Evaluation of heat-labile enterotoxins type IIa and type IIb in the pathogenicity of enterotoxigenic Escherichia coli for neonatal pigs

Type II heat-labile enterotoxins (LT-II) have been reported in Escherichia coli isolates from humans, animals, food and water samples. The goal here was to determine the specific roles of the antigenically distinguishable LT-IIa and LT-IIb subtypes in pathogenesis and virulence of enterotoxigenic E. coli (ETEC) which has not been previously reported. The prevalence of genes encoding for LT-II was determined by colony blot hybridization in a collection of 1648 E. coli isolates from calves and pigs with diarrhea or other diseases and from healthy animals. Only five isolates hybridized with the LT-II probe and none of these isolates contained genes for other enterotoxins or adhesins associated with porcine or bovine ETEC. Ligated intestinal loops in calves, pigs, and rabbits were used to determine the potential of purified LT-IIa and LT-IIb to cause intestinal secretion. LT-IIa and LT-IIb caused significant secretion in the intestinal loops in calves but not in the intestinal loops of rabbits or pigs. In contrast, neonatal pigs inoculated with isogenic adherent E. coli containing the cloned genes for LT-I, LT-IIa or LT-IIb developed severe watery diarrhea with weight loss that was significantly greater than pigs inoculated with the adherent, non-toxigenic parental or vector only control strains. The results demonstrate that the incidence of LT-II appeared to be very low in porcine and bovine E. coli. However, a potential role for these enterotoxins in E. coli-mediated diarrhea in animals was confirmed because purified LT-IIa and LT-IIb caused fluid secretion in bovine intestinal loops and adherent isogenic strains containing cloned genes encoding for LT-IIa or LT-IIb caused severe diarrhea in neonatal pigs.