Age-Specific Concentrations and Seroprevalence of Antibodies Against Salmonella Enteritidis (O:9) and Salmonella Typhimurium (O:4,5) Across 3 Sites in Kenya

BACKGROUND: Seroprevalence studies can indicate the age distribution of first infection with nontyphoidal Salmonella and estimate the rate of infection by age. METHOD: We collected 1254 paired samples of serum and stool from healthy children and adults in Kilifi, Nairobi, and Siaya counties in Kenya, areas of low, medium, and high incidence of invasive nontyphoidal Salmonella disease, respectively. We quantified the age- and site-specific geometric mean concentrations (GMCs) of IgG and IgA antibodies against S Enteritidis O-antigen (O:9, serogroup D) and S Typhimurium O-antigen (O:4,5, serogroup B) using an in-house standardized enzyme-linked immunosorbent assay. Serum concentrations were estimated against a previously calibrated reference serum. RESULTS: Maternally derived O:9 IgG and O:4,5 IgG antibodies were detectable in 100% of neonates, and the GMC decreased by 40% (95% CI, 25%-52%) per month in the first 6 months of life. GMCs of IgA were low in neonates. After age 6 months, the O:9 and O:4,5 IgG and IgA GMCs increased sharply with age across all sites, reaching a plateau in early adulthood. The rate of increase in IgG GMCs by age was highest for O:9 in Nairobi and for O:4,5 in Kilifi. Mixture modeling defined a threshold of 14.1 antibody units for O:9 IgG and 28.2 antibody units for O:4,5 IgG. Seroprevalence also increased by age. The GMCs of O:4,5 IgG were 2 times higher for carriers of serogroup B Salmonella than noncarriers. DISCUSSION: Maternal antibodies to nontyphoidal Salmonella decay rapidly from 0 to 5 months, after which incident infection with both serogroups occurs. To be effective, control efforts should be implemented before primary infection.