Verotoxigenic Escherichia coli O157:H7 from Swedish cattle; isolates from prevalence studies versus strains linked to human infections--a retrospective study

Abstract

Background: Several cases of human infection caused by verotoxin-producing Escherichia coli (VTEC) O157:H7 in Sweden have been connected with cattle farm visits. Between 1996 and 2002, 18 farms were classified as the source of human cases with isolation of EHEC (Enterohaemorrhagic Escherichia coli) after VTEC O157:H7 had been isolated from cattle on those farms.

Results: Characterization by phage typing and molecular methods of the strains isolated from these 18 farms, including PCR for virulence genes (vtx1, vtx2 and variants thereof, eaeA and EHEC-hlyA) and Pulsed-Field Gel Electrophoresis (PFGE), demonstrated a cluster of very similar strains from 16 farms. All were of phage type 4, carried the genes encoding the verotoxins VT2 and VT2c, intimin, EHEC-haemolysin and flagellin H7 as shown by PCR, and had identical or very similar PFGE patterns. When analysing strains in a prevalence study of VTEC O157:H7 from cattle at slaughter as well as from an on-farm prevalence study of dairy cattle, using the same typing methods, a rather wide variation was observed among the isolated VTEC O157:H7 strains.

Conclusions: In Sweden, a limited group of genetically similar and highly pathogenic VTEC O157:H7 strains seem to predominate in direct or indirect transmission from cattle to man.

Figure 1

Localization of cattle farms implicated in human cases versus origin of randomly isolated VTEC O157 strains. Localization of cattle farms implicated in human EHEC cases viz-á-vis a map illustrating the origin of the randomly isolated VTEC O157 strains. (a) farms implicated in EHEC cases (*); (b) prevalence studies based on slaughterhouse sampling (■), prevalence study of on-farm dairy cattle (□). Arrow in (a) indicates the county of Halland.

Figure 2

Origin of VTEC O157 strains being PT4, vtx₂ and vtx_2c. Map of (a) cattle farms implicated in human EHEC cases, viz-á-vis a map illustrating the geographical origin of the (b) randomly isolated VTEC O157 strains. Strains being PT4, vtx₂ and vtx_2c marked (●); other (○)

Figure 3

Origin of randomly isolated VTEC O157:H7 strainsbeing PT4, vtx₂ and vtx_2c from two different time periods. Map (a) illustrating the geographical origin of the 37 VTEC O157 isolates from the slaughterhouse survey performed between 1996-97, viz-à-viz a map (b) illustrating the geographical origin of the 29 VTEC O157 strains isolated in the 2002 slaughterhouse prevalence study. Strains being PT4, vtx₂ and vtx_2c marked (●); other (○).