Prevalence and Quinolone Susceptibilities of Salmonella Isolated from the Feces of Preharvest Cattle Within Feedlots that Used a Fluoroquinolone to Treat Bovine Respiratory Disease

Abstract

Salmonella is an important foodborne pathogen and antimicrobial resistance can be a human health concern. The objectives of this cross-sectional study were to (1) determine the prevalence and quinolone susceptibility of Salmonella in feces of preharvest commercial feedlot cattle and (2) determine if the prevalence and susceptibility of Salmonella isolates were associated with previous fluoroquinolone use within pens. Five feedlots in western Kansas and Texas were selected based on their use of a commercially licensed fluoroquinolone for initial treatment of bovine respiratory disease (BRD). Twenty pen floor fecal samples were collected from each of 10 pens from each feedlot during early summer of 2012. Salmonella isolation was performed and microbroth dilution was used to determine susceptibility of isolates to nalidixic acid and ciprofloxacin. Prior antimicrobial treatment data were retrieved from feedlots' operational data. Generalized linear mixed models were used to assess associations between Salmonella prevalence and the number of fluoroquinolone treatments within pens while taking into consideration cattle demographic and management factors, as well as the hierarchical structure of the data. Overall, cumulative fecal prevalence of Salmonella was 38.0% (380/1000), but prevalence varied significantly (p < 0.01) among the five feedlots: 0.5% (1/200), 17.5% (35/200), 37.0% (74/200), 58.5% (117/200), and 76.5% (153/200). Salmonella serogroups included C1 (49.3%), E (36.4%), C2 (13.8%), and D (0.6%). There was no significant association (p = 0.52) between Salmonella prevalence and the frequency of fluoroquinolone treatments within a pen. All Salmonella isolates (n = 380) were susceptible to ciprofloxacin, while one isolate exceeded the human breakpoint (≥32 μg/mL) for nalidixic acid. In conclusion, Salmonella fecal prevalence in preharvest cattle was highly variable among feedlots. Nearly all Salmonella isolates were susceptible to quinolones, despite the fact that a fluoroquinolone was used as the primary therapeutic antimicrobial to treat BRD in these feedlot populations.

Introduction

The Centers for Disease Control and Prevention (CDC) reports that Salmonella is one of the leading causes of foodborne illnesses in the United States (CDC, 2012). Salmonella outbreaks usually occur due to the consumption of contaminated food, water, and raw milk (CDC, 2012). Although beef is not generally considered a primary source of human salmonellosis, feedlot cattle can be asymptomatic carriers of Salmonella and shed these bacteria in their feces (Fedorka-Cray et al., 1998; Dargatz et al., 2003; Dodd et al., 2011b). In addition, the recovery of Salmonella from lymph nodes in beef carcasses has become a significant concern for the U.S. beef industry (Arthur et al., 2008; Haneklaus et al., 2012; Gragg et al., 2013). The average prevalence of Salmonella in feces of feedlot cattle and beef cows in the United States is generally low, but it can vary tremendously both within and among feedlots or herds (Fedorka-Cray et al., 1998; Dargatz et al., 2000, 2003; Dodd et al., 2011b).

Antimicrobials are effectively used to treat and control diseases in feedlot cattle. Some classes of antimicrobials, such as quinolones, are used to treat bovine respiratory disease (BRD) in cattle as well as foodborne illnesses in humans. With the growing concern over antimicrobial resistance, the use of antimicrobial drugs in livestock is highly scrutinized (Emmerson and Jones, 2003; Page and Gautier, 2012). Risk assessments are often used to evaluate antimicrobial use in livestock and its risk to humans; however, data necessary to develop valid and informative quantitative risk assessment models are often lacking. Current knowledge regarding the prevalence of Salmonella in U.S. beef cattle and their antimicrobial susceptibilities, particularly to quinolones, is limited. National studies of feedlots and beef cows have shown that resistance of Salmonella to quinolones is extremely rare, but may cluster within a few herds for reasons not fully defined (Fedorka-Cray et al., 1998; Dargatz et al., 2000, 2003; Dodd et al., 2011a). The objectives of this study were to (1) determine the prevalence and quinolone susceptibility of Salmonella in feces from preharvest pens of commercial feedlot cattle and to (2) determine if the prevalence and susceptibility of Salmonella in fecal isolates differ among feedlots or among pens with different fluoroquinolone treatment histories.

