Influence of weaning on fecal shedding of pathogenic bacteria in dairy calves.

The objectives of the current research were to determine the effect of weaning on fecal shedding of Salmonella and Escherichia coli O157:H7 in dairy calves and to examine cultured isolates (to include Enterococcus) for antimicrobial susceptibility. This research was conducted on one large commercial dairy (>3000 head) in the southwestern United States. Two collections were made, during the winter (January 2009) and summer (July 2009) seasons. For the winter collection, two groups of calves were sampled (group 1: n = 18 pens, 69 head, ∼12 weeks of age; group 2: n = 19 pens, 75 head, ∼10 weeks of age). Fecal samples were collected from all calves via rectal palpation 2 days pre- and again 2 days postweaning. For the summer collection, one group of calves housed in 40 pens were utilized and 79 and 76 calves sampled 7 days pre- and 5 days postweaning, respectively. Fecal samples were collected into sterile palpation sleeves, placed on ice, and shipped to our laboratory for bacterial culture of E. coli O157:H7, Salmonella, and Enterococcus. Antimicrobial susceptibility was determined on select isolates. No differences (p > 0.10) in prevalence of Salmonella or E. coli O157:H7 were observed due to weaning in the winter collection. In the summer collection, more (p < 0.01) fecal samples were Salmonella positive preweaning (15.2%) as compared to postweaning (2.6%). No differences were observed for antimicrobial susceptibility in isolates collected pre- as compared to postweaning in either winter or summer collections, with the exception that multidrug-resistant Enterococcus faecium isolates preweaning were resistant to six antibiotics compared to seven or eight antibiotics postweaning (summer collection). Results of the current research indicate that the weaning of dairy calves does not increase the prevalence of pathogenic bacteria or substantially modify antimicrobial susceptibility profiles of these bacteria.

Influence of wet distiller's grains on prevalence of Escherichia coli O157:H7 and Salmonella in feedlot cattle and antimicrobial susceptibility of generic Escherichia coli isolates.

The current research examined the inclusion of 20% wet distiller's grains (WDG) fed with steam-flaked corn (SFC) or dry-rolled corn (DRC) in diets fed to feedlot cattle on fecal prevalence of Escherichia coli O157:H7 and Salmonella. Crossbred beef heifers (n = 272; average initial body weight (BW) = 354 kg) were blocked by BW and pen size and randomly assigned to treatment. Fecal samples from freshly voided fecal pats were collected from each pen on the day cattle shipped for slaughter (237 fecal samples: 72, 125, and 40 from cattle 132, 160, and 181 days on feed, respectively). Fecal samples were cultured quantitatively and qualitatively for the above pathogens. Populations of E. coli O157:H7 and Salmonella were generally low with very few samples containing quantifiable populations. Similarly, after enrichment, few samples were E. coli O157:H7 positive in any collection with no treatment differences (p > 0.10). More samples were Salmonella positive during the first collection with an increased (p < 0.05) prevalence observed in the SFC and DRC treatments compared with DRC + WDG treatment. No other treatment differences were observed for Salmonella. Putative fecal coliform isolates (18 per treatment; first collection) were examined for antimicrobial susceptibility, and the majority were susceptible to all of the antibiotics examined. Most of the resistance was observed in the SFC (n = 3) and DRC (n = 4) treatments, and only one isolate in each of the two WDG treatments demonstrated resistance (one antibiotic each, streptomycin and tetracycline). All multidrug resistance (2-4 antibiotics) was observed in isolates cultured from the DRC and SFC treatments (n = 2 isolates in each treatment). Results of the current research found no significant effect of feeding WDG to feedlot cattle on fecal prevalence, at time of shipment for slaughter, of E. coli O157:H7, and only modest differences (decreases) in Salmonella prevalence with no apparent affect on antimicrobial susceptibility of fecal coliform isolates.

Influence of beta-agonists (ractopamine HCl and zilpaterol HCl) on fecal shedding of Escherichia coli O157:H7 in feedlot cattle.

