SUMMARYEscherichia coli 0157. H7 was found in 10 of 3570 (0 28 %) faecal samples from dairy cattle in 5 of 60 herds (8-3%). Several tentative associations with manure handling and feeding management practices on dairy farms were identified. Faecal/urine slurry samples, bulk milk samples, and milk filters from dairy herds were negative for E. coli 0157. H7. E. coli 0157. H7 was also isolated from 10 of 1412 (0-71 %) faecal samples from pastured beef cattle in 4 of 25 (16%) herds. The prevalence ofE. coli 0157. H7 excretion in feedlot beef cattle was 2 of 600 (0 33 %). The identification of cattle management practices associated with colonization of cattle by E. coli 0157. H7 suggests the possibility that human E. coli 0157. H7 exposure may be reduced by cattle management procedures.

Escherichia coli O157 shedding in 14 cattle herds was determined by faecal culture at intervals of approximately 1 month for up to 13 months. The overall prevalence was 1.0% (113/10832 faecal samples) and 9 of the 14 herds were detected as positive. Herds positive 2 years previously (n = 5) had a higher prevalence of positive cattle (median = 1.9%) than herds which had been negative on a previous sampling (n = 8, median = 0.2%). Weaned heifers had a higher prevalence (1.8%) than did unweaned calves (0.9%) or adults (0.4%). For all herds the highest prevalence occurred in the summer months, which resulted in most of the positive faecal samples being collected on a minority of sampling visits.

This study examined the sensitivity, specificity, predictive values, and classification accuracy of 3 commonly used screening tests for failure of passive transfer: the sodium sulfite turbidity test, the zinc sulfate turbidity test, and refractometry relative t o serum immunoglobulin G1 (IgG,) concentrations determined by radial immunodiffusion. Serum samples were obtained from 242 calves ranging from 1 t o 8 days of age. Using a serum concentration of 1,000 mg/dL IgG, t o define adequate passive transfer, the zinc sulfate test had a sensitivity of 1.00 and a specificity of 0.52 in the detection of inadequate passive transfer. The endpoint of the test appeared t o be higher than desired; calves testing negative had mean serum lgG, concentration of 955 mg/ dL and a large proportion of calves with adequate passive transfer were misclassified as positive for failure of passive transfer. Using the qualitative zinc sulfate test, the percentage of calves correctly classified with regard t o passive transfer status was less than that observed with either the sodium sulfite test or refractornetry. The sensitivity of the sodium sulfite assay was 0.85 at a 1 + endpoint and 1.00 at alves are born without appreciable serum concentra-C tions of immunoglobulin (Ig). Ig requirements are satisfied by ingesting and absorbing colostral 1gs.l.' Adequate passive transfer of Igs is a critical determinant of calf health and s~r v i v a l .~~' Several assays have been used to measure serum Ig concentrations in neonatal livestock. These include refractometer determination of total serum protein concentration, zinc sulfate and sodium sulfite turbidimetry, immunoassays, and the glutaraldehyde clot test.'.*-'' Assay procedures should be adapted to a practice setting. Immediacy of results permits practitioners to provide feedback to calfrearing personnel and make clinical decisions regarding the necessity of targeted intervention in individual calves with failure of passive transfer (FPT).Immunoassay procedures detecting bovine IgG have not been successfully adapted to a practice setting. Additionally, the time constraints of sample collection, sample submission, and assays procedures often relegate the results of immunoassays to historical interest. Labor and expense also have limited the widespread on-farm application of immunoassay procedures. The recently developed and marketed whole blood glutaraldehyde coagulation test attempted to address these timing and economic constraints; however, the sensitivity and specificity of the assay proved inadequate for routine diagnostic use." The primary goal of this study was to compare the sensitivity, specificity, and predictive values of 3 commonly used screening tests for the detection of FPT in calves. These test procedures included refractometry, the qualitative zinc sulfate turbidity test, and the semiquantitative sodium sulfite turbidity test. All 3 assays are readily adapted to a farm or practice setting. Assay results were compared with serum IgG I concentrations determined by radial immu...

The prevalence of antimicrobial drug-resistant bacteria is typically highest in younger animals, and prevalence is not necessarily related to recent use of antimicrobial drugs. In dairy cattle, we hypothesize that antimicrobial drug-resistant, neonate-adapted bacteria are responsible for the observed high frequencies of resistant Escherichia coli in calves. To explore this issue, we examined the age distribution of antimicrobial drug-resistant E. coli from Holstein cattle at a local dairy and conducted an experiment to determine if low doses of oxytetracycline affected the prevalence of antimicrobial drug-resistant E. coli. Isolates resistant to tetracycline (>4 g/ml) were more prevalent in <3-month-old calves (79%) compared with lactating cows (14%). In an experimental trial where calves received diets supplemented with or without oxytetracycline, the prevalence of tetracycline-resistant E. coli was slightly higher for the latter group (P ‫؍‬ 0.039), indicating that drug use was not required to maintain a high prevalence of resistant E. coli. The most common resistance pattern among calf E. coli isolates included resistance to streptomycin (>12 g/ml), sulfadiazine (>512 g/ml), and tetracycline (>4 g/ml) (SSuT), and this resistance pattern was most prevalent during the period when calves were on milk diets. To determine if prevalence was a function of differential fitness, we orally inoculated animals with nalidixic acid-resistant strains of SSuT E. coli and susceptible E. coli. Shedding of SSuT E. coli was significantly greater than that of susceptible strains in neonatal calves (P < 0.001), whereas there was no difference in older animals (P ‫؍‬ 0.5). These data support the hypothesis that active selection for traits linked to the SSuT phenotype are responsible for maintaining drug-resistant E. coli in this population of dairy calves.

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