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Canadian Journal of Veterinary Research logoLink to Canadian Journal of Veterinary Research
. 2008 Mar;72(2):101–108.

Antimicrobial resistance in generic fecal Escherichia coli obtained from beef cattle on arrival at the feedlot and prior to slaughter, and associations with volume of total individual cattle antimicrobial treatments in one western Canadian feedlot

Sylvia L Checkley 1,✉, John R Campbell 1, Manuel Chirino-Trejo 1, Eugene D Janzen 1, John J McKinnon 1
PMCID: PMC2276894  PMID: 18505198

Abstract

A prospective observational study was carried out to examine antimicrobial resistance patterns of fecal Escherichia coli isolates of calves on arrival at the feedlot, and then evaluate the associations between the total volume of antimicrobial used for disease treatment and changes in antimicrobial resistance, during the feeding period. No macrolides or tetracyclines were administered in the feed during this study. On arrival, at the animal level, all 3 isolates obtained from 36.6% [95% confidence interval (CI): 29.0 to 44.8] of all cattle sampled (n = 153), were susceptible to all antimicrobials, while 5.9% (95% CI: 2.7 to 10.9) of cattle had at least 1 isolate that was resistant to ≥ 3 antimicrobials out of the 7 antimicrobials tested. The most frequent antimicrobials for which resistance was observed were sulphamethoxazole, ampicillin, and tetracycline where, of all cattle, 44.4% (95% CI: 36.4 to 52.7), 20.3% (95% CI: 14.2 to 27.5), and 17.7% (95% CI: 12.0 to 24.6), respectively had at least 1 resistant isolate. All cattle received antimicrobial metaphylaxis on arrival at the feedlot. Antimicrobial use was described for a cohort of 95 cattle. Antimicrobials were given to 42 of the 95 cattle during the feeding period, to treat disease. Amongst the 42 treated cattle, there were a total of 133 animal daily doses (ADDFeedlot), where 1 ADDFeedlot represented 1 day of antimicrobial treatment received by a feedlot animal at the approved dose. Only 1 ADDFeedlot was given in the 100 days immediately prior to slaughter. There were no associations found between antimicrobial use and antimicrobial resistance in this study.

Introduction

Antimicrobial resistance (AMR) has been perceived as an important component of both animal health and food safety in the international community and specifically by countries that import beef (1–4). Health Canada has examined the use of antimicrobials in agriculture (5); however, the pathways by which antimicrobial use (AMU) in cattle could affect human health were not understood (5–7). More information was also needed about the extent to which the use of antimicrobials in animal agriculture was related to AMR in human infections (5,8,9). In Canada, the beef industry and the veterinarians serving that industry, were aware of these issues and thus developed prudent use guidelines for antimicrobials (10–12). The objectives of this study were to examine AMR patterns of fecal Escherichia coli isolates of auction market derived, newly weaned calves on arrival at the feedlot, and then evaluate the associations between the total volume of parenteral antimicrobials used for disease treatment and changes in antimicrobial resistance, during the feeding period.

Materials and methods

The University Committee on Animal Care and Supply approved this study and the guidelines of the Canadian Council on Animal Care were followed. This study was carried out at the same time as another trial described previously (13).

In the fall of 2001, backgrounding of 447 newly weaned, auction market derived steers was carried out at the University of Saskatchewan Research Feedlot (1 time capacity 800 head) with 12 adjacent pens of 37 or 38 steers. The steers were crossbred beef calves with an average weight of 249 kg (s = 17 kg). They were divided into weight groups and randomly allocated at processing to create pens of approximately equal weights. The routine vaccination, parasiticide, and implant strategy was described previously (13). A metaphylactic antimicrobial injection of long-acting oxytetracycline (Liquamycin LA-200; Pfizer Canada Animal Health Group, Kirkland, Quebec), 1 mL/10 kg bodyweight (BW), SC, was given to each animal with a body temperature < 41°C. The rest of the cattle received tilmicosin (Micotil; Provel, Division Eli Lilly Canada, Guelph, Ontario), 1 mL/30 kg BW, SC, for disease treatment. Booster vaccinations and the 2nd hormonal implant were given at 94 days on feed (DOF), 18 d before the steers moved to the finishing feedyard. The steers were finished in 2 adjacent pens in a large commercial feedlot with a 1 time capacity of approximately 30 000 head. These cattle remained in their respective study groups until slaughter.

