Objective—To evaluate administration of chlortetracycline in feed of cattle as a method to select for tetracycline resistance among enteric bacteria in feedlot settings.
Procedures—Steers were randomly assigned to an exposed cohort (n = 10) or an unexposed cohort (control cohort; 10). Chlortetracycline (22 mg/kg) in cottonseed meal was administered to the exposed cohort on days 0 through 4, 6 through 10, and 12 through 16. The control cohort was administered only cottonseed meal. Fecal samples were collected from 16 steers on days −7, 0, 2, 6, 8, 12, 14, 19, 22, 26, and 33, and Escherichia coli and Enterococcus spp were isolated. Minimum inhibitory concentration (MIC) of selected antimicrobials was estimated.
Results—Overall, 56.0% and 31.4% of E coli and Enterococcus isolates, respectively, were resistant to tetracycline. Exposure to chlortetracycline was associated with a significant temporary increase in log2 MIC for both genera but returned to preexposure values by day 33. Averaged across time, the proportion of tetracycline-resistant E coli and Enterococcus isolates was significantly greater in exposed than in unexposed steers. Although all ceftiofur-resistant E coli isolates were coresistant to tetracycline, exposure to chlortetracycline led to a significant decrease in the proportion of E coli resistant to ceftiofur during exposure.
Conclusions and Clinical Relevance—Exposure to chlortetracycline was associated with a temporary increase in the likelihood of recovering resistant bacteria. Exposure to chlortetracycline decreased the likelihood of recovering ceftiofur-resistant E coli isolates, even though isolates were coresistant to tetracycline. These findings warrant further investigation.
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.
Objective—To evaluate the effect of feeding aspirin and
supplemental vitamin E on growth performance, lung
lesions, plasma concentrations of 3-methylindole (3MI),
and 3-methyleneindolenine (3MEIN)-adduct concentrations
in blood and pulmonary tissues of feedlot cattle.
Animals—256 crossbred steers; 64 cattle were used in
experiment 1 and 192 cattle were used in experiment 2.
Procedures—A 2 × 2 factorial design was used for
each experiment. Treatment factors were aspirin (0 or
3 g daily) and vitamin E (200 or 1,500 IU daily). Steers
were housed in pens (8 steers/pen). Steers were
slaughtered on days 59 and 138 for experiments 1
and 2, respectively. Lungs were grossly evaluated.
Plasma 3MI concentration was determined, and
3MEIN-adduct concentrations were measured in
blood and pulmonary tissues.
Results—Treatment was not associated with
improvement or adverse effects on weight gain, drymatter
intake, or feed efficiency in experiment 2. In
experiment 1, 36 of 63 (57.1%) steers had lung
lesions. Lesions were not associated with treatment
or concentrations of 3MI and 3MEIN-adduct. Plasma
3MI concentration and concentrations of 3MEINadduct
in blood and pulmonary tissues were
3.11 µg/mL, 0.51 U/µg of protein, and 0.49 U/µg of
protein, respectively. Aspirin was associated with
increased blood concentrations of 3MEIN-adduct for
diets that did not contain supplemental vitamin E.
Conclusions and Clinical Relevance—Differences in
performance of feedlot steers were not associated with
treatment diet. It is possible that concurrent exposure of
feedlot cattle to other factors typically associated with
development of respiratory tract disease would affect
these findings. (Am J Vet Res 2002:63:1641–1647)
Objective—To determine associations between in vitro minimum inhibitory concentrations (MICs) of tilmicosin against Mannheimia haemolytica and Pasteurella multocida and in vivo tilmicosin treatment outcome among calves with clinical signs of bovine respiratory disease (BRD).
Animals—976 feeder calves with clinical signs of BRD enrolled in 16 randomized clinical trials.
