Submitted to: International Symposium and Workshop on Shiga Toxin ... Escherichia coli
Publication Type: Abstract Only
Publication Acceptance Date: August 20, 2002
Publication Date: N/A
Technical Abstract: Fecal shedding of E. coli O157:H7 was examined in large dairy herds (> 1000 hd, n = 5) in the southwestern United States over a two year period. Rectal grab samples were analyzed using an immunomagnetic bead separation technique and selected isolates examined for antimicrobial susceptibility by broth microdilution. The percentage of cows shedding E. coli varied considerably among farms ranging from less than 1% to 56%. Lactating cows shed more (P < 0.05) than non-lactating (66 vs 34%, respectively) and on one farm cows in early lactation tended (P = 0.10) to shed more than late lactation (> 60 days in milk) cows. Further examination of early vs late lactation on a different farm found no difference in E. coli O157:H7 shedding. It should be noted, however, that the majority of cows were shedding Salmonella. As we observed on this farm and others, when Salmonella shedding is high, E. coli O157:H7 shedding is low. The effect of heat stress was examined on a smaller (200 head) dairy by sampling individual cows (n = 80) in the morning (coolest part of the day) and again in the afternoon (hottest). While more animals were found to shed E. coli O157:H7 in the afternoon (11.1 vs 2%), the number of shedders was low overall and almost 100% of the cows were shedding Salmonella at both sampling times. Fifteen E. coli isolates were examined for susceptibility to antibiotics commonly used by United States veterinarians. At least one isolate was resistant to each of the antibiotics on the panel with the exception of ceftiofur and trimethoprim/sulphamethoxazole. Multiple resistance was observed in two-thirds of the E. coli isolates. Six isolates were resistant to five or less antibiotics, three were resistant to nine or less, and four isolates showed resistance to ten or more antibiotics.