Table 3 Relative risk of Pontiac fever associated with Legionella concentrations in the hot water of showers according to the exposure scenarios and the detection methods
Exposure scenariosCulture method*FISH method†
Concentration (CFU/l water)‡Cases or PF-like eventsPerson-daysRR (95% CI)p ValueConcentration (Legionella cells/l water)‡Cases or PF-like eventsPerson–daysRR (95% CI)p Value
Scenario 1 (worst case exposure to water Legionella)<100 (DL)§21¶736911.00<6000 (DL)§312811.00
⩾10211**264771.46 (0.70 to 3.02)0.20⩾600021††663881.24 (0.55 to 2.79)0.39
⩾1035166841.05 (0.40 to 2.79)0.54⩾10420624791.25 (0.55 to 2.84)0.38
⩾104458722.39 (0.82 to 6.96)0.11⩾10513227882.23 (0.92 to 5.38)0.05
⩾105121021.67 (0.22 to 12.41)0.46⩾106571722.73 (0.89 to 8.33)0.08
Scenario 2 (average maximal exposure to water Legionella)<10021736911.00<104‡‡9312811.00
⩾10211**264771.46 (0.70 to 3.02)0.20⩾10420††624791.11 (0.51 to 2.44)0.48
⩾1035133511.31 (0.50 to 3.49)0.37⩾10513227881.98 (0.85 to 4.64)0.08
⩾104130191.16 (0.16 to 8.64)0.59⩾106565582.65 (0.89 to 7.91)0.08
⩾105113812.54 (0.34 to 18.89)0.34
Scenario 3 (frequency weighted exposure to water Legionella)<10022740691.00<104‡‡9585361.00
⩾10210**260991.29 (0.61 to 2.72)0.31⩾10420††571252.28 (0.89 to 7.91)0.03
⩾1035120631.40 (0.53 to 3.69)0.32⩾10512219013.56 (1.50 to 8.46)3×10−3
⩾104126411.27 (0.17 to 9.46)0.55⩾106548926.65 (2.23 to 19.84)3×10−3
⩾105112572.68 (0.36 to 19.87)0.32
  • *Culture on agar media expressed in colony forming unit (CFU) per litre.

  • †Fluorescent in situ hybridization (FISH) expressed in Legionella cells per litre.

  • ‡Concentration in Legionella spp CFU/l of water for culture and in Legionella FISH/l of water for FISH.

  • §DL, detection limit of the method.

  • ¶Subjects with PF-like clinical symptoms; these are not PF cases, in absence of contaminated water or aerosol.

  • **Only the cases associated with contaminated water using the culture technique are here enumerated, which explains why their number is not 29.

  • ††Same comment as above, using the FISH technique; three events are missing because the microscopic preparations could not be read.

  • ‡‡No clinical event was detected for concentrations below DL; reference category is consequently <104 Legionella FISH/l.