Shiga toxin-producing Escherichia coli in beef heifers grazing an irrigated pasture

Shiga toxin-producing Escherichia coli (STEC) produce toxins that have been associated with several human illnesses. E. coli O157:H7 is the most well- studied STEC and was first associated with consumption of improperly cooked ground beef in 1982. E. coli O157:H7 is not the only foodborne STEC becaus e other STEC serotypes are also associated with human illnesses. The object ive of this study was to assess prevalence of STEC in 23 yearling beef (Ang us) heifers grazing an irrigated grass pasture in spring (April), summer (J uly), fall (October), and winter (December) of 1999. A total of 86 fecal sa mples were rectally collected and were subjected to microbiological testing for the presence of STEC. Nine E. coli isolates from five heifers (one in spring and fall and three in winter) were toxic to Vero cells. Of these iso lates, four were E. coli O157:H7, two belonged to the serogroup O6, one O39 :NM, one O113:H-, and the final isolate was untypable. The STEC prevalence rate in our herd ranged from 4% (spring) to 15% (winter). Based on detectin g both O157:H7 and non-O157:H7 STEC in our heifers, it is clear that screen ing fecal samples should not be limited to E. coli O157:H7. Identification of STEC-positive cattle prior to slaughter should help in reducing the risk of beef contamination with such foodborne pathogens if pre- and/or posthar vest control measures are applied to such animals.