As the Temperature Rises, So Does Incidence of Surgical Site Infections

Among the most common healthcare-associated infections are surgical site infections (SSIs), and new research coming from the University of Iowa has found that these infections may be seasonal; meaning that during summer months, as the temperatures rise, so do the number of SSI infections.

In fact, the researchers found that the odds for hospitalization due to surgical site infection increased by 28.9% when temperatures were over 90°F versus when temperatures were below 40°F.

“We show that seasonality of SSIs is strongly associated with average monthly temperature. As temperature rises, risk increases,” senior study author Philip M. Polgreen, MD, Director of the Innovation Lab at the Signal Center for Health Innovation and associate professor of Internal Medicine and Epidemiology at the University of Iowa explained in a recent press release. “However, the odds of any one person getting an infection are still small, and due to the limitations of our data, we still do not know which particular surgeries or patients are at more risk from higher temperature.”

The researchers’ central aim was to see if the seasonality of SSIs can be explained by changes in temperature. Using the “largest database of discharges from hospitals in the United States,” the Nationwide Inpatient Sample, the researchers identified all adult hospitalizations due to SSI spanning 13 years, from January 1998 to November 2011. The researchers then used the longitude and latitude of each hospital included in the study to identify weather stations that were in close proximity to each respective hospital. After statistically analyzing data from these stations—including temperature, rainfall, and wind speed—the researchers were able to analyze SSI incidence “in a linear time trend” in order to identify the role that seasonality plays in the equation.

What did the researchers find? SSIs are seasonal; the number of SSI-associated hospital discharges during the time period were 26% higher in the month of August than they were in January, where incidence appears to be lowest. In fact, the authors found that the incidence of many infections is seasonal. For example, perhaps not surprisingly, there are more respiratory infections in the winter months, and the incidence of tick- or mosquito-borne diseases tends to be highest in the summer. In addition, although there is not much research available on the seasonality of healthcare-associated infections, the authors noted past research that has found that there is seasonality in the incidence of Clostridium difficile infections as well, with more infections occurring in the winter and spring. Furthermore, catheter-related bloodstream infections peak during summer months.

The implications of these findings point to the possibility that “a 25% reduction in the average number of at-risk surgeries in the months of July and August would be associated with a decrease of nearly 1,700 SSIs” annually, according to the study authors.

Furthermore, according to Christopher A. Anthony, MD, first author of the study and surgery resident physician at the University of Iowa Roy J. and Lucille A. Carver College of Medicine, “These results tell us that we need to identify the patients, surgeries, and geographic regions where weather-related variables are most likely to increase patients’ risk for infections after surgery.” He continued, “This way, we can identify the patients at the greatest risk for surgical site infections during warmer summer months.”

The authors stress in their articles that “more granular data including exact surgery date and specific procedures” is needed to “help determine whether shifting the timing of some surgeries away from peak SSI months can help reduce SSIs in patients with specific procedures.”