| The purpose of this research was to investigate how Escherichia coli O157:147 disseminates among cattle herds. The first part was an evaluation of pulsed-field gel electrophoresis (PFGE) as a measure of relatedness. The correlation between similarity coefficients using two enzymes was 0.53. Four additional restriction enzymes were used on a subset of isolates. The average correlations between similarity coefficients from pairwise comparisons using sets of one, two, and three enzymes were 0.405, 0.568, and 0.65, respectively. Southern blot analysis demonstrated that same sized bands sometimes contain nonhomologous genetic material. These findings indicated that if epidemiologic relationships between isolates must be inferred from PFGE data, six or more restriction enzymes would be needed to measure genetic relatedness.; Second, we compared the geographic and genetic distance of a set of international bovine Escherichia coli O157:117 isolates. Six different restriction enzyme digests were used to generate an average Dice similarity coefficient for each isolate comparison. The Mantel correlation between genetic similarity and the logarithm of geographic distance in kilometers was −0.21 (P < 0.01). The finding of a significant Mantel correlation confirms that transmission over a short distance is more likely to occur than over a long distance, however the low magnitude suggests that transmission over long distances is common. A composite dendrogram was consistent with this conclusion. Cattle feed shipments are the most plausible vehicle for dissemination, compared to alternatives such as human or animal movements.; To more directly test the hypothesis that feeds contribute to populations of pathogens on farms, E. coli 0157:117 and Salmonella isolates from feed samples were compared to bovine fecal isolates from the same farm using PFGE. Four of 2,365 component feed samples and one of 226 feed mill samples were positive for E. coli 0157:117. Twenty of 2,405 component feed samples were positive for Salmonella . Component feed Salmonella isolates were found to have indistinguishable PFGE profiles compared to fecal isolates. The evidence from Salmonella PFGE typing and the isolation of E. coli O157:147 from a feed mill provide strong evidence that feeds serve as a vehicle for these pathogens among farms. |
