Fecal swabs obtained from 1,300 healthy lambs in 93 flocks in Spain in 1997 were examined for Shiga toxin-producing Escherichia coli (STEC). STEC O157:H7 strains were isolated from 5 (0.4%) animals in 4 flocks, and non-O157 STEC strains were isolated from 462 (36%) lambs in 63 flocks. A total of 384 ovine STEC strains were characterized in this study. PCR showed that 213 (55%) strains carried the stx ₁ gene, 10 (3%) possessed the stx ₂ gene, and 161 (42%) carried both the stx ₁ and the stx ₂ genes. Enterohemolysin ( ehxA ) and intimin ( eae ) virulence genes were detected in 106 (28%) and 23 (6%) of the STEC strains, respectively. The STEC strains belonged to 35 O serogroups and 64 O:H serotypes (including 18 new serotypes). However, 72% were of 1 of the following 12 serotypes: O5:H−, O6:H10, O91:H−, O117:H−, O128:H−, O128:H2, O136:H20, O146:H8, O146:H21, O156:H−, O166:H28, and ONT:H21 (where NT is nontypeable). Although the 384 STEC strains belonged to 95 different seropathotypes (associations between serotypes and virulence genes), 49% of strains belonged to only 11. O91:H− stx ₁ stx ₂ (54 strains) was the most common seropathotype, followed by O128:H− stx ₁ stx ₂ (33 strains) and O6:H10 stx ₁ (25 strains). Three strains of serotypes O26:H11, O156:H11, and OX177:H11 had intimin type β1; 5 strains of serotype O157:H7 possessed intimin type γ1; and 15 strains of serotypes O49:H−, O52:H12, O156:H− (12 strains), and O156:H25 had the new intimin, intimin type ζ. The majority (82%) of ovine STEC strains belonged to serotypes previously found to be associated with human STEC strains, and 51% belonged to serotypes associated with STEC strains isolated from patients with hemolytic-uremic syndrome. Thus, this study confirms that healthy sheep are a major reservoir of STEC strains pathogenic for humans.

In Spain, as in many other countries, verotoxin-producing Escherichia coli (VTEC) strains have been frequently isolated from cattle, sheep, and foods. VTEC strains have caused seven outbreaks in Spain (six caused by E. coli O157:H7 and one by E. coli O111:H^– [nonmotile]) in recent years. An analysis of the serotypes indicated serological diversity. Among the strains isolated from humans, serotypes O26:H11, O111:H^–, and O157:H7 were found to be more prevalent. The most frequently detected serotypes in cattle were O20:H19, O22:H8, O26:H11, O77:H41, O105:H18, O113:H21, O157:H7, O171:H2, and OUT (O untypeable):H19. Different VTEC serotypes (e.g., O5:H^–, O6:H10, O91:H^–, O117:H^–, O128:H^–, O128:H2, O146:H8, O146:H21, O156:H^–, and OUT:H21) were found more frequently in sheep. These observations suggest a host serotype specificity for some VTEC. Numerous bovine and ovine VTEC serotypes detected in Spain were associated with human illnesses, confirming that ruminants are important reservoirs of pathogenic VTEC. VTEC can produce one or two toxins (VT1 and VT2) that cause human illnesses. These toxins are different proteins encoded by different genes. Another virulence factor expressed by VTEC is the protein intimin that is responsible for intimate attachment of VTEC and effacing lesions in the intestinal mucosa. This virulence factor is encoded by the chromosomal gene eae. The eae gene was found at a much less frequency in bovine (17%) and ovine (5%) than in human (45%) non-O157 VTEC strains. This may support the evidence that the eae gene contributes significantly to the virulence of human VTEC strains and that many animal non-O157 VTEC strains are less pathogenic to humans.