The shift in seasonality of legionellosis in the USA

Abstract

According to the Centers for Disease Control and Prevention (CDC), from 2000 to 2014, reported cases of legionellosis per 100 000 population increased by 300% in the USA, although reports on disease seasonality are inconsistent. Using two national databases, we assessed seasonal patterns of legionellosis in the USA. We created a monthly time series from 1993 to 2015 of reported cases of legionellosis from the CDC, and from 1997 to 2006 of medical claims of legionellosis-related hospitalisation in older adults from the Centers for Medicaid and Medicare Services (CMS). We split the study time interval into two segments (before and after 2003), and applied a Poisson harmonic regression model to each dataset and each segment. The time series of monthly counts exhibited a significant shift of seasonal peaks from mid-September (9.676 ± 0.164 months) before 2003 to mid-August (8.452 ± 0.042 months) after 2003, along with an alarming increase in the amplitude of seasonal peaks in both CDC and CMS data. The lowest monthly reported cases of legionellosis in 2015 (281) exceed the maximum value reported before 2003 (206). We also observed a discrepancy between CDC and CMS data, suggesting that not all cases of legionellosis diagnosed by hospital-based laboratories were reported to the CDC. Improved reporting of legionellosis is required to better inform the public and organise disease prevention.

Keywords: Legionnaire's disease; Surveillance; hospitalisation; older adults; seasonality.

Fig. 1.

Histogram and calendar plot of legionellosis cases reported to the Centers for Disease Control and Prevention from January 1993 to December 2015. Cool to warm colour scale represents a low to high scale of monthly per cent of reported cases. Low monthly percentage of yearly reported counts is represented by a dark blue tone with red symbolizing the other end of the scale.

Fig. 2.

Monthly time series of reported cases of legionellosis from the Centers for Disease Control and Prevention with model results superimposed as a solid blue line. The dashed vertical line at December 2002 marks two periods.

Fig. 3.

Trend by age groups of reported legionellosis cases from the Centers for Disease Control and Prevention from years 1996 to 2015.

Fig. 4.

Monthly time series of reported cases of legionellosis in older adults (65+) from the Centers for Medicare and Medicaid from January 1998 to December 2006 with model results superimposed. The dashed line at month 60 or December 2002 separates periods 1a and 2a.

Fig. 5.

Peak timing estimates for pre- and post-periods for three models. The k timing estimates for pre- and post-periods for three models.