Materials and Methods

Study population

A convenience sample of five commercial feedlots in western Kansas and Texas that used a fluoroquinolone (Baytril 100^® [enrofloxacin]; Bayer HealthCare LLC, Animal Health, Shawnee Mission, KS) as first-line therapy for the treatment of BRD was selected for this research. Feedlots were selected based on previous fluoroquinolone use and their willingness to participate in our study. Ten study pens within each feedlot were selected in consultation with feedlot managers based on the pens' projected slaughter dates, allowing for pens with both high and low respiratory disease morbidity (thus antimicrobial use) to be selected when available. Health and treatment data on study pens, including the number of animals treated with a fluoroquinolone or any other antimicrobials, were retrieved from the feedlots' operational database.

Twenty freshly voided fecal samples were collected from the pen floors of each of the 10 pens of cattle (∼1–2 weeks before slaughter) within each feedlot (total samples per feedlot = 200) during May–July 2012. Cattle fecal samples were collected using plastic spoons and placed in Whirlpak bags (Nasco, Inc., Fort Atkinson, WI). The samples were then transported on ice to the Kansas State University (KSU) Preharvest Food Safety laboratory for isolation of Salmonella.

Isolation of Salmonella

Ten grams of feces from each sample was placed in 90 mL tetrathionate (TT) broth (Becton-Dickinson, Sparks, MD) and incubated for 24 h at 37°C. From the TT broth, 100 μL of the inoculum was transferred to 10 mL Rapport–Vassiliadis broth (Becton-Dickinson) and incubated at 42°C for 24 h. The enriched sample was then streaked for isolation on Hektoen enteric (HE) agar plates (Becton-Dickinson) and incubated at 37°C for 24 h. Three presumptive colonies with morphology consistent with Salmonella (blue–green colony with black center) were streaked onto blood agar (Remel, Lenexa, KS) and incubated at 37°C for 24 h. Isolates were subjected to slide agglutination with the Salmonella polyvalent O antisera for Salmonella serogroups B, C1, C2, D1, D2, and E. Isolates were considered Salmonella based on hydrogen sulfide production on HE agar and agglutination with the polyvalent O antisera. These isolates were stored in cryoprotection beads (Hardy Diagnostics, Santa Maria, CA) at −80°C until further tested. Serotyping of isolates found to be resistant based on microbroth dilution methods (described below) was performed by the National Veterinary Services Laboratory (Ames, IA).

Susceptibility testing

Minimum inhibitory concentrations (MICs) of ciprofloxacin and nalidixic acid were determined for each isolate by the microbroth dilution method (CLSI, 2008) using human Salmonella breakpoints (CLSI, 2010). Stock solutions of ciprofloxacin and nalidixic acid (Sigma-Aldrich, St. Louis, MO) were prepared in sterile distilled water at a concentration of 1 mg/mL based on the potency of the antibiotic. Nalidixic acid was tested at concentrations of 100, 50, 25, 12.5, 6.25, 3.125, 1.56, 0.78, 0.39, and 0.195 μg/mL and ciprofloxacin was tested at concentrations of 3.125, 1.56, 0.78, 0.39, 0.195, 0.098, 0.024, 0.012, and 0.006 μg/mL. For the preparation of the inoculum, a single colony picked from a plate was inoculated into 10 mL Mueller–Hinton II broth (Becton-Dickinson) and incubated for 6 h and the cell turbidity was adjusted using the 0.5 McFarland turbidity standard. The antimicrobial susceptibilities were performed in 96-well microtiter plates (Becton-Dickinson). Plates were incubated at 37°C for 24 h and results were recorded as growth or no growth within each well.

This process was performed separately for each antibiotic (ciprofloxacin and nalidixic acid) and each Salmonella isolate was tested in quadruplicate. Quality control organism, Escherichia coli (ATCC 25922), was used to ensure validity of the test. The MIC was determined by the lowest concentration of antimicrobial agent that completely inhibited the growth of the organism. Salmonella isolates were considered susceptible to nalidixic acid and ciprofloxacin if the MIC was ≤16 and ≤1 μg/mL, respectively, and considered resistant if the MIC was ≥32 and ≥4 μg/mL, respectively (CLSI, 2010). Isolates considered resistant were tested for susceptibility using Sensititre^® plates that contained 17 antimicrobial drugs and a semi-automated testing system (Sensititre; TREK Diagnostics, Westlake, OH).

