Objective—To evaluate site-to-site variation within
fecal pats from cattle with regard to detection of
Escherichia coli O157 and determine the effect on the
accuracy of prevalence estimates of assay of multiple
samples collected from the same fecal pat.
Sample Population—120 freshly voided fecal pats
collected from 2 beef feedlots.
Procedures—5 samples were systematically collected
from each fecal pat and analyzed for E coli O157 via
selective preenrichment techniques, immunomagnetic
separation, and biochemical tests. Presumptive isolates
were definitively identified via agglutination
assays and polymerase chain reaction techniques. Best
estimators of prevalence were calculated from the distribution
of E coli O157–positive samples per pat.
Results—Of the 120 fecal pats, 96, 13, 4, 2, 3, and 2
fecal pats had 0, 1, 2, 3, 4, and 5 E coli O157–positive
samples, respectively. The greatest estimate of E coli
O157 prevalence (20%) was achieved when all 5 samples
were assessed; this estimate represented a 2.4-
fold increase in prevalence, compared with that provided
via analysis of 1 sample/pat (8.2%). Compared
with assessment of 5 sites/pat, the relative sensitivity
of detecting an E coli O157–positive fecal pat via
analysis of 1 site/pat was 40.1%.
Conclusions and Clinical Relevance—Results suggest
that estimates of E coli O157 prevalence derived
from sampling of 1 location/pat are likely underestimates
of the true prevalence of this pathogen in fecal
pats (and by extension, cattle). Additional research is
warranted to confirm these results in situations of
high and low prevalence and across different feedlots.
(Am J Vet Res 2005;66:2023–2027)
Objective—To determine effects of administration of ceftiofur crystalline-free acid (CCFA) on antimicrobial susceptibility of Escherichia coli in feedlot cattle.
Animals—61 feedlot steers.
Procedures—A cohort study was conducted. Steers were housed in pens (5 pens with 10 steers and 1 pen with 11 steers). Five steers in each pen were administered CCFA, and 5 served as control steers (1 pen had 6 control steers). The CCFA administration included a single-dose regimen (6.6 mg/kg, SC, on day 0), two-thirds–dose regimen (4.4 mg/kg, SC, on day 0), and 3-dose regimen (6.6 mg/kg, SC, on days 0, 6, and 13). Fecal samples were collected on days 0, 2, 6, 9, 13, 16, 20, and 28. Fecal samples were collected immediately before CCFA administration. Minimum inhibitory concentrations of 15 antimicrobials were determined for 3 E coli isolates/fecal sample. Escherichia coli were enumerated by use of direct-plating techniques.
Results—Resistance to 1 or more antimicrobials was detected in 986 of 1,441 (68.4%) isolates recovered. Administration of CCFA was associated with a transient increase in the population of ceftiofur-resistant isolates. Susceptibility returned to day 0 values (ie, samples collected immediately before CCFA administration) approximately 2 weeks after completion of CCFA administration. Agreement between ceftiofur resistance and coresistance to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline was almost perfect (κ 0.97). We did not detect variation in susceptibility of E coli recovered from commingled control steers.
Conclusions and Clinical Relevance—Administration of CCFA provided selection pressure that favored transient expansion of multiple-resistant variants.