Salmonella Bacteremia in Maryland, 2001-2010

BACKGROUND:  Non-typhoidal Salmonella (NTS), a major cause of gastroenteritis in the United States, is most commonly identified by stool culture. NTS can also cause bacteremia. We examined the epidemiology of these infections in Maryland, their outcomes, and associations with demographic factors.

METHODS: The Foodborne Disease Active Surveillance Network (FoodNet) coordinates active population-based surveillance for multiple pathogens, including Salmonella, in 10 states. Data from Maryland from 2001 through 2010 were analyzed to compare stool culture confirmed cases of NTS with blood culture confirmed cases. If a patient had positive stool and blood cultures, we considered the case a bloodstream infection. Statistical analyses included the calculation of incidence rates and rate ratios and Pearson’s chi-squared tests.

RESULTS:  676 isolates of Salmonellawere cultured from blood (1.21 cases per 100,000), and 6,720 from stool (12.01 per 100,000). Yearly incidence of bacteremia ranged from 0.73 per 100,000 in 2001 to 1.48 per 100,000 in 2005. The 10-year incidence of bacteremia was significantly higher in males than females (1.31 vs. 1.10 per 100,000, p= 0.026).  Males 65 years old and older had a significantly higher incidence of bacteremia than females the same age (IRR=1.88, p=0.0013), but no significant difference existed between males and females under 65 (IRR=1.14, p=0.127). No significant difference was found in the incidence of gastrointestinal infection between sexes of all ages or when stratifying by age. Approximately 75% of cases with bacteremia were hospitalized, compared to 30% of cases with gastrointestinal infections (p>0.001). This varied by sex: 80% of men and 70% of women with bacteremia were hospitalized compared to 28% of men and 31.5% of women with gastrointestinal infections. While the overall mortality rate during the study period was low, cases with bacteremia had much higher odds of dying (N=32, OR=22.6, p<0.001).

CONCLUSIONS:  Blood invasiveness of non-typhoidal Salmonella infections appears to be a function of age and gender:  incidence is higher among males than females of all ages, and this difference is even greater between those 65 years old and older. This indicates that males 65 years old and older are the group at highest risk for NTS bloodstream infections. Compared to younger males or females, regardless of age, a higher percentage of older males with bloodstream infections were hospitalized.  Further investigation is needed to determine what factors influence this higher risk among older males and if there are factors that can be modified to reduce the incidence of these infections.