Shigella flexneri O-antigen specific enzyme immunoassay: a prospective study of class-specific antibody titres against lipopolysaccharide antigens in Vietnamese children and adults with serotype 1b or 2a dysentery

Abstract

A total of 278 sera were collected from 97 patients with a bacteriologically verified Shigella flexneri serotype 1b or 2a infection. Of the patients, 65 were children below the age of 5 years and admitted to the Department of Infectious Diseases at St. Paul's Hospital, Hanoi, Vietnam; 32 were adults, aged 20–24 years, and infected during a dysentery outbreak at Son Tay technical school, Hanoi. The sera were analysed for their specific immunoglobulin A (IgA), M (IgM) and G (IgG) titres against phenol-water-extracted and chemically defined lipopolysaccharide (LPS) antigens of S. flexneri and other shigellae by an enzyme immunoassay (EIA). The titres estimated in sera from patients were compared with titres seen in sera from age-matched healthy people living in the Tu Liem district, Hanoi. Positive titres were defined as greater than the mean titre+ 2 SD in sera from healthy persons. Children 1–2 years old and infected with S. flexneri serotype 1b responded with significantly elevated IgA titres up to 60 days after falling ill ( P-values 0·06 to <0·001). The IgG titres against the homologous S. flexneri serotype 1b were significantly elevated in samples collected up to 6 months after children up to 5 years old had fallen ill ( P-values ranging from 0·007 to <0·001). IgM titres were elevated, but not significantly higher than those seen in healthy individuals. The results suggested that children younger than 3 years who responded with IgA, IgG and, to a lesser extent, IgM increases were experiencing their first S. flexneri dysentery, whereas older children and adults who only showed IgG increases had had experience with previous S. flexneri infections and displayed an anamnestic response. In adults, significant titre increases were seen only in individuals with low pre-infection titres. The clinical data and laboratory analyses of the strains suggest that a virulent S. flexneri serotype 1b clone was prevalent in the Hanoi area in 1982–1983. Young children infected with S. flexneri serotype 2a also responded with elevated IgA and IgG titres, but there were too few children to permit statistical analyses.