Sorbitol-Negative Shiga Toxin-Producing Escherichia coli in Beef Heifers Grazing Rangeland Forages

Abstract

Escherichia coli O157:H7 has been a major cause of human illness outbreaks in the U.S., mainly attributable to the consumption of contaminated beef. Worldwide, however, other shiga toxin-producing E. coli (STEC) have been responsible for major outbreaks of human illnesses. Because of the lack of sorbitol fermentation and β-glucuronidase activity, E. coli O157:H7 is relatively easier to detect than other STEC. The objective of this experiment was to examine prevalence of sorbitol-negative STEC in beef heifers grazing rangeland forages over the four seasons of 1999. However, because of the loss of heifers and vegetation to wild fires during the summer, the study was terminated, and the results of two seasons (winter and spring) were reported. A random number of heifers (22 in winter; 23 in spring) from a herd of 70 were fecal-sampled; nine were sampled in both seasons. A random sample (n = 115) of potential sorbitol-negative STEC isolates was screened for the presence and expression of toxin genes. Fifty-six STEC isolates were detected in feces of five heifers in the winter and two heifers in the spring with occurrence rates of 22.7 and 8.7%, respectively. Of these isolates, 37 had and expressed Shiga Toxin 1(Stx1), 14 had and expressed Shiga Toxin 2 (Stx2), and five had but did not express stx2. Only non-O157:H7 STEC (O118:H–[non-motile] and O138:H–) were detected. Because E. coli O118:H–is pathogenic to humans, it is important to determine its incidence in cattle and to determine the risk of beef contamination with such a pathogen.