Statistical analysis

Associations between antimicrobial use and demographic characteristics with Salmonella pen-level prevalence were evaluated using generalized linear mixed models, which were fitted using a binomial distribution, maximum likelihood estimation, complimentary log–log link, Kenward–Roger degrees of freedom, and Newton–Raphson and Ridging optimization procedures (Proc GLIMMIX SAS 9.3; SAS Institute, Inc., Cary, NC). A random intercept for feedlot was included to account for the clustering effect of pens nested within feedlots. Overdispersion was adjusted for by including a multiplicative parameter (scale parameter).

The outcome variable was the number of Salmonella-positive samples in a pen (events)/number of samples collected per pen (trials). Independent variables included the number of BRD fluoroquinolone treatments administered, body weight at arrival, days on feed, sex (steers/heifers), and antimicrobial metaphylaxis use in the pen (use/no use). Due to variability in pen size, the variable pertaining to the number of fluoroquinolone treatments was coded as the number of treatments per 100 cattle. All continuous variables (number of fluoroquinolone treatments per 100 cattle, body weight, and days on feed) were categorized in quartiles to avoid violation of the linearity assumption.

Pearson's and Spearman's correlation analyses were performed to identify highly correlated variables (≥|0.80|) before initiating a multivariable model building process. Initially, variables unconditionally associated with the outcome in the univariable screen (p < 0.40) were included in the main effects model. The number of fluoroquinolone treatments per 100 cattle, considered our exposure variable of interest, was forced in our multivariable model regardless of its significance. The following variables, considered a priori confounders, were tested: sex, metaphylaxis use, and body weight. If their removal caused a 20% or greater change in the coefficient of significant variables, they were considered analytical confounders and were kept in the model. Two-way interactions between potential a priori confounders that were deemed significant at the 5% level (p < 0.05) were kept in the model. Following forward selection, nonsignificant variables (p > 0.05) that were not acting as confounders or effect modifiers were removed from the multivariable model. Furthermore, to evaluate whether the within-pen prevalence of Salmonella differed among feedlots, feedlot was included as a fixed effect in a logistic regression model. Residual diagnostics included the evaluation of the best linearized unbiased predictors for feedlot-level residuals and Pearson and deviance residuals for observations at the pen level.

Descriptive statistics were used to evaluate the MIC data for both nalidixic acid and ciprofloxacin for samples collected within each pen across the five feedlots. Since each Salmonella isolate was tested in quadruplicate, the mode MIC value was reported for each sample. When only two MIC values were available and the mode could not be computed, the MIC value with the highest concentration was used. If four different MIC values were reported, the isolate was retested and either the mode or the highest MIC value was reported.

Results

Cattle demographics

The mean number of cattle per pen and the mean number of days on feed for cattle across all feedlots are shown in Table 1. The mean body weight of cattle in study pens upon arrival at the feedlot ranged from 289 to 317 kg across all five feedlots. The majority (56.0%; 28/50) of the cattle pens comprised steers and one pen contained both steers and heifers. The frequency of fluoroquinolone treatments across all feedlots ranged from 5 to 23 treatments within a pen (Table 1). All feedlots had at least one pen where all cattle were treated metaphylactically for BRD with a range of 2–10 study pens receiving metaphylaxis (Table 1).

Table 1.

Summary of Demographic Characteristics for Pens of Cattle Within Feedlots that Were Tested for Salmonella

Feedlot	No. of pens (steers/heifers)	Mean no. of cattle per pen	Mean no. of days on feed at sampling (range)	Mean cattle body weight, in kg, at feedlot arrival (range)	No. of pens that received an antimicrobial for metaphylaxis ^a (product)	Mean no. of fluoroquinolone ^b treatments per 100 cattle (range)
A	10 (9/1)	158.4	150.0 (139–162)	317.0 (273–362)	2 (Tilmicosin)	5.3 (1–22)
B	10 (10/0)	125.2	183.0 (148–209)	290.5 (248–318)	10 (Tulathromycin)	19.2 (4–32)
C	10 (4/6)	96.3	178.2 (137–240)	288.9 (215–344)	2 (Oxytetracycline)	10.1 (0–36)
D	10 (3/7)	198.6	163.9 (102–203)	300.0 (259–352)	8 (Tulathromycin)	22.8 (5–66)
E	10 (2/7; one mixed)	117.3	155.5 (118–210)	288.7 (226–382)	10 (Tulathromycin)	7.7 (1–15)

Used for control of bovine respiratory disease in high-risk cattle.