During the study, all of the aforementioned animal health products were used according to label instructions and detailed individual animal records were maintained. Both feedlots were typical of those found in western Canada, with dirt floors, shared waterers, and a central alley for feeding. Feedlot staff checked all cattle daily for signs of disease. Any cattle that appeared depressed, gaunt, or distinctly different from their penmates were pulled and treated according to the standard treatment protocols in use at the feedlot. Cattle were weighed before treatments, and all medications were used at the approved dose per kg body weight. The feedlot staff was blinded as to the allocation of the treatment groups and to the specific objectives of the study. Steers were also individually weighed at the feedlot within 24 h of slaughter at 245 to 260 DOF.

Pens were randomly entered into 1 of 2 feeding programs, with different diet compositions and feeding methods, used at the University feedlot during the backgrounding period (13). Monensin sodium 3% (Rumensin; Elanco Animal Health, Guelph, Ontario) was fed to all cattle during the backgrounding period in the total mixed ration at 27–28 ppm DM. All cattle were fed a high grain diet ad libitum with decoquinate at a dose of 0.5 mg/kg BW (Deccox 6% Premix; Alpharma, Mississauga, Ontario) during the finishing period.

Calculations of sample size were performed for the 2 main objectives (Win Episcope v2; University of Edinburgh, Edinburgh, Scotland). Fecal samples from 150 steers were sufficient to characterize AMR in as few as 5% to 10% of cattle with 80% power and 95% confidence. Sample size for the cohort study depended on the total number of treated cattle during the study, estimated between 15% and 45%, based on previously reported bovine respiratory disease (BRD) treatment rates (14). Fifty cattle per group, based on 80% power and 95% confidence, were needed to detect a significant relative risk of 1.5 to 2.

On arrival, fecal samples were collected from the rectums of all cattle using a new obstetrical glove for each animal. The samples were placed into individual, clean foam cups with lids, labeled, and then transported to the laboratory for direct storage at −80°C. One hundred and fifty of the 447 mixed-breed steers were chosen randomly for characterization of AMR in fecal E. coli isolates on arrival. At the end of the study, a random sample of cattle that were treated with antimicrobials for disease during the feeding period, and a sample of those that were not treated, were chosen for inclusion in the cohort study. The arrival samples from these cattle were then retrieved from the frozen, stored samples and sent for laboratory analysis. A 2nd fecal sample was collected from the cohort of cattle within 24 h of slaughter. All preslaughter fecal samples were collected in the same manner as the arrival samples, and transported to the laboratory for culture the same day.

Laboratory analysis

The feces were thawed (arrival samples only) and cultured overnight on MacConkey’s agar. Identification of E. coli was confirmed by standard biochemical tests. Three individual colonies from each animal, with the characteristic phenotype of E. coli, were chosen randomly for subculture on blood agar. From each blood agar plate, E. coli were inoculated into sterile phosphate-buffered saline (PBS) to make standard solutions of 0.5 MacFarland. This solution was delivered onto Mueller-Hinton agar using the replicator technique. The minimum inhibitory concentrations (MICs) of 7 antimicrobials were determined using the agar dilution method. The antimicrobials tested were ampicillin (AMP), enrofloxacin (ENR), tetracycline (TCY), gentamycin (GEN), sulphamethoxazole (SMX), trimethoprim/sulfanilamide (TMP/SSS), and trimethoprim (TMP). The Mueller-Hinton plates were cultured at 37°C and antimicrobial susceptibilities were read between 18 and 24 h. A control strain of E. coli ATCC 25922 was included with each plate. Antimicrobial breakpoints and interpretation were from the CLSI standards (15,16) (Table I). All laboratory procedures were carried out according to CLSI standards.

Table I.

Concentration range and breakpoints of antimicrobials tested

Antimicrobiala Breakpoint for resistance (μg/mL) Concentration range measured (μg/mL)
AMP ≥ 32 < 4, 4, 8, 16, 32, > 32
ENR ≥ 2 < 0.5, 0.5, 1, 2, 4, > 4
GEN ≥ 16 < 4, 4, 8, 16, 32, > 32
SMX ≥ 512 < 125, 125, 256, 512, > 512
TCY ≥ 16 < 2, 2, 4, 8, 16, 32, > 32
TMP/SSS ≥ 4/76 < 1/19, 1/19, 2/38, 4/76, 8/152, > 8/152
TMP ≥ 16 < 4, 4, 8, 16, 32, > 32
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a

AMP — Ampicillin; ENR — Enrofloxacin; GEN — Gentamicin; SMX — Sulphamethoxazole; TCY — Tetracycline; TMP/SSS = Trimethoprim/sulfanilamide; TMP — Trimethoprim.

