Abstract.A total of 720 Escherichia coli strains isolated from diarrheic piglets on 756 swine farms were screened for the presence of the enteroaggregative E. coli heat-stable enterotoxin 1 (EAST1) gene by polymerase chain reaction (PCR). Escherichia coli strains that carried EAST1 genes were also tested by PCR for the presence of 4 fimbriae (F4, F5, F6, F41), 2 heat-stable enterotoxins (STa and STb), and 1 heat-labile enterotoxin (LT) gene. One hundred sixty-four (22.7%) of the 720 E. coli isolates carried genes for EAST1. Of these 164 isolates, 62 (37.8%) carried EAST1 genes only, 11 (6.7%) carried genes for at least 1 of the fimbrial adhesins, 51 (31.1%) carried genes for at least 1 of the enterotoxins, and 40 (23.8%) carried genes for at least 1 of the fimbrial adhesins and enterotoxins. Forty-six percent of strains that carried EAST1 genes carried STa genes, and 16% of strains that carried EAST1 genes carried F4. The isolation rate of enterotoxigenic E. coli strains carrying genes for EAST1 gene was 63%. The 6 major genotypes observed in this study (in decreasing order) were EAST1ϩ, EAST1ϩSTaϩ, EAST1ϩSTaϩSTbϩ, EAST1ϩSTaϩF5ϩ, EAST1ϩSTaϩF4ϩ, and EAST1ϩSTbϩF4ϩ. EAST1 is widely prevalent among diarrheagenic strains of E. coli and may represent an important virulence determinant in the pathogenesis of enteric colibacillosis of preweaned pigs.Escherichia coli is a major cause of diarrhea in human beings and animals. The 4 major groups are enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), enteropathogenic E. coli (EPEC), and enterohemorrhagic E. coli (EHEC). 5 Recently, evidence has accumulated to implicate a fifth distinct diarrheagenic category, enteroaggregative E. coli (EAggEC). The term enteroaggregative is derived from the pattern of adherence of bacteria to HEp-2 cells in culture. 6 EAggEC strains are a major cause of persistent diarrhea in children in the developing world. The aggregative adherence phenotype is associated with a bundle-forming pilus referred to as AAF/1. The genes for AAF/1 and a heat-stable enterotoxin 1 (EAST1) are carried on a 60-MD plasmid. EAST1 is genetically and immunologically distinct from heat-stable enterotoxin a (STa) produced by ETEC strains. 14,15 In addition to EAggEC, the EAST1 gene is also carried by human, porcine, and bovine ETEC strains. 1,10,15 The EAST1 gene sequence of F4ϩ ETEC strains is slightly different from that of human-colonizing ETEC strains. [14][15][16] The gene encoding EAST1 is highly prevalent among other groups of diarrheagenic E. coli isolated from humans, e.g., EHEC, ETEC, and EAggEC. 10 Although enteric colibacillosis is common in piglets, little is known concerning the prevalence of EAST1-positive E. coli and, thus, its potential significance in isolates from this species. The purpose of this study was to investigate the prevalence of the EAST1 gene in E. coli isolated from diarrheic piglets and to determine its relationship with other fimbrial (F4, F5, F6, and F41) and enterotoxin (STa, STb, and LT) genes by using polymerase chain reac...