Warm Weather Increases Hospital Infections, And What That Might Mean For Climate Change

What makes hospital-acquired infections so intractable? There’s no question that some of the organisms that cause them are tricky: MRSA hangs out on the skin and and in the nostrils, and E. coli resides in the gut, making it easy for them to be carried into hospitals undetected. Hospital workers’ poor performance on hand-washing is well-documented. And recently, researchers have begun to wonder whether hospitals have missed an opportunity by not emphasizing environmental cleaning —- of rooms, computers and equipment, for instance -— given how persistently some bacteria can linger.

A new paper in PLoS One, though, says there’s another factor contributing to the problem, one that has missed consideration until now: weather. An 8-year study of infection data from 132 hospitals finds that as outside temperatures rise, in-hospital infections with some of the most problematic pathogens rise also.

The analysis is a warning to healthcare institutions to be additionally on guard when it is warm outside. But the authors say it’s also a warning to the rest of us: If global climate change raises ambient temperatures, it could increase the likelihood of deadly hospital infections as well.

The study — by researchers from the University of Iowa, University of Maryland, Princeton University and the nonprofit Center for Disease Dynamics, Economics and Policy — used a privately maintained national database of almost 212,000 clinical bloodstream cultures taken between Jan. 1999 and Sept. 2006. It plotted the infections’ incidence against data on mean temperature and dew point and total precipitation from the US National Climate Data Center. It accounted for the potentially confounding effect of seasonal variation in hospital admissions.

And it found: From winter to summer, Gram-negative bacteria, the most problematic hospital pathogens, rose anywhere from slightly to dramatically. E. coli infections rose 12.2 percent; Pseudomonas infections rose 28.1 percent; Klebsiella infections rose 28.6 percent; and Acinetobacter infections rose 51.8 percent.

Moreover, for every 10-degree Fahrenheit rise in mean temperature, there was a rise in infections with those same Gram-negatives. The increase varied from 3.5 percent for E. coli to 10.8 percent for Acinetobacter, independent of any changes in the season, the humidity or amounts of precipitation. Changes in temperature also affected S. aureus and MRSA, but much less: Those infections rose 2.2 percent for every 10-degree change.

Why should this be? Since hospitals in the United States are climate-controlled, the authors say it most likely represents the influence of the environment outside the hospital. Warmer weather leads to more bacterial growth; the presence of more bacteria increases the likelihood that some of them will be carried into hospitals by staff or visitors. An earlier study by this team found summer peaks in infections. So did studies by other researchers, though they were looking either for just one organism or in just a few hospitals at a time.

The obvious response to this finding is to recommend that hospitals ramp up their infection surveillance in warmer months. The not-obvious question that follows is: What do they do when it is warmer all the time?

Because unlike the earlier studies, which were season-specific, this study found a rise in infections whenever there was a rise in temperature — leading to the natural conclusion that if temperature rises around the year, expensive and deadly hospital infections will rise too. Notably, there is evidence that Gram-negative bacteria not only proliferate at warmer temperatures; thanks to a temperature-related change in the outer membrane, they may become more virulent as well.

“If average temperatures are rising (independent of cause), then we would expect to see more Gram-negative infections in the future,” lead author Eli Perencevich told me by email. “This could mean more difficult-to-treat infections, and also higher likelihood of outbreaks when temperatures are warm.”

Cite: Eber MR et al. Seasonal and Temperature-Associated Increases in Gram-Negative Bacterial Bloodstream Infections among Hospitalized Patients. PLoS ONE 6(9): e25298. doi:10.1371/journal.pone.0025298



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