A. O’Connor1, C. Poppe2, S.W. Martin1 and S.A. McEwen1 1Department of Population Medicine, University of Guelph and 2Laboratory for Foodborne Zoonoses, Health Canada us to estimate effects on resistance of in-feedmedication alone, and in combination with injectable In this feedlot bull study, the proportions of E. coli isolates resistant to ampicillin, kanamycin, neomycin,oxytetracycline, spectinomycin, streptomycin, and Materials and Methods
sulfamethoxazole increased after in-feed treatment A nested case-control study was conducted in fall with chlortetracycline. In spite of the fact that all 1998 with the co-operation of Beef Improvement study animals received the in-feed treatment, Ontario, at the University of Guelph Arkell bull test treatment of individual cattle with oxytetracycline station. Case animals required treatment for either was also associated with increased prevalence of undifferentiated bovine respiratory disease or resistance to oxytetracycline, spectinomycin and infectious interdigital dermititis with injectable long- sulfamethoxazole. These findings demonstrate that acting oxytetracycline (200mg/ml) used according to treatment of feedlot animals for clinical disease does the manufacturer’s instructions. At the time of select for antimicrobial resistance among enteric treatment a freshly voided faecal sample, referred to bacteria. Producers and veterinarians should be aware as the selection sample, was collected. Animals were of this and use antimicrobials prudently.
selected as cases between the 30th October and the23rd December 1998. For each case animal, a freshly- Introduction
voided faecal sample was also collected from twountreated animals to act as controls. For convenience, Until recently, the principal food safety and quality control animals were usually selected from animals in problem arising from antimicrobial use in food animals was the presence of residues in edible tissues.
World-wide, concerns about antimicrobial resistancein humans are rapidly changing this situation. Much On the 26th of December 1998, due to the diagnosis of greater scrutiny is now placed on antimicrobial use several animals with chronic respiratory disease, in- and development of resistance in commensal and feed antibiotics were prescribed for all animals in the pathogenic organisms that may be transmitted from facility for a period of three weeks. Aureomycin food animals to humans. This is a controversial and (chlortetracycline) was added in the feed at the rate of complex issue and the public health effects are not 8 mg/lb body weight per day for approximately 8 days well understood. Even if these effects are minor and then at 3 mg/lb body weight per day for however, veterinary and producer groups should approximately 8 days. On the 9th of February 1999, encourage prudent antimicrobial use practices that faecal samples (referred to as the final sample) were achieve intended animal health goals while collected from all animals in the facility. After minimizing resistance problems. In order to collection of the samples, they were transported to the accomplish this, we need to better understand how laboratory for culture and sensitivity testing.
different routes and timing of antimicrobialadministration affect resistance. Five faecal E. coli were isolated from each sampleusing standard methods and representative isolates We had the opportunity to investigate resistance tested for resistance using the “SensitreR” system.
prevalence among fecal E coli before and after Each E coli isolate was tested for resistance as treatment of cattle with injectable oxytetracycline.
follows: ampicillin 32 mg/ml, amikacin 16 mcg/ml, During the course of the study, the entire group of apramycin 16 mcg /ml, ceftifur 8 mcg/ml, ceftriaxone cattle was treated for respiratory disease with 32 mcg/ml, ciprofloxacin 0.125 mcg/ml, gentamycin chlortetracycline in the feed. Thus, this study enabled 16 mcg/ml, kanamycin 64 mcg/ml, florfenicol 100mcg/ml, nalidixic acid 32 mcg/ml, nitrofurantoin impact on resistance than is observed in some studies.
64 mcg/ml, polymixin B 16 mcg/ml, streptomycin 64 In this investigation, treatment of the entire group with in-feed chlortetracycline appeared to increasethe proportion of resistant strains to to ampicillin, Proportions of resistant isolates for cases and controls n e o m y c i n , o x y t e t r a c y c l i n e, at time of selection and final sampling were compared spectinomycin, streptomycin, and sulfamethoxazole.
using non-parametric tests and regression analyses, While it was not possible to completely separate and the effects of different types of treatment were individual from group-level treatment effects, injection of some animals with oxytetracyclineappeared to further increase resistance to some of Results and Discussion
these drugs. Clearly, industry-sponsored programsthat are intended to encourage the prudent use ofantimicrobials should provide for both types of Nine hundred and five (905) E coli isolates were treatment at the individual and group level. used in the final analysis. Prior to treatment, nodifference existed in the number of animals identified Conclusions
with no resistant strains for all antimicrobials tested.
However at the final sampling, cattle that had In this feedlot bull study, the proportions of E. coli received injectable oxytetracycline (cases) were more isolates resistant to ampicillin, kanamycin, neomycin, likely to have at least one oxytetracycline resistant oxytetracycline, spectinomycin, streptomycin, and strain than those animals not treated (controls) sulfamethoxazole increased after in-feed treatment (Fig.1). More cases had sulfamethoxazole resistant with chlortetracycline. In spite of the fact that all strains and there was some indication of increased study animals received the in-feed treatment, resistance to spectinomycin. For the other treatment of individual cattle with oxytetracycline antimicrobials tested, the odds ratio included unity was also associated with increased prevalence of (i.e. no difference). In-feed medication increased the resistance to oxytetracycline, spectinomycin and proportion of strains resistant to ampicillin, sulfamethoxazole. These findings demonstrate that k a n a m y c i n , n e o m y c i n, o x y t e t r a c y c li n e , treatment of feedlot animals for clinical disease does spectinomycin, streptomycin, and sulfamethoxazole, select for antimicrobial resistance among enteric but not ceftiofur, gentamycin, florfenicol, nalidixic acid, nitrofurantoin, and polymixin B (all strains weresensitive to amikaycin, apramycin, ceftriaxone and Significance to the Industry
Producers and veterinarians should use antimicrobials Antimicrobials exert a potent selective pressure on prudently and in ways that maximize therapeutic bacteria and it is no surprise that resistance emerges benefit and minimize resistance. This will help to in populations of exposed animals – that this maintain the efficacy of these drugs for treatment of sometimes happens has been known for decades. The cattle diseases and reduce any risks to public health.
speed and extent with which resistance occurs in In the past, the principal food quality and safety individuals and groups of cattle is less well concern with antimicrobial use has been residues.
understood, however. Conventional wisdom has it Resistance has the potential to be a much more that in-feed medications tend to have a greater impact important food safety issue for the industry.
on resistance than individual-animal treatments. Thisis, of course, a generalization as there are many other Acknowledgements
factors to consider, including the drug administered,dose, age of animal, etc. In-feed medications tend to Financial support for this study was provided by the be administered to larger numbers of animals for Ontario Cattlemen’s Association, the Ontario longer periods of time, and at lower doses than Ministry of Agriculture, Food and Rural Affairs and individual animal treatments. These factors along with the pharmacological properties of the drugsinvolved are probably responsible for the greater Figure 1 : Frequency distribution of the proportion (r/n) of faecal E coli isolates resistant to oxytetracycline
at 64 mcg/ml in case and control animals after treatment with injectable oxytetracycline at the Arkell bull
testing station, Fall 1998.



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