Salmonella Serogroup C: Current Status of Vaccines and Why They Are Needed

Abstract

Nontyphoidal Salmonella (NTS; i.e., Salmonella enterica organisms that do not cause typhoid or paratyphoid) are responsible for 94 million infections and 155,000 deaths worldwide annually, 86% of which are estimated to be foodborne. Although more than 50 serogroups and 2,600 serovars have been described, not all Salmonella serovars cause disease in humans and animals. Efforts are being made to develop NTS vaccines, with most approaches eliciting protection against serovars Typhimurium and Enteritidis (serogroups B [O:4] and D [O:9], respectively), as they are widely considered the most prevalent. Here, we show that serogroup C (O:6,7, O:6,8, or O:8 epitopes) is the most common serogroup in the United States, and the prevalence of serovars from this serogroup has been increasing in Europe and the United States over the last decade. They are also the most commonly isolated serovars from healthy cattle and poultry, indicating the underlying importance of surveillance in animals. Four out of the 10 most lethal serovars in the United States are serogroup C, and reports from African countries suggest that strains within this serogroup are highly antibiotic resistant. Serogroup C consists of highly diverse organisms among which 37 serovars account for the majority of human cases, compared to 17 and 11 serovars for serogroups B and D, respectively. Despite these concerning data, no human vaccines targeting serogroup C NTS are available, and animal vaccines are in limited use. Here, we describe the underestimated burden represented by serogroup C NTS, as well as a discussion of vaccines that target these pathogens.

FIG 1

Frequency of nontyphoidal Salmonella serogroups associated with human infections, by year. (A) NTS serovars in the United States from 1995 to 2012. (B) NTS serovars in Europe from 2005 to 2013. Data were obtained from the U.S. Centers for Disease Control and Prevention and the European Surveillance System (TESSy).

FIG 2

Serogroups of Salmonella isolates from animals in the United States in 2012. Data depict the 20 most common serovars for cattle (A), poultry (B), and swine (C), identified in healthy animals or sick animals and reported to the U.S. Centers for Disease Control and Prevention.

FIG 3

Correlation between invasiveness of a serovar and hospitalization rate. Hospitalization rates were defined as the percentages of salmonellosis patients who were hospitalized within 7 days following specimen collection. The invasive index was defined as the ratio of the number of specimens isolated from normally sterile sites and the total number of isolates of a serovar. Data for S. Choleraesuis (empty symbol) and S. Dublin (filled symbol) are shown on a separate scale, as they are much more invasive than other NTS. Data obtained from reference .

FIG 4

Number of serovars within each serogroup accounting for 99% of NTS cases in 2012. Data are from the U.S. Centers for Disease Control and Prevention.

FIG 5

Contribution of individual serovars within serogroups isolated in the United States in 2012. Only serovars responsible for 2% or more of cases within each serogroup are represented individually. In 2012 in the United States, serovar I 4,[5],12:i:− accounted for 15.8% of all cases associated with serogroup B NTS. These data are included within the S. Typhimurium slice of the pie chart. Data are from the U.S. Centers for Disease Control and Prevention.