Abstract
Shiga toxin-producing Escherichia coli (STEC) are one of the leading causes of human gastrointestinal disease in the United States and other developed countries worldwide. Illnesses caused by STEC range from mild non-bloody diarrhea to hemorrhagic colitis that can lead to life threatening renal failure, a condition known as hemolytic-uremic syndrome (HUS). The most common reservoir of STEC is cattle. Therefore, the majority of human outbreaks reported have been associated with the consumption of undercooked ground beef and other beef products (Arthur et al. 2002; Gyles 2007; Hussein et al. 2003). There are two main groups of cattle that are used in the beef manufacturing industry, feedlot cattle and adult range-grazing cattle. Earlier investigations of STEC prevalence have focused primarily on the O157:H7 Escherichia coli serotype in cattle on feedlots. This is problematic, as over 100 other serotypes have been identified as having pathogenic potential in humans (Gioffre et al. 2002; Gyles et al. 1998; Law 2000), and not all cattle that enter the food chain spend time on feedlots. Adult range-grazing cattle are removed from their herd once past their reproductive prime, and sent directly to meat processing. They are not grain fed on a feedlot like younger cattle. Research that uses feedlot cattle for their investigations of STEC prevalence should not be applied to non-feedlot cattle. The two groups of cattle are managed differently and should be investigated separately. The aim of this study was to determine the prevalence levels of all types of STEC in healthy adult range-grazing cattle from Northern California, specifically Sacramento County and neighboring counties. Our hypothesis was that healthy adult range-grazing cattle would have a prevalence of STEC lower than what previous studies utilizing feedlot cattle have reported. In this study, we also assessed the effects of precipitation and temperature on the prevalence of STEC in one isolated cattle herd over a sixteen-month time period, and then analyzed our data using linear regression analysis. To test our hypothesis, fresh fecal samples from 152 healthy range-grazing cattle were collected and evaluated for the presence of the Shiga toxin genes by PCR. It was determined that 71.7% of the samples tested positive for the Shiga toxins genes. It was also determined that no statistically significant relationship existed between the cattle herd’s STEC prevalence and the herd’s size or density. Finally, the results from our seasonal prevalence study suggested that precipitation and temperature did not have a statistically significant relationship on the prevalence of STEC in adult range-grazing cattle. Taken together, our findings suggest that other environmental factors besides temperature and precipitation are contributing to the prevalence of STEC in range- grazing cattle, and that the percent of STEC in range-grazing cattle is much higher than originally hypothesized.