Serum
immunoglobulin G antibodies to purified heat-labile
enterotoxin (LT) from human (LTh) and porcine (LTp) Escherichia coli strains and
cholera enterotoxin (CT) were measured by an
enzyme-linked
immunosorbent assay. Sera from patients with LTh E. coli
infection showed a prominent response with LTh, an intermediate response with LTp, and a meager response with CT. Of 47 persons with clinical LTh-producing E. coli (herein shortened to LTh E. coli)
infections, significant rises in
antitoxin were detected against LTh in 36 (77%), against LTp in 30 (64%), and against CT in only 13 (28%) patients; seroconversions also occurred in 11 of 14 (79%) patients with subclinical LTh E. coli
infections. In North Americans with experimental LTh E. coli
infection, anti-Lth did not remain at high levels for more than 3 months. Persons with
cholera manifested
antitoxin responses that were similarly potent against all three toxin
antigens; in fact, net optical density values were often slightly higher against LTh than against CT. The ratio of CT/LTh ELISA net optical density in convalescent sera proved to be a sensitive means to differentiate LT E. coli from
cholera infection. All 11
cholera patients tested had CT/LTh ratios of greater than 0.70, whereas in only 1 of 47 LTh E. coli
infections did the ratio exceed that value (it was 0.71) (P less than 0.0000000001). In single serum specimens, a net optical density of greater than or equal to 0.30 against LTh was shown to be a useful cutoff in screening sera for recent LTh E. coli or past
cholera infection. The CT/LTh ratio was then used to differentiate definitively. Sera from healthy 3- to 5-year -olds and 15- to 19-year-olds in Maryland, Chile, and Bangladesh were tested against LTh and CT. The serological results fit known epidemiological observations. (i) LTh
infections are rare in the United States (only 2 of 60 sera had LTh net optical density values of >/= 0.30. (ii) In contrast, evidence of recent LTh E. coli
infections was very common in Chilean (69%) and Bangladeshi (57%) 3- to 5-year-olds and not uncommon in 15- to 19-year-olds (38 and 31%, respectively) in those countries. (iii) Only Bangladeshi sera showed serological evidence of
cholera infections (CT/LTh ratios of > 0.70). The
immunoglobulin G enzyme-linked
immunosorbent assay measuring
antibodies to purified LTh and CT represents a practical and effective tool for the serological study of LTh E. coli and
cholera diarrheal
infections.