Ractopamine HCl and zilpaterol HCl, beta-agonists recently approved for use in feedlot cattle to improve performance traits and carcass leanness, were examined for their effects on fecal shedding of Escherichia coli O157:H7 in feedlot cattle. Fecal samples (n = 2,454) were obtained from four experiments (one ractopamine HCl, three zilpaterol HCl) over the course of a 3-year period, either by rectal palpation (ractopamine HCl experiment) or from pen-floor fecal pats. Samples were cultured quantitatively and qualitatively for E. coli O157:H7. No significant treatment differences were detected for fecal prevalence of E. coli O157:H7 in the ractopamine HCl experiment. Zilpaterol HCl feeding had no effect (P > 0.20) on fecal shedding in the first or second experiments, with overall E. coli O157:H7 prevalence relatively low (<7%). In the third zilpaterol HCl experiment, the percentage of fecal samples that were E. coli O157:H7 positive following qualitative culture was higher (P < 0.05) in the zilpaterol HCl treatment (10.3%) than for the control (6.1%). The current research showed minimal effects of beta-agonists on fecal shedding of E. coli O157:H7 and indicated that these compounds (fed immediately prior to slaughter) are not a cause for concern from a food safety standpoint.

Influence of exogenous melatonin on horizontal transfer of Escherichia coli O157:H7 in experimentally infected sheep.

The objective of the current research was to determine if exogenous melatonin would exert a "protective" effect on the gastrointestinal tract of sheep and prevent or reduce the horizontal transfer of Escherichia coli O157:H7 from experimentally infected to noninfected or "naïve" sheep. Sixteen cross-bred ewes were housed indoors and adapted to a high concentrate ration. Ewes were randomly assigned to one of four rooms and treatment (three ewes/room, six ewes/treatment) and received either control (gelatin capsule only) or melatonin (5.0 mg/kg body weight [BW]/d). Four additional ewes served as "carrier" sheep (one/room) and were experimentally infected via oral gavage with E. coli O157:H7. Three days post-challenge, carrier ewes were housed with naïve sheep and remained with them for the remainder of the experimental period. Treatments were administered to the naïve sheep 1 day prior to introduction of the carrier sheep and on each of the remaining 7 days of the experimental period. Fecal samples were collected via rectal palpation from the carrier sheep daily throughout experiment and from the naïve sheep daily for 5 days, starting 2 days following introduction of the carriers. On day 8 of the experiment, all ewes were euthanized and tissues from the rumen, ileum, cecum, colon, and rectum as well as their respective lumen contents collected. The carrier sheep quickly infected the naïve ewes, which had similar fecal concentrations as the carrier animals throughout the 5-day sampling period. Melatonin treatment had no effect (p > 0.10) on daily fecal shedding, luminal content concentrations, or in the percentage of gastrointestinal tract tissue positive for the inoculated strain of E. coli O157:H7.

Influence of vitamin D on fecal shedding of Escherichia coli O157:H7 in naturally colonized cattle.

Three experiments were conducted to evaluate the influence of vitamin D on fecal shedding of Escherichia coli O157:H7 in cattle. In the first experiment, two groups of cattle (beef and dairy) were assigned to a control treatment or to receive 0.5 × 10(6) IU vitamin D per day via oral bolus for 10 days. Fecal samples were collected before and throughout the dosing period for culture of E. coli O157:H7. No differences were observed for fecal shedding of E. coli O157:H7 among treatments for either beef or dairy animals. Serum concentrations of vitamin D were markedly higher (P < 0.0001) in treated beef cattle but only tended to be higher (P = 0.09) in the dairy cattle. In the second experiment, three successive vitamin D dosages (2,400, 4,800, and 9,600 IU/day; 14 days each) were administered to 14 dairy steers (7 steers served as controls), fecal samples were collected daily, and serum samples were collected weekly throughout the 42-day experimental period. No significant differences in fecal prevalence or serum vitamin D concentrations were observed for any of the vitamin D dosages. A third experiment sampled feedlot cattle (winter and summer) to determine whether serum vitamin D concentrations were correlated with fecal shedding of E. coli O157:H7. A fecal sample and a blood sample were obtained in each season from 60 randomly selected animals (total of 120 fecal samples and 120 corresponding blood samples). As expected, season was highly correlated (r = 0.66) with serum vitamin D concentration with higher concentrations (P < 0.01) observed in the summer. E. coli O157:H7 prevalence (percentage of positive samples) was not highly correlated (r = 0.16) with season, although the correlation tended to be significant (P = 0.08). The proportion of cattle shedding E. coli O157:H7 was 16.7 and 6.7% for the summer and winter collections, respectively. Results of this research do not support a correlation between vitamin D intake and E. coli O157:H7 shedding in cattle.