While it is way too early to advise patients to have elective surgery in the dead of winter, researchers are unraveling some telling indications that surgical site infections (SSIs) are much more likely to occur in the hottest months of the year.
If confirmed, that means one of the most prevalent healthcare-associated infections is seasonal, and the difference is quite dramatic. In a national study they found that 26.5% more SSIs occurred in August than in January.1 Put another way, temperatures above 90 degrees F were linked to a 29% increased risk for an (SSI) compared to temperatures less than 40°F.
“I think this work has implications for surveillance because if these surgical site infections are seasonal that would be something infection preventionists should know about,” says Philip M. Polgreen, MD, senior author of the study and associate professor of internal medicine and epidemiology at the University of Iowa. “If one was trying to do an intervention and rates went up or down, that might not be due to the intervention but due to the seasonal incidence. So one can imagine starting an intervention in the fall and if rates [decline] that may not be due to the intervention but to the underlying seasonal pattern.”
That said, caveats and questions abound, and Polgreen and colleagues are already conducting a second study to try to bring the connection into sharper focus.
“I think it’s too early to make any specific recommendations,” he tells Hospital Infection Control & Prevention. “We still have work underway now that will hopefully allow us to not only describe the seasonal risk of individual surgeries, but also the [patient] populations that may be at more risk. For example, there may be an interaction between specific risk factors that we know exist and seasonal effects. One could imagine that in particular subsets of patients it might be advised to delay surgery. That’s speculating, but the goal of our work is to define how risky is it and whether it more risky for certain patients than others. In some patients it may not matter at all.”
To determine whether the seasonality of SSIs can be explained by changes in temperature the authors conducted a retrospective cohort analysis. All hospital discharges with a primary diagnosis of SSI from 1998 to 2011 were considered cases. Discharges with a primary or secondary diagnoses of specific surgeries commonly associated with SSIs from the previous and current month served as an “at risk” cohort. They modeled the national monthly count of SSI cases both nationally and stratified by region, sex, age, and type of institution. They used data from the National Climatic Data Center to estimate the monthly average temperatures for all hospital locations. All discharge data were extracted from the Nationwide Inpatient Sample (NIS), the largest all-payer database of hospital discharges in the United States. They identified every adult hospitalization with a primary diagnosis of SSI from January 1998 to November 2011.
Using each hospital’s longitude and latitude, they identified all weather stations within 100 kilometers of the hospital, then extracted the average temperature and other meteorological data. The results show “that SSIs are seasonal for men, women, all age groups, and all geographic regions,” the researchers concluded. “… By incorporating weather into our analysis, we have demonstrated that the average temperature in the month of a hospitalization is an important risk factor for SSIs and that higher temperatures are associated with higher odds of SSI.”
Of course many infections, particularly of the viral variety, are expressed in the ebb and flow of seasons. This pattern has been less clear for HAIs, though the authors cite studies that suggest Clostridium difficile infections peak during winter and spring. Similarly, some research has suggested the incidence of catheter-related bloodstream infections peak during the summer months. The link between SSIs and warmer weather has been made previously in smaller studies, though it is sometimes explained by non-weather factors.
“So one could make the argument that maybe this is just because hardly any surgeries happen in December and in January,” Polgreen says. “That did not explain our findings.
Another common argument that there is a “learner effect,” as more surgeries are done by medical students in, for example, July.
“We accounted for that in our data set – there is a code for teaching and non-teaching hospitals – and again that did not seem to be the case,” he says. “We need more granular data. Work is underway so we will have the exact data, the exact weather conditions, and the exact follow-up of patients and see if there is increased risk for not only specific surgeries but also specific weather conditions.”
In the interim, the reason that SSIs peak in the summer is unclear, but it coincides with other observations of a spike in skin and soft-tissue infections in the warmer months.
“Skin infections in general are seasonal,” he says. “We found a striking seasonal pattern with cellulitis and so it helps us sort of think of this in a scientific sense. SSIs are different than cellulitis, however they involve the skin. We actually started working on cellulitis first and found that pattern and then went to the SSI patients.”
Elevated levels of bacteria may be found in certain anatomic locations with higher temperatures. Could the explanation be the certain bacteria thrive in hot weather and retreat during colder months?
“Not too much is known about the seasonality of colonization, but we do know that the majority of SSIs are caused by bacteria that is often on patient skin,” Polgreen says. [Thus], one approach to reducing surgical site infection is to decolonize patients prior to [elective] surgery.”
- Anthony CA, Peterson RA, Polgreen LA, et al. The Seasonal Variability in Surgical Site Infections and the Association With Warmer Weather: A Population-Based Investigation. Infection Control & Hospital Epidemiology, 2017;1-8. doi:10.1017/ice.2017.84