Invasive Nontyphoidal Salmonella Infections in the United States, 1980 to 2012

BACKGROUND:   Salmonella causes an estimated 1.2 million human infections, 23,000 hospitalizations, and 450 deaths in the United States annually. Almost all Salmonella serotypes are classified as nontyphoidal, indicating that infection typically presents as gastroenteritis, not typhoid fever.  However, nontyphoidal serotypes can cause invasive infections, where the organism is isolated from an extra intestinal site, including blood. 

METHODS:   We analyzed laboratory-confirmed human nontyphoidal Salmonella (NTS) infections with isolates from blood reported to the Centers for Disease Control and Prevention (CDC) from 1980 to 2012.  To examine the effect of age on risk of invasive NTS infection, we used the following age groups: <1 year, 1 to 4 years, 5 to 17 years, 18 to 64 years, 65 to 84 years, and 85 years and older; the age group 18 to 64 years was used as the referent to calculate odds ratios (OR) and 95% confidence intervals (95% CI).  We calculated a blood invasiveness ratio (BIR) as the ratio between the number of blood and all reported isolates (excluding those with an unknown specimen site) for serotypes with more than 100 isolates from blood during the study period. 

RESULTS:   From 1980 to 2012, the percent of invasive NTS ranged from 3–7%.  Patients who were 65 to 84 years old (OR: 2.37, 95% CI: 2.30-2.43) or who were 85 years and older (OR: 2.81, 95% CI: 2.67-2.97) were more likely to have an invasive NTS infection.  There were forty-two serotypes with more than 100 isolates from blood reported in the study period, with a mean BIR of 4.7.  The serotypes with the highest BIR were the known invasive serotypes Choleraesuis (including var. Kunzendorf) (58) and Dublin (51).  The BIR among the other serotypes ranged from 2–14; nineteen had a BIR greater than 4.7, including Urbana (14), Sandiego (12), and Schwarzengrund (11).  Serotypes Choleraesuis and Dublin had the highest BIR for all age groups.  Among other serotypes, those with the highest BIR varied by age group as follows: Heidelberg (< 1 year), Urbana (1 to 64 years), Bovismorbificans (65 to 84 years), and Brandenburg (85 years and older). 

CONCLUSIONS:   The proportion of reported NTS infections that are invasive has been relatively stable over the past 3 decades.  The risk for invasive NTS infection is increased for those over 64 years old.  The patterns of invasive NTS infection vary by age and serotype.