Statistical analysis

Data were entered into a commercial spreadsheet and descriptive statistics were generated (SPSS v. 15.0.0 for Windows; SPSS, Chicago, USA). An animal was considered resistant if it had at least 1 resistant isolate. Exact confidence intervals for animal-level prevalence estimates were calculated (PEPI v 4; Sagebrush Press, Salt Lake City, USA) (17). The pen allocation changed after 112 DOF from 12 pens to 2 pens. No adjustment for clustering was used as there was no clustered pen structure representative of the entire feeding period.

The measurement, ADDFeedlot, was used to quantify the number of actual antimicrobial treatments given at the approved dose of the antimicrobial. This measurement accounted for the dosage and duration of action of the antimicrobial (Table II). The concept of ADDFeedlot was based on that of defined daily dose (DDD) used in human studies (18–21). Each antimicrobial treatment was described as 0, 1, 2, or 3 ADDFeedlot (Table II). An example calculation follows:

Table II.

Antimicrobials used in this study and Animal Defined Dose (ADDFeedlot) equivalent for a feedlot animal

Antimicrobials used in study — concentration Dose equivalent (mg/kg BW)a ADDFeedlot equivalent
Ceftiofur — 50 mg/mL; intramuscular injection (Excenel Sterile Powder; Pharmacia Animal Health, Orangeville, Ontario) 1 1
Oxytetracycline — 200 mg/mL; intramuscular injection (Liquamycin × LA-200; Pfizer Canada Animal Health Group, Kirkland, Quebec) 20 3
Tilmicosin — 300 mg/mL; subcutaneous injection (Micotil; Provel, Division Eli Lilly Canada, Guelph, Ontario) 10 3
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a

Actual body weight of the animal at the time of treatment.

Example: An animal was treated with 27 cc of long acting oxytetracycline on November 9 (BW 270 kg) and 40 cc of oxytetracycline on January 19 (BW 400 kg).

20 mg/kg×270 kg=5400 mg, 5400 mg/(200 mg/mL)=27 mL (actual dose given)

This was a long-acting treatment, equivalent to 3 ADDFeedlot

20 mg/kg×400 kg=8000 mg, 8000 mg/(200 mg/mL)=40 mL (actual dose given)

This was a long-acting treatment, equivalent to 3 ADDFeedlot.

The total ADDFeedlot actually given to this animal, therefore, was 6.

Two dichotomous outcomes were examined: conversion to TCY resistance and conversion to AMP resistance. Resistance conversion was defined as an animal having more resistant isolates present preslaughter than at arrival. These outcomes were chosen because TCY is the antimicrobial class most commonly used in animals (22), and AMP resistance was thought to be associated with TCY resistance in previous feedlot work.

Possible explanatory variables included Total ADDFeedlot (continuous), diet (dichotomous), antimicrobial treatment (dichotomous), conversion to AMP resistance for the outcome TCY resistance (dichotomous), and conversion to TCY resistance (dichotomous) as an explanatory variable for the outcome AMP resistance. Total ADDFeedlot was the sum of the actual doses received by the animal; this was a quantitative way to assess this potential association. Diet was examined to ensure the concurrent trial had no effect on the outcomes of this study. Antimicrobial treatment was a dichotomous variable representing whether the animal was treated or not. Tetracycline and AMP resistances were thought to be associated in previous feedlot studies, so AMP conversion was examined as an explanatory variable for the outcome TCY resistance, and TCY conversion was examined as a variable for AMP resistance.

There were no biological reasons to force any variables into the model. First, explanatory variables were each screened individually using unconditional logistic regression (SPSS v. 15.0.0 for Windows, SPSS). Variables with a statistical association at a level P < 0.15 were considered for entry into a multivariable model. When no variables were found to be significant through unconditional associations, the variables were also screened for entry into a multivariable model using a backwards stepwise approach. This was done because the effect of an important explanatory variable could be masked in the unconditional association due to uncontrolled confounding. The liberal P-value, P < 0.15, for entry into the model, was also used for this reason. A value of P < 0.05 was considered statistically significant and was necessary for a variable to stay in the model.

