Occurrence of verotoxin-producing Escherichia coli in dairy heifers grazing an irrigated pasture

Abstract

Verotoxin-producing Escherichia coli (VTEC) produce one or two toxins known as VT1 and VT2. These toxins have been associated with several human illnesses. Dairy cattle harboring VTEC represent a potential health hazard because they enter the food chain as ground beef. The objective of this study was to assess the occurrence of VTEC in dairy heifers. A total of 91 fecal samples were rectally collected during four periods (spring, summer, fall, and winter of 1999) from 23 heifers. A random sample ( n=530) of potential VTEC isolates were tested for verotoxicity and were screened by the polymerase chain reaction (PCR) for presence or absence of VT1 and/or VT2 genes. Thirteen isolates from two heifers (from the winter collection) were verotoxic and were confirmed as E. coli. VTEC were only detected during winter with an occurrence rate of 9.5%. Using PCR, five isolates had the VT1 gene while the remaining eight had the VT2 gene. The sequence and expression of VT1 and VT2 genes were confirmed. No E. coli O157:H7 was detected, but serotyping revealed that the five VT1-positive isolates were O26:NM (a non-motile strain of O26). The remaining eight isolates were untypeable. Identification of VTEC-positive cattle before slaughter is a critical step in any on-farm strategy to minimize the risk of beef contamination with such pathogens.