Sepsis Incidence Rates May Vary by Season and Region

Key Point

The incidence and severity of sepsis and severe sepsis may be seasonal and linked to viral respiratory rates, although these associations may also vary by region.

ATLANTA—The incidence and mortality of sepsis and severe sepsis may be seasonal and correspond with viral respiratory infection rates, reported Pajman A. Danai, MD, and colleagues.¹ Incidence of sepsis was highest during the winter and was predominantly related to respiratory causes, although these associations may vary according to region.

Dr. Danai, of the Department of Medicine at Emory University in Atlanta, and colleagues studied data from the National Hospital Discharge Survey that included records for patients hospitalized for sepsis or viral respiratory infection between 1979 and 2003 in acute care nonfederal US hospitals. There were 12.5 million cases of sepsis and 3.8 million cases of severe sepsis during the 25-year period.

The researchers found a low of 41.7 cases of sepsis per 100,000 persons in the fall and a high of 48.6 cases per 100,000 persons during the winter, which corresponded to a 16.5% seasonal increase. There was a statistically significant increase in seasonal rates for severe sepsis, as well: There were 13 fall cases per 100,000 persons and 15.3 winter cases per 100,000 persons, which corresponded to a 17.7% seasonal increase. The incidence of sepsis with respiratory sources rose 40% from fall to winter, which represented the greatest change in sepsis incidence observed in the study. Despite the fact that the severity of sepsis was similar between winter and summer, mortality rates due to sepsis during the winter were 13% greater than rates in the summer and 10% greater than rates in the fall.

“The seasonality in the incidence and fatality rates for sepsis may be due in part to the effect of viral infection,” Dr. Danai and colleagues stated. “Influenza epidemics tend to occur between December and March, and respiratory syncytial virus epidemics often overlap the influenza season.” The highest seasonal incidence rate of influenza was 15.8 winter cases per 100,000 persons, while the lowest seasonal rate was 1.3 summer cases per 100,000 persons. Although the seasonal change in viral pneumonia incidence was not as strong as that of influenza, it was still significant. The highest rate of viral pneumonia incidence was 7.6 winter cases per 100,000 persons, while the lowest rate was 1.7 summer cases per 100,000 persons.

Regional Variations

The Northeastern, Southern, Midwestern, and Western regions of the US showed large summer-to-winter increases in the incidence of influenza and viral pneumonia, and annual sepsis rates in the West did not differ significantly from corresponding Midwestern rates. However, annual Western sepsis rates were significantly lower compared with rates in the Northeast and South. In addition, the Northeast showed the greatest seasonal variation of sepsis: 50.2 fall cases per 100,000 persons versus 65.2 winter cases per 100,000 persons, which corresponded to a 30% increase. By contrast, the South showed the least seasonal sepsis variation. Rates in the South increased 7%, from 43.7 fall cases per 100,000 persons to 47 winter cases per 100,000 persons. Despite higher sepsis rates and more seasonal sepsis variation in the Northeast, the incidence of influenza and viral pneumonia was not higher in the Northeast compared with the South. These regional variations suggest that “viral respiratory infections [are] an incomplete explanation for the seasonal variations in sepsis,” the researchers pointed out.

The investigators considered other factors that might contribute to regional variations in sepsis incidence, such as seasonal changes in host susceptibility to sepsis and infection caused by differences in the light-dark cycle. “Photo-periodicity influences leukocyte function, at least in part via melatonin,” Dr. Danai’s team noted. “In addition to influencing the development of sepsis, melatonin may also modulate the pathophysiology and subsequent outcomes, as suggested by both preclinical evidence and human studies.” Other possible influences include variations in climate, population density, or air quality.

Clinical Implications

“The seasonal variation in sepsis incidence and outcomes could have important implications for providers of healthcare services in allocating limited critical care resources,” Dr. Danai’s team pointed out. “Our finding of an increased incidence of sepsis in the winter season likely contributes to a greater requirement for ICU resources during that time.”

In an accompanying editorial, Jean-Jacques Parienti, MD, and Fabrice Carrat, MD, PhD, stated that Dr. Danai and colleagues’ findings could also improve the design and understanding of future sepsis research.² “If the pre-intervention sepsis rate is measured in the winter and the intervention occurs in the spring, then the decrease in the sepsis rate may be due to the seasonal trend and not to the intervention,” the editorialists noted.

—John Merriman

Reference
1. Danai PA, Sinha S, Moss M, et al. Seasonal variation in the epidemiology of sepsis. Crit Care Med. 2007;35:410-415.
2. Parienti JJ, Carrat F. Viral pneumonia and respiratory sepsis: association, causation, or it depends? Crit Care Med. 2007;35:639-640.

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