Results

Missing data

Three of the total 150 arrival samples had only 2 E. coli isolates cultured, and another 3 had only 1 isolate cultured. An extra 3 arrival samples were chosen randomly to be part of the study. This gave a total of 450 isolates from 153 cattle on arrival and represented an E. coli recovery rate of 3 isolates per animal from 96.1% of animals.

Fifty treated and 50 untreated cattle were chosen to be in the cohort analysis; 5 substitutes were also chosen if needed. Of the 55 treated cattle, 13 were excluded from the analysis; 8 lost their individual identification, and 5 died or were euthanized during the study due to chronic disease conditions. Of the 55 untreated cattle, 2 were excluded because they lost their individual identification preslaughter. The 3 extra, untreated cattle were left in the analysis. Overall, 42 treated cattle and 53 untreated cattle (n = 95) cattle were included in the cohort analysis.

Arrival

The most frequent antimicrobials for which resistance was observed were SMX, AMP, and TCY where, of all cattle, 44.4% (95% CI: 36.4% to 52.7%), 20.3% (95% CI: 14.2% to 27.5%), and 17.7% (95% CI: 12.0% to 24.6%), respectively had at least 1 resistant isolate (Table III). All isolates from an animal obtained from 36.6% (95% CI: 29.0% to 44.8%) of all cattle sampled (n = 153), were susceptible to all antimicrobials, while 5.9% (95% CI: 2.7% to 10.9%) of cattle had at least 1 isolate that was resistant to 3 or more antimicrobials out of the 7 antimicrobials tested (Table III).

Table III.

Antimicrobial resistance in fecal Escherichia coli isolates from steers on arrival at the feedlot

Antimicrobial resistance Count resistant (% resistant) n = 153 Exact lower confidence limit (%) Exact upper confidence limit (%)
Resistance to antimicrobialsa
 AMPb resistance 31 (20.3) 14.2 27.5
 SMX resistance 68 (44.4) 36.4 52.7
 TCY resistance 27 (17.6) 12.0 24.6
 TMP/SSS resistance 7 (4.6) 1.9 9.2
 TMP resistance 1 (0.7) 0.02 3.6
Multidrug Resistancec
 Resistant to 0 antimicrobials 56 (36.6) 29.0 44.8
 Resistant to 1 antimicrobial 71 (46.4) 38.3 54.6
 Resistant to ≥ 1 antimicrobials 97 (63.4) 55.2 71.0
 Resistant to ≥ 2 antimicrobials 17 (11.1) 6.6 17.2
 Resistant to ≥ 3 antimicrobials 9 (5.9) 2.7 10.9
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a

An animal was considered “resistant” if 1 or more isolate was resistant to the antimicrobial.

b

AMP — Ampicillin, SMX — Sulphamethoxazole, TCY — Tetracycline, TMP/SSS — Trimethoprim/sulfanilamide, TMP — Trimethoprim.

c

The number of antimicrobials to which an animal was resistant, was characterized by the isolate with resistance to the most number of antimicrobials.

Antimicrobial use

Sixty-six of the 447 cattle (14.8%) were given antimicrobials for disease treatment during the feeding period. Of the 447 cattle, 420 (94.0%) received a metaphylactic injection of long-acting oxytetracycline (Pfizer Canada Animal Health Group) on arrival. The other 27 (6.0%) were treated with tilmicosin (Division Eli Lilly Canada) on arrival due to undifferentiated fever or other early symptoms of disease.

Overall, amongst the 42 treated cattle used in the analysis, there was a total of 133 ADDFeedlot antimicrobials used for disease treatment; only 1 ADDFeedlot was administered during the last 100 d immediately prior to slaughter. There were 81 ADDFeedlot of tilmicosin (Micotil; Provel, Division Eli Lilly Canada) given to 26 cattle, 49 ADDFeedlot of ceftiofur (Excenel Sterile Powder; Pharmacia Animal Health, Orangeville, Ontario) given to 20 cattle, and 3 ADDFeedlot of oxytetracycline (Pfizer Canada Animal Health Group) given to 1 animal. Some cattle received treatment with more than 1 antimicrobial.

