Summer increase of Legionnaires' disease 2010 in The Netherlands associated with weather conditions and implications for source finding

Abstract

During August and September 2010 an unexpected high number of domestic cases of Legionnaires' disease (LD) were reported in The Netherlands. To examine this increase, patient characteristics and results of source finding and environmental sampling during the summer peak were compared to other domestic cases in 2008-2011. This analysis did not provide an explanation for the rise in cases. A similar increase in LD cases in 2006 was shown to be associated with warm and wet weather conditions, using an extended Poisson regression model with adjustment for long-term trends. This model was optimized with the new data from 2008 to 2011. The increase in 2010 was very accurately described by a model, which included temperature in the preceding 4 weeks, and precipitation in the preceding 2 weeks. These results confirm the strong association of LD incidence with weather conditions, but it remains unclear which environmental sources contributed to the 2010 summer increase.

Fig. 1.

Number of domestic Legionnaires' disease (LD) cases notified in The Netherlands with onset in 2010 compared to the average number of domestic cases in 2008, 2009 and 2011.

Fig. 2

[colour online]. Incidence rate of domestic Legionnaire's disease cases per municipal health region in weeks 32–38 (2010) and the location of the De Bilt weather station.

Fig. 3.

Univariate Poisson regression analysis of 6-week mean temperature, 2-week rainfall intensity, 2-week precipitation, and 2-week rainfall duration. Incidence rate ratio (IRR; symbol) and 95% confidence interval (line) per category.

Fig. 4.

Actual number of notified domestic, sporadic Legionnaires' disease (LD) cases by year and week of infection, 2003–2011 (grey bars) and predicted number of LD cases for 2003–2011 with the adjusted Poisson regression model, based on the 4-week mean temperature, the 2-week rain intensity, 1-week rain duration (line), and long-term correction (loess, dotted line). For this graph the model was fitted on data from weeks 16–44 (2003–2009) (191 weeks, n = 769 cases) and run to predict LD cases in 2010–2011.