Rain Ups Legionnaire's Risk

NEW YORK (Reuters Health) - While Legionnaire's disease occurs with marked summertime seasonality, epidemiologists have discovered that it's wet, humid weather, rather than increased temperature, that best predicts the acute occurrence of the disease.

A sharp increase Legionnaire's disease in the greater Philadelphia metropolitan area in recent years, which had been characterized by unusually heavy rainfall, led local epidemiologists to investigate the seasonality of legionellosis and the potential role of weather.

When they looked at 240 cases of legionellosis -- 90 percent of which were sporadic -- that had occurred in Philadelphia, Bucks, Montgomery, Chester and Delaware counties, they found that these cases occurred predictably and overwhelmingly in the summer months.

"Legionella bacteria grow much better in warmer conditions so summertime occurrence makes sense," Dr. David N. Fisman of Princeton University in New Jersey told Reuters Health. "But when we looked at the data in a way that allowed us to control for season, we found that it was day to day changes in rainfall and humidity, which go together, which explain short-term surges in Legionnaire's disease," he added.

According to Fisman, "when it rains, you get a 2.5-fold increase in the occurrence of Legionnaire's disease over the next 6 to 9 days. Because this time lag is longer than the incubation period of Legionnaire's (thought to be less than 6 days, usually), it seems as though the effect of rain must have something to do with the amount of Legionella bacteria in the environment."

Rainfall acutely increases microbial contamination and sediments in drinking water, Fisman's team explains in The Journal of Infectious Diseases. Organic sediments and the presence of other microorganisms increase Legionella replication, and sediments diminish the effectiveness of chlorination.

SOURCE: The Journal of Infectious Diseases, December 15, 2005.