Cattle treated for respiratory disease were treated with the same fluoroquinolone.

Salmonella prevalence and susceptibility results

Overall, sample-level prevalence of Salmonella across all feedlots and pens was 38.0% (380/1000). Sample-level prevalence ranged from 0.5% (1/200) to 76.5% (153/200) across all pens within feedlots and ranged from 0.0% (0/20) to 100.0% (20/20) within pens. The prevalence of Salmonella varied significantly (p < 0.01) among the five feedlots (Table 2). The most common Salmonella serogroups isolated from pen floor fecal samples across the five commercial feedlots were C1 (49.3%; 179/363), followed by E (36.4%; 132/363), C2 (13.8%; 50/363), and D (0.6%; 2/363) (Table 2). Of the 380 isolates tested, 17 did not test positive for any of the six serogroups evaluated.

Table 2.

Sample-Level Fecal Prevalence and Presumptive Serogroups of Salmonella for Five U.S. Commercial Feedlots

Feedlot	Prevalence	95% CI	Serogroup C1 ^a	Serogroup C2 ^a	Serogroup D ^a	Serogroup E ^a
A	0.5% (1/200)	0.1–3.6	0.0% (0/1)	0.0% (0/1)	0.0% (0/1)	100.0% (1/1)
B	17.5% (35/200)	12.7–23.6	33.3% (11/33)	3.0% (1/33)	0.0% (0/33)	63.6% (21/33)
C	37.0% (74/200)	30.4–44.1	28.2% (20/71)	28.2% (20/71)	2.8% (2/71)	40.8% (29/71)
D	58.5% (117/200)	51.4–65.3	52.4% (55/105)	3.8% (4/105)	0.0% (0/105)	43.8% (46/105)
E	76.5% (153/200)	70.0–82.0	60.8% (93/153)	16.3% (25/153)	0.0% (0/153)	22.9% (35/153)
Total	38.0% (380/1000)		49.3% (179/363)	13.8% (50/363)	0.6% (2/363)	36.4% (132/363)

Of the 380 isolates tested, 17 isolates did not test positive for any of the serogroups (B, C1, C2, D1, D2, and E) evaluated.

CI, confidence interval.

Potential associations between pen-level Salmonella prevalence and the number of fluoroquinolone treatments per 100 cattle, sex, metaphylaxis use, arrival body weight, and days on feed were evaluated and these data are shown in Table 3. No significant unconditional associations were identified, except for arrival body weight (p = 0.02; Table 3). The variable pertaining to the number of fluoroquinolone treatments per 100 cattle was not statistically significant (p = 0.52), but was forced in the multivariable model as it was considered our main variable of interest. The multivariable model included body weight and fluoroquinolone treatments and provided very similar results to the univariable model results (data not shown). Arrival body weight was the only variable significantly associated with Salmonella prevalence (p = 0.02). Pens of cattle that weighed 266–298 kg at arrival had significantly lower prevalence of Salmonella (15.5%; 95% confidence interval [CI] 2.4–70.0) than those that weighed 215–265 kg (28.4%; 95% CI 4.7–90.4; p = 0.01) or 320–382 kg (32.3%; 95% CI 5.5–93.4; p < 0.01). No other differences were found among other weight categories.

Table 3.

Results from Univariable Mixed Models of Associations Between Pen-Level Risk Factors and Within-Pen Prevalence of Salmonella

Variable, units	No. of pens	Mean prevalence (%)	95% CI	p
No. of fluoroquinolone treatments for bovine respiratory disease per 100 cattle (in quartiles)				0.52
0–2	13	20.3	3.5–76.8
3–8	12	26.7	4.6–87.0
9–23	13	26.9	4.6–87.3
24–66	12	31.4	5.6–91.6
Sex^a				0.35
Steer	28	24.3	4.1–84.2
Heifer	21	29.3	5.1–90.0
Metaphylaxis use				0.33
No	18	21.9	3.9–78.8
Yes	32	28.9	5.0–89.5
Body weight at arrival, kg (in quartiles)				0.02
215–265	12	31.2^b	5.0–93.5
266–298	13	15.7^c	2.4–70.5
299–319	11	25.7^bc	4.1–88.0
320–382	14	30.2^b	4.8–92.7
Days on feed (in quartiles)				0.94
102–145	11	25.3	4.1–86.7
146–161	14	25.3	4.2–86.0
162–192	12	25.5	4.1–87.0
193–240	13	28.4	4.7–90.2

Models included a random effect to account for lack of independence among pens within feedlots. Row values with different superscripts (b and c) differ (p < 0.05).

