Seasonal and regional patterns of lower leg cellulitis in Western Australia

Lower leg cellulitis (LLC) is a common infection that is usually caused by Streptococcus pyogenes or other beta-haemolytic streptococci. We hypothesised that in Western Australia (WA), LLC is a summer disease and would be more common in the northern, tropical regions.

We conducted a statewide data linkage of adult patients presenting to WA hospitals with a first ever diagnosis of LLC, from January 2002 through December 2013 according to the region and season.

A total of 36 276 cases presented with a primary episode of LLC. The northern regions of the Kimberley (2.26 (2.13-2.39), P < 0.001) and midwest (1.13 (1.06-1.20), P < 0.001) had higher incidence rates than the Perth metropolitan region, while the southern regions of Southwest, Great Southern and Goldfields had lower incidence rates (0.89 (0.85-0.93), P < 0.001; 0.81 (0.75-0.88), P < 0.001; and 0.77 (0.71-0.83), P < 0.001, respectively). The total number of primary cases was higher in summer (10 570 (29.1%, 95% confidence interval 28.7-29.6), P < 0.0001) and autumn (9306 (25.7%, 95% confidence interval 25.2-26.1), P = 0.004). Seasonality of LLC was observed in all WA regions except those in the Kimberley, Pilbara and Great Southern regions.

In most non-tropical regions of WA, LLC is a summer disease, while in the warmer tropical regions of WA where year-round temperatures are higher, no seasonality was observed, but overall incidence of LLC presentations were higher. These findings may have important implications for public health messaging and research around prevention of LLC.