Also read: Legionnaire-Disease

Legionellosis Incidence Rises With
Abundant Rain in Warm Weather

Neil Osterweil

Medscape Medical News 2004. © 2004 Medscape

Oct. 4, 2004 (Boston) — The incidence of legionellosis appears to increase during wet weather, suggesting that clinicians and public health officials need to be on the lookout for signs of the infection and possible methods for reducing environmental exposures during the rainy season, researchers from the Centers for Disease Control and Prevention (CDC) and public health officials said.

Legionellosis includes both Pontiac fever, a self-limited flu-like illness, and Legionnaire's disease (LD), a severe pneumonia responsible for an estimated 8,000 to 18,000 hospitalizations in the U.S. each year, according to CDC data.

The causative pathogen, Legionella pneumophila, is a waterborne organism, so it is not surprising that the bug would flourish under wet conditions, according to Lauri Hicks, DO, a medical epidemiologist in the division of respiratory disease at the CDC in Atlanta, Georgia. But little is known about risk factors or transmission of sporadic LD, which accounts for about 80% of all cases.

When a sporadic outbreak of legionellosis infection occurred after record rainfall in the mid-Atlantic states in 2003, Dr. Hicks and colleagues in the state health departments of North Carolina, Maryland, and Virginia wanted to know why.

The investigators used data reported to the CDC along with annual Census Bureau population estimates to calculate monthly LD incidence in five mid-Atlantic states: Delaware, Maryland, North Carolina, Pennsylvania, and Virginia. They also obtained meteorologic data, including rainfall and temperature information for each state, from the National Climatic Data Center. The investigators evaluated the relationship between legionellosis rates and climatic variables using Pearson's correlation coefficient (r) and a Poisson regression model.

They found that from January to August 2003, there were 374 reported sporadic cases in the five states compared with an average of 161 cases (range, 107-225) during the same months over the previous 13 years. The correlations for legionellosis were 0.40 for rainfall, 0.39 for temperature, and 0.53 for the combination of temperature and rain (P < .01 for all). Using the Poisson model, they found a linear relationship between rainfall and relative risk for legionellosis. The model showed that an increase of 2.6 inches in the monthly average rainfall from May to September 2003 corresponded to a 24% increased risk of legionellosis. The results held up even when they controlled for variables such as changes in surveillance practices or in the urine antigen test now widely used to screen for Legionella infections.

The investigators speculated that the increase in legionellosis could be due to a temporary compromise of public water supplies.

"What we found is consistent with the ecology of the organism. We know that Legionella is a waterborne organism and we know that it grows fast in warm temperatures, so it makes sense that if water quality changes, perhaps related to disinfectants in the water supply or an increase in particulate matter that supports growth of Legionella, that legionellosis cases may increase," Dr. Hicks told Medscape.

"We cannot say that there is absolutely a causal relationship at this point, and heavy rainfall does not lead directly to an increase in legionellosis cases, but we can see that it may lead to some sort of change in conditions that would lead to legionellosis," she said.

An epidemiologist from a notoriously soggy state told Medscape that larger climatic factors than rainfall alone may be at work.

"I think it's very interesting the way that rates [of legionellosis] do seem to change in certain situations, and I think it probably is multifactorial and may be related to actual change in climate than to the rainfall itself, because you would expect that in western Washington, where we have a much higher average rainfall than they do in say, Pennsylvania, that we would have lots of cases of Legionella, said Jo Hofmann, MD, an epidemiologist for communicable diseases for the state of Washington. "It's interesting, but it would be more interesting to follow it for a longer period of time as climate changes throughout the world." Dr. Hofmann attended the presentation and commented on it for Medscape. She was not involved in the study.

Dr. Hicks and colleagues recommend increased vigilance for legionellosis during times of abnormally high rainfall and suggest that "identification of the specific environmental sources that support transmission during heavy rainfall may lead to opportunities for prevention," according to their abstract.

IDSA 2004 Annual Meeting: Abstract 751. Presented Oct. 2, 2004.

Reviewed by Gary D. Vogin, MD

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