One pen had a mix of heifers and steers and was removed from this analysis.

Of the 380 isolates tested for susceptibility to nalidixic acid and ciprofloxacin, only one isolate (1/380; 0.3%) was resistant (MIC ≥32 μg/mL) to nalidixic acid and all isolates were susceptible (MIC ≤1 μg/mL) to ciprofloxacin (380/380; 100.0%). As indicated previously, further testing was only performed on the resistant isolate, which was found to be pan-susceptible to amoxicillin/clavulanic acid, ampicillin, azithromycin, ceftiofur, ceftriaxone, chloramphenicol, ciprofloxacin, doxycycline, gentamicin, kanamycin, minocycline, streptomycin, sulfisoxazole, tetracycline, and trimethoprim/sulfamethoxazole. Most of the isolates (81.6%; 310/380) had an MIC value for nalidixic acid of 6.25 μg/mL. However, some isolates had nalidixic acid MIC values of 1.56 μg/mL (n = 1), 3.125 μg/mL (n = 64), 12.5 μg/mL (n = 3), 25 μg/mL (n = 1), and >32 μg/mL (n = 1). The majority (83.2%; 316/380) of the Salmonella isolates had an MIC value for ciprofloxacin of 0.012 μg/mL, whereas other isolates had MIC values of ≤0.006 μg/mL (n = 31), 0.024 μg/mL (n = 31), 0.098 μg/mL (n = 1), and 0.39 μg/mL (n = 1).

Discussion

The results of our study indicated that the overall prevalence of Salmonella in feedlot cattle feces before harvest was highly variable across the five feedlots (0.5–76.5%) and across pens within a feedlot (0–100.0%). These data are consistent with previously reported Salmonella prevalence data obtained from commercial feedlot cattle (Dargatz et al., 2000; Alam et al., 2009; Dodd et al., 2011b). Salmonella isolates recovered from pens of cattle that were treated with fluoroquinolones were susceptible to both nalidixic acid and ciprofloxacin regardless of the number of fluoroquinolone treatments administered to the cattle within pens.

There are numerous factors that have been shown to be associated with Salmonella prevalence in feedlot cattle, including geographic location, season, environmental stress, number of days in the feedlot, and diet (Losinger et al., 1997; Barham et al., 2002; Dargatz et al., 2003; Edrington et al., 2010; Dodd et al., 2011b). In the current study, the Salmonella prevalence varied tremendously among feedlots, with the highest sample-level prevalence (76.5%) in a Texas feedlot and the lowest prevalence (0.5%) in a western Kansas feedlot. Having lower prevalence in the most northern region is consistent with the literature (Dargatz et al., 2003; Green et al., 2010); however, there were too few feedlots (5) to determine potential feedlot-level risk factors as this was not a primary objective of this current study.

At the pen level, it is important to note that the number of fluoroquinolone treatments per 100 cattle, the number of days the cattle were in the feedlot, metaphylaxis use (whole pen use of an antimicrobial to control respiratory disease), and sex were not found to be associated with Salmonella prevalence within pens. However, there was a significant association between cattle body weight at feedlot arrival and Salmonella prevalence within pens. Specifically, pens of cattle with a mean weight range of 266–298 kg had a significantly lower Salmonella prevalence than the smallest body weight category (215–265 kg) and the heaviest body weight category (320–382 kg). Cattle that are lighter at feedlot arrival generally require a longer feeding period and may have increased morbidity due to higher probability of stress, suboptimal immunity, and exposure to pathogens in the feedlot environment (Losinger et al., 1997; Fedorka-Cray et al., 1998; Babcock et al., 2010). Heavier cattle, in our study, spent fewer days on feed, received fewer antimicrobial treatments, and had fewer pens that were given metaphylaxis. However, the mean prevalence of Salmonella was ∼30% for both the lightest and heaviest weight categories. A previous study evaluating Salmonella in 30 commercial feedlot cattle cohorts found that mean body weight at feedlot arrival was not associated with preharvest fecal prevalence of Salmonella (Dodd et al., 2011b), so it seems that cattle body weight at feedlot entry is not a consistent predictor of Salmonella prevalence at harvest.