Conversion of cattle from nonresistant to resistant between arrival and preslaughter was described for each antimicrobial. Resistance conversion was stratified by the number of treated (ADD ≥ 1) and untreated cattle (ADD ≤ 1) (Table IV), and by the number of ADDFeedlot per animal (Table V). Tetracycline and AMP had the highest levels of resistance conversion during the study at 72/95 (75.8%, 95% CI: 65.9% to 84.0%) and 46/95 (48.4%, 95% CI: 38.0% to 58.9%) cattle, respectively (Table IV). The SMX conversion occurred in 11/95 (11.6%, 95% CI: 5.9% to 19.8%) cattle (Table IV). Overall, 34 of the 72 cattle (47.2%, 95% CI: 35.3% to 59.3%) that showed TCY conversion were treated for disease during the feeding period with a total of 107 ADDFeedlot (between 2 and 9 ADDFeedlot each). Thirty-eight of these 72 cattle (52.8%, 95% CI: 40.7% to 64.7%) that showed TCY conversion during the feeding period had no antimicrobial treatments during the feeding period, but all received oxytetracycline for metaphylaxis on arrival at the feedlot (Table V). Similarly, 24 of the 46 cattle with positive AMP conversion (52.2%, 95% CI: 36.9% to 67.1%) were treated with a total of 70 ADDFeedlot during the feeding period and 22 (47.8%, 95% CI: 32.9% to 63.1%) were not, other than metaphylaxis (Table V).

Table IV.

Antimicrobial treatments stratified by whether animals converted to a resistant status to specific antimicrobials during the feeding period

Antimicrobial resistance to: Treatment during feeding period Resistance conversionb No conversion Total cattle
AMPa No 22 31 53
Yes 24 18 42
Total 46 49 95
TCY No 38 15 53
Yes 34 8 42
Total 72 23 95
SMX No 6 47 53
Yes 5 37 42
Total 11 84 95
TMP/SSS No 2 51 53
Yes 0 42 42
Total 2 93 95
TMP No 3 50 53
Yes 0 42 42
Total 3 92 95
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a

AMP — Ampicillin; SMX — Sulphamethoxazole; TCY — Tetracycline; TMP/SSS — Trimethoprim/sulfanilamide; TMP — Trimethoprim.

b

Resistance conversion was defined as an animal having more resistant isolates present preslaughter than at arrival.

Table V.

Number of ADDFeedlot per animal level stratified by whether animals converted to a resistant status to specific antimicrobials during the feeding period

Change in antimicrobial resistance pattern ADDFeedlot total Grand total
0 2 3 5 6 9
No TCYa conversion 15 1 6 0 1 0 23
TCY conversion 38 8 22 2 1 1 72
No AMP conversion 31 4 10 2 1 1 49
AMP conversion 22 5 18 0 1 0 46
No SMX conversion 47 6 27 2 1 1 84
SMX conversion 6 3 1 0 1 0 11
No TMP/SSS conversion 51 9 28 2 2 1 93
TMP/SSS conversion 2 0 0 0 0 0 2
No TMP conversion 50 9 28 2 2 1 92
TMP conversion 3 0 0 0 0 0 3
Grand total 53 9 28 2 2 1 95
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a

AMP — Ampicillin; SMX — Sulphamethoxazole; TCY — Tetracycline; TMP/SSS — Trimethoprim/sulfanilamide; TMP — Trimethoprim.

Associations between antimicrobial use and antimicrobial resistance

No statistically significant associations were found between the outcomes and explanatory variables described (Table VI); therefore, no multivariable analyses were undertaken.

Table VI.

Univariate associations for antimicrobial resistance in cattle that had an increase in resistant isolates to tetracycline preslaughter

Variable Variable (categorical)a β SE Wald df P
TCYb Treatment group −0.517 0.497 1.082 1 0.298
Diet −0.374 0.482 0.600 1 0.439
Total ADDFeedlot (overall) — — 1.411 2 0.494
AMP conversion −0.497 0.488 1.039 1 0.308
AMP Treatment group −0.631 0.418 2.273 1 0.132
Diet 0.128 0.411 0.097 1 0.756
Total ADDFeedlot (overall) — — 2.284 2 0.319
TCY conversion 0.497 0.488 1.039 1 0.308
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a

β — Beta coefficient; SE — standard error of Beta coefficient; Wald — test statistic; df — degrees of freedom; P — probability value.

b

AMP — Ampicillin; TCY — Tetracycline.