Procedures—Records of clinical trials from October 26, 1996, to November 15, 2004, were searched to identify calves with BRD from which a single isolate of M haemolytica or P multocida was identified via culture of deep nasal swab samples prior to treatment with tilmicosin (10 mg/kg [4.5 mg/lb], SC) and for which MICs of tilmicosin against the isolate were determined. The MICs of tilmicosin against recovered isolates and response to tilmicosin treatment were evaluated.
Results—Tilmicosin resistance among M haemolytica and P multocida isolates was uncommon (6/745 [0.8%] and 16/231 [6.9%], respectively). Treatment outcome, defined as success or failure after tilmicosin treatment, did not vary with the MIC of tilmicosin against recovered isolates. The proportion of treatment failures attributed to M haemolytica isolates categorized as resistant (MIC of tilmicosin, ≥ 32 μg/mL) or not susceptible (MIC of tilmicosin, ≥ 16 μg/mL), was 0.2% and 0.5%, respectively.
Conclusions and Clinical Relevance—Recovery of tilmicosin-resistant M haemolytica or P multocida isolates was rare, and no association was detected between MIC of tilmicosin and treatment response.
OBJECTIVE To investigate the effects of dietary supplementation with the β-adrenoceptor agonists ractopamine hydrochloride and zilpaterol hydrochloride on ECG and clinicopathologic variables of finishing beef steers.
DESIGN Randomized controlled trial.
ANIMALS 30 Angus steers.
PROCEDURES Steers were grouped by body weight and randomly assigned to receive 1 of 3 diets for 23 days: a diet containing no additive (control diet) or a diet containing ractopamine hydrochloride (300 mg/steer/d) or zilpaterol hydrochloride (8.3 mg/kg [3.8 mg/lb] of feed on a dry-matter basis), beginning on day 0. Steers were instrumented with an ambulatory ECG monitor on days −2, 6, 13, and 23, and continuous recordings were obtained for 72, 24, 24, and 96 hours, respectively. At the time of instrumentation, blood samples were obtained for CBC and serum biochemical and blood lactate analysis. Electrocardiographic recordings were evaluated for mean heart rate and arrhythmia rates.
RESULTS Steers fed zilpaterol or ractopamine had greater mean heart rates than those fed the control diet. Mean heart rates were within reference limits for all steers, with the exception of those in the ractopamine group on day 14, in which mean heart rate was high. No differences in arrhythmia rates were identified among the groups, nor were any differences identified when arrhythmias were classified as single, paired, or multiple (> 2) beats.
CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that dietary supplementation of cattle with ractopamine or zilpaterol at FDA-approved doses had no effect on arrhythmia rates but caused an increase in heart rate that remained within reference limits.
Objective—To compare concentrations of 3-methyleneindolenine
(3MEIN) in lung tissues obtained from
feedlot cattle that died as a result of acute interstitial
pneumonia (AIP) and cattle that died as a result of
other causes and to compare blood concentrations of
3MEIN in healthy feedlot cattle and feedlot cattle with
Study Population—Blood samples and lung tissues
collected from 186 cattle housed in 14 feedlots in the
western United States.
Procedure—Samples of lung tissues were collected
during routine postmortem examination and submitted
for histologic, microbiologic, and toxicologic examination.
Blood samples were collected from cattle with
clinical manifestations of AIP and healthy penmates.
Histologic diagnoses were categorized as AIP, bronchopneumonia
(BP), control samples, and other disorders.
Concentrations of 3MEIN were determined in
lung tissues and blood samples, using an ELISA.
Results—Concentrations of 3MEIN in lung tissues
were significantly greater in AIP and BP samples,
compared with control samples. Absorbance per
microgram of protein did not differ between BP and
AIP samples. Blood concentrations of 3MEIN were
significantly greater in cattle with AIP, compared with
healthy cattle or cattle with BP. Odds of an animal with
AIP being a heifer was 3.1 times greater than the
odds of that animal being a steer.
Conclusion and Clinical Relevance—Increased
pulmonary production of 3MEIN may be an important
etiologic factor in feedlot-associated AIP. (Am J Vet