Despite the fact that all five feedlots used a fluoroquinolone as the primary line of therapy for the treatment of BRD (the only legal indication for fluoroquinolone use at the time of the study), nearly all of the Salmonella isolated for this study (379/380) were susceptible to both nalidixic acid and ciprofloxacin, with only one isolate found to be resistant to nalidixic acid. These results are consistent with previous studies demonstrating an extremely low prevalence of quinolone-resistant Salmonella isolated from feedlot cattle (Beach et al., 2002; Sorensen et al., 2002; Edrington et al., 2010). However, the results reported here are unique as this is the first study in which the use of fluoroquinolones was an inclusion criterion to select feedlots for the study population.

Several studies completed over the last decade have shown that less than 1.0% of the Salmonella isolates recovered from cattle feces, hides, and carcasses have been resistant to nalidixic acid and ciprofloxacin (Dargatz et al., 2000, 2003; Beach et al., 2002; Fluckey et al., 2007; Alam et al., 2009). The 2011 National Antimicrobial Resistance Monitoring System (NARMS) Meat Retail Report showed a similar trend in the number of quinolone-resistant isolates recovered from ground beef (FDA, 2014). Salmonella isolates recovered from retail ground beef (n = 1320) collected from 11 U.S. states were all susceptible to both ciprofloxacin and nalidixic acid (FDA, 2014). These results, and those first reported here, are important as Salmonella is a leading cause of foodborne illnesses in the United States (CDC, 2012), and quinolones are considered a critically important class of antimicrobials for human health (FDA, 2003).

Follow-up serotype testing of the one resistant isolate recovered during our study revealed that it was Salmonella serotype Cerro. This serotype has been isolated from both US beef and dairy cattle and has been associated with human salmonellosis (Kunze et al., 2008; Cummings et al., 2010; Hoelzer et al., 2011). However, the incidence of human salmonellosis cases due to resistant Salmonella Cerro has been extremely low, with the majority of the resistance being attributed to non-fluoroquinolone drugs (Kunze et al., 2008; Cummings et al., 2010; Hoelzer et al., 2011).

Cross-sectional pen-level studies designed to evaluate the prevalence of foodborne pathogens in preharvest feedlot cattle and their antimicrobial susceptibilities may be useful in assessing the potential human health risks. However, data collected from this type of study have limited value for causal inferences because the sequence of events that lead to the observed outcome cannot be determined (Mann, 2003). Lack of history of fluoroquinolone use before cattle's entry into the feedlot and lack of knowledge regarding the temporal dynamics of Salmonella within cattle and within their environment are limitations of the current study. However, this study provides estimates of the preharvest fecal prevalence of Salmonella and the susceptibility of recovered isolates to two quinolones that are used for human therapy. Importantly, the results also demonstrate no evidence of an association between the number of previous fluoroquinolone treatments administered to cattle and the within-pen-level fecal prevalence of Salmonella before harvest.

This cross-sectional study indicates that Salmonella prevalence in preharvest feedlot cattle is highly variable among feedlots and across pens within feedlots. The prevalence of quinolone-resistant Salmonella in this study population was extremely low, with all but one of the 380 recovered isolates being susceptible to both nalidixic acid and ciprofloxacin. Given that we purposely selected feedlots that used a fluoroquinolone for treating cattle with BRD, this study population could have been considered potentially higher risk for resistance; yet, the level of Salmonella resistance was negligible. While Salmonella as a foodborne pathogen and the use of antimicrobial drugs in livestock both remain critically important issues, this study found no evidence of an association between fluoroquinolone treatments administered to feedlot cattle and the within-pen-level fecal prevalence of Salmonella before harvest.

Footnotes

Acknowledgments

This research was supported by Bayer Healthcare, LLC, and the Department of Diagnostic Medicine/Pathobiology, College of Veterinary Medicine, Kansas State University. Investigators would like to thank the participating feedlots and their staff, as well as staff and students at the Kansas State University, College of Veterinary Medicine, Preharvest Food Safety Laboratory. This is contribution number 15-324-J from the Kansas Agricultural Experiment Station.

Disclosure Statement

The first author (A.B.S.) is a graduate student of Kansas State University and an employee of Bayer Healthcare, LLC, which produces a commercially licensed fluoroquinolone for the treatment of BRD. For the remaining authors, no competing financial interests exist.

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