Discussion

Escherichia coli was chosen as the indicator organism in this study because it was a commensal bacterium in cattle that was ubiquitous, easy to culture, and one of the major carcass contaminants at slaughter (23). Escherichia coli was considered a potential reservoir of resistance genes that could transfer resistance to other zoonotic or commensal organisms that might cause disease in cattle or people (24–27). It was therefore judged important to ensure that the last fecal sample was collected during the last 24 h prior to shipping for slaughter, to be representative of bacteria at the stage of production that might ultimately affect the consumer.

There were few published studies on AMU and AMR in feedlot cattle. Direct comparison of results between species or even between different stages in the production cycle was not appropriate as AMU and management norms were extremely different (28). In general, calves arrive at the feedlot with relatively low proportions of resistant bacterial isolates. In 2005, surveillance of generic E. coli isolates from colon samples at Canadian abattoirs suggested that the prevalence of isolates resistant to ≥ 1 antimicrobials was about 27% in beef cattle, 85% in swine, and 77% in chickens (29). Numbers in this study suggested a slightly higher prevalence, but only because it was calculated for animals not isolates, where an animal was considered resistant if it had ≥ 1 resistant isolates. Even in comparable studies in feedlot cattle, differences may exist between methodologies for sample collection, culture, and determination of antimicrobial susceptibilities, analysis and presentation of results due to differing underlying purposes or interests (30). The use of veterinary specific antimicrobial susceptibility methods and standards from the CLSI Subcommittee on Veterinary Antimicrobial Susceptibility Testing, in this study and in the Canadian and American national surveillance systems, narrows some of these differences. However, it is important to keep in mind that CLSI methods and interpretive criteria were developed for approved indications of the antimicrobial for specific pathogens, but have been applied to surveillance of commensal bacteria (29,31).

The AMR profiles of generic E. coli isolated from cattle feces on arrival at the feedlot in western Canada have not been previously characterized. Similarly, baseline levels of resistant bacteria in the feces of newly weaned, auction market derived calves on arrival at the feedlot in western Canada are not well documented. The AMR in fecal E. coli on arrival at the feedlot represents the antimicrobial resistant strains of bacteria that the calves were carrying prior to antimicrobial treatment at the feedlot, and may be related to resistance patterns and AMU in their herd of origin. In this study, there was no resistance found to ENR (a fluoroquinolone) and GEN (an aminoglycoside), both of which represented drug classes used in human medicine. Both ENR and GEN were not approved for use in cattle in Canada at the time of this study, but were evaluated due to concerns about cross-resistance and coresistance, mechanisms by which resistance to 1 antimicrobial may be associated with resistance to another, related or unrelated, antimicrobial (22). Enrofloxacin (ENR) has now been licensed for use in cattle for the treatment of bovine respiratory disease. Another feedlot study in the USA also found no resistance to GEN or ciprofloxacin, a fluoroquinolone like ENR (32). The proportion of TCY resistant E. coli isolates was similar to that found in newly weaned calves on pasture in an American study where 13 to 17% of fecal samples contained E. coli resistant to TCY in at least 1 of the 5 isolates per fecal sample (33).

This study examined individual animal antimicrobial usage in the absence of feed antimicrobials other than coccidiostats. The use of feed antimicrobials for disease prophylaxis and treatment was a common practice in feedlots in western Canada, and elsewhere in North America; growth promotion was not always the primary objective of this use (34,35). There were no published studies on the quantities of antimicrobials used in western Canadian feedlots, but regional feedlot consultants describe their use of feed antimicrobials, other than ionophores, as primarily for disease prophylaxis (35). In western Canadian feedlots, use of antimicrobials by approved instructions was the operating norm (35). Coccidiostats, including ionophores, were antimicrobials but they have not been used in human medicine and the importance of these agents with respect to AMR remains unclear.

There were concerns that DDD in humans do not represent the prescribed daily dose (PDD) in humans at the national level (18,36). This was not a valid concern for the use of ADDFeedlot in this study where each ADDFeedlot was calculated for the actual antimicrobial dose received.

