 10 days (see Figure below). The Figure shows the incubation period distribution reported for the Melbourne outbreak (grey bars) and the resulting maximum likelihood estimate for the gamma distribution (solid black stepped line, with resulting parameters a=4.96 and b=1.27). The Japanese outbreak [4] shows a smaller range of 2 - 14 days with only 5% of cases in excess of 10 days, athough the incubation period definition inclusion criteria may have introduced biases due to most recent visit being used.

In summary, analysis of the limited quantitative evidence found to date supports an average median incubation period of 6 days, with the majority of cases suffering a 2-10 day incubation period. The distribution of incubation periods from an outbreak appears to be effectively described by a range of parametric mathematical probability distributions [5]. About 10% of cases might be expected to have an incubation period longer than 10 days, and therefore limiting the case definition and case travel history to only 10 days might limit the effectiveness of identifying the source. Collecting case information on travel and activities for 14 days prior to onset of symptoms offers a reasonable and pragmatic compromise. This will cover the potential exposure source for 90% of cases, compared to 80% when history is sought for only 10 days. When these variations in incubation periods occur, clinical discretion should be used to agree a cut-off point so that the epidemiological follow-up of these cases can be completed [6].

Reference List

1. BARTRAM J. (2007) Legionella and the prevention of legionellosis WHO, Geneva ISBN 92 4 156297 http pdf
2. 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
3. GREIG J. E., CARNIE J. A., TALLIS G. F., RYAN N. J., TAN A. G., GORDON I. R., ZWOLAK B., LEYDON J. A., GUEST C. S. & HART W. G. (2004) An outbreak of Legionnaires' disease at the Melbourne Aquarium, April 2000: investigation and case-control studies The Medical Journal of Australia. 180(11), pp.566-572. http
4. SASAKI T., MATSUMOTO N., NAKAO H., KATOH T., FUKUDA Y., NAKAZATO M. & OKAYAMA A. (2008) An outbreak of Legionnaires' disease associated with a circulating bathwater system at a public bathhouse. I: a clinical analysis. Japanese Society of Chemotherapy and The Japanese Association for Infectious Diseases 14, pp.117 - 122 http
5. EGAN J., HALL I., LEMON D. J., LEACH S. A. (2011) Modelling Legionnaires' disease outbreaks: estimating the timing of an aerosolized release using symptom-onset dates Epidemiology 22(2), pp. 188-198 http
6. JOSEPH C., PARANTHAMAN K. & NAIK F. (2010) Guidance on the Control and Prevention of Legionnaires' Disease in England. Technical Paper 1 Disease Surveillance Respiratory Diseases Department HPA Centre for Infections. http