Recent studies in western Canada examined associations between AMR and AMU in Campylobacter spp. (37,38). In one of the studies, the authors suggest that the increased proportion of cattle with tetracycline resistant isolates of Campylobacter spp. preslaughter might be associated with the use of oxytetracycline or chlortetracycline in the feed (38). Other attempts to evaluate the association between individual animal AMU and AMR have been confounded by the concurrent use of antimicrobials in the feed (39,40). Therefore, the opportunity to evaluate associations in this study, in the absence of feed antimicrobials, such as oxytetracycline or tylosin, was considered important.

In the cohort of cattle examined, there were treated and untreated cattle with no resistant isolates on arrival, but ≥ 1 resistant isolates preslaughter. There were also cattle that had at least 1 resistant isolate on arrival but none preslaughter. No associations were found in this study between individual animal AMU and AMR. This was somewhat different than findings from other studies where associations were found between AMR in fecal E. coli isolates in pigs and cattle and individual AMU when feed antimicrobials were also used (39,40). In 1 study, individual animal use of GEN was associated with the farm-level prevalence of GEN resistance in fecal E. coli isolates in pigs (39). In another study, the use of injectable oxytetracycline in individual cattle receiving chlortetracycline in the feed was associated with increased prevalence of resistance to chloramphenicol and sulfisoxazole in fecal E. coli isolates (40). In both these cases, associations were found with antimicrobials (GEN, chloramphenicol and sulfisoxazole) not used for mass medication of cattle in the feed. It was suggested that an association of AMR with feed medication might obscure the relationship between individual animal antimicrobial treatment and AMR if this relationship did indeed exist (39,40). Studies in pre-weaned dairy calves have shown associations between AMR and AMU. In the 1 study, calves treated within 5 d prior to sampling were more likely to have multiple drug resistant fecal E. coli than those not treated during that time frame (41). This may indicate that individual antimicrobial treatment has a transient effect that would no longer be present in the current study, at the time immediately preslaughter when the 2nd sample was taken. Only 1 animal was treated in the 100 d immediately preslaughter. Again, the focus of this study was the change in AMR over the feeding period to understand the effect of antimicrobial agents over that time. It should also be noted that AMR does occur in the absence of AMU pressure (24,42).

A weakness in this study was that metaphylactic antimicrobial injections were given to all cattle; there were several reasons for this. The goal of the study was evaluation of associations between the volume of individual animal AMU and AMR. In western Canada, metaphylactic usage of antimicrobials was estimated at 20% to 50% of calves on arrival at the feedlot across all groups, based on the risk profiles of each group of cattle on arrival (28). Metaphylaxis use in this study was representative of commercial feedlot management; only 1 AMU variable was removed, the in-feed use of oxytetracycline or tylosin. It is likely that metaphylaxis was associated with AMR in this study; considerable TCY and AMP conversion occurred in both groups in the cohort study, not associated with the individual animal antimicrobial disease treatments. The volume of individual animal antimicrobial doses was a logical way to evaluate antimicrobial use quantified beyond that used for metaphylaxis. Using the variable Total ADDFeedlot, not ADDFeedlot calculated for specific antimicrobials, could have masked associations between AMU of specific antimicrobials and AMR (43). This study was not designed examine these specific associations.

In conclusion, calves arrived at the feedlot with relatively low numbers of resistant bacteria compared with other species (29). Basic description of AMU in 1 group of calves in a western Canadian commercial feedlot was given. Understanding of resistance patterns existing on arrival at the feedlot as well as AMU during the feeding period was essential to understanding the changing patterns of antimicrobial resistant isolates at the feedlot and interpreting resistance patterns preslaughter. No association was found between total AMU for individual animal disease treatment and AMR in fecal E. coli isolates. This study allowed a unique opportunity to evaluate these associations in cattle that were not fed antimicrobials, other than a coccidiostat, during the entire feeding period. The results suggest that metaphylaxis and AMR patterns should be evaluated further, and that the evaluation of conversion to resistance during the feeding period is important when assessing associations of AMR attributable to AMU during the feeding period.

Acknowledgments

The authors thank the management and staff of the University of Saskatchewan Feedlot and PoundMaker AgVentures for their cooperation during this study. The Saskatchewan Agriculture Development Fund and Novartis Animal Health Canada are also thanked for their support of this study.

Footnotes

This study was part of the PhD thesis of Dr. S.L. Checkley through the Department of Large Animal Clinical Sciences at the Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan.

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