Overview of the use of antimicrobial drugs for the treatment of bacterial infections in horses

The use of antimicrobials in veterinary medicine is under great scrutiny with the emergence of antimicrobial resistance in the human population. Equine veterinarians rely on antimicrobials as an essential tool for the treatment of infections in horses, but there is much criticism of some use, particularly prophylaxis. While the appropriate use of antimicrobials can be justified in equine medicine, the misuse cannot. The definition of appropriate use is complex and involves the indication for therapy, antimicrobial selection, dosing regimen and timing and route of administration, duration of therapy and modification of therapy based on microbial susceptibility and clinical response. The aim of this article is to provide guidance on these factors to assist equine veterinarians in determining what constitutes appropriate antimicrobial use in horses.     

Antimicrobial use in the surgical patient.

Antimicrobials are often used in the surgical patient in an effort to prevent infection (prophylactic) or to treat established infection (therapeutic). To be effective, prophylactic antimicrobials at appropriate concentrations must be present in tissues at the surgical site at the time of contamination to prevent bacterial growth and subsequent infection. Therapeutic antimicrobials are used to treat established localized or systemic infection. Selection of antimicrobial agents for prophylactic or therapeutic use should be based on knowledge of expected flora, ability of the antimicrobial to reach the target tissue at appropriate concentrations, bacterial resistance patterns, drug pharmacokinetics, and culture and susceptibility testing results (therapeutic use). Failure of antimicrobial therapy to prevent or treat infection in the surgical patient may result from poor antimicrobial selection, inappropriate dosage or frequency, or inappropriate duration of therapy.     

Improving clinical outcomes via responsible antimicrobial use in horses

The growing problem of antimicrobial resistance affects veterinarians on a daily basis. Antimicrobial stewardship and responsible prescribing are essential for a future with effective antimicrobials, as it is unlikely that new antimicrobials will become available for use in horses in the near future. Extended-spectrum β-lactamase (ESBL) producing Enterobacteriaceae, methicillin-resistant Staphylococcus aureus (MRSA) and Salmonella spp. are pathogens of significant concern but there are also other opportunistic pathogens such as Pseudomonas spp., α-haemolytic Streptococcus spp., Enterococcus spp. and Acinetobacter spp. which, due to their high intrinsic resistance, have limited treatment options in adult horses. It is essential that highest priority critically important antimicrobials such as ceftiofur, enrofloxacin, rifampicin and polymyxin B are used prudently in horses and ideally based on culture and antimicrobial susceptibility testing (AST). For example, the use of polymyxin B at a low anti-endotoxic dose rather than at a higher antimicrobial dose in horses for the treatment of systemic inflammatory response syndrome is a potential driver for resistance to colistin (polymyxin E), an antimicrobial used as a last resort in the treatment of multidrug resistant (MDR) Enterobacteriaceae infections in humans. Serum procalcitonin levels are used in humans to distinguish noninfectious inflammatory conditions from inflammation caused by bacteria and other infectious agents and are also used to guide cessation of antimicrobial treatment. Although no such studies have been performed in horses, this or other markers may prove to be helpful in guiding antimicrobial treatment decisions in the future. Optimising sampling techniques and good communication with the microbiology laboratory are essential for generating the accurate culture and AST results that underpin appropriate antimicrobial use. Additionally, there is clearly a need for national and international harmonisation of laboratory methods in order to improve the reliability and consistency of results reported by different laboratories.     

Nosocomial infections and antimicrobial resistance in critical care medicine

Objective: To review the human and companion animal veterinary literature on nosocomial infections and antimicrobial drug resistance as they pertain to the critically ill patient. Data sources: Data from human and veterinary sources were reviewed using PubMed and CAB. Human data synthesis: There is a large amount of published data on nosocomially‐acquired bloodstream infections, pneumonia, urinary tract infections and surgical site infections, and strategies to minimize the frequency of these infections, in human medicine. Nosocomial infections caused by multi‐drug‐resistant (MDR) pathogens are a leading cause of increased patient morbidity and mortality, medical treatment costs, and prolonged hospital stay. Epidemiology and risk factor analyses have shown that the major risk factor for the development of antimicrobial resistance in critically ill human patients is heavy antibiotic usage. Veterinary data synthesis: There is a paucity of information on the development of antimicrobial drug resistance and nosocomially‐acquired infections in critically ill small animal veterinary patients. Mechanisms of antimicrobial drug resistance are universal, although the selection effects created by antibiotic usage may be less significant in veterinary patients. Future studies on the development of antimicrobial drug resistance in critically ill animals may benefit from research that has been conducted in humans. Conclusions: Antimicrobial use in critically ill patients selects for antimicrobial drug resistance and MDR nosocomial pathogens. The choice of antimicrobials should be prudent and based on regular surveillance studies and accurate microbiological diagnostics. Antimicrobial drug resistance is becoming an increasing problem in veterinary medicine, particularly in the critical care setting, and institution‐specific strategies should be developed to prevent the emergence of MDR infections. The collation of data from tertiary‐care veterinary hospitals may identify trends in antimicrobial drug resistance patterns in nosocomial pathogens and aid in formulating guidelines for antimicrobial use.     

Antimicrobial resistance in commensal faecal Escherichia coli of hospitalised horses

The objective of this study was to examine the impact of hospitalisation and antimicrobial drug administration on the prevalence of resistance in commensal faecal E. coli of horses. Faecal samples were collected from ten hospitalised horses treated with antimicrobials, ten hospitalised horses not treated with antimicrobials and nine non-hospitalised horses over a consecutive five day period and susceptibility testing was performed on isolated E. coli. Results revealed that hospitalisation alone was associated with increased prevalence of antimicrobial resistance and multidrug resistance in commensal E. coli of horses. Due to the risk of transfer of resistance between commensal and pathogenic bacteria, veterinarians need to be aware of possible resistance in commensal bacteria when treating hospitalised horses.     

Post operative infection, pyrexia and perioperative antimicrobial drug use in surgical colic patients

Reasons for performing study: Infections are common complications in post operative colic patients. It is the impression of some surgeons that pyrexia in the early post operative period is a sign of infection and appropriate timing of perioperative antimicrobials will decrease the incidence of post operative infection. Objective: To determine the association between 1) post operative pyrexia and development of infection and 2) perioperative antimicrobial drug use and infection rate in post operative colic patients. Methods: Medical records of patients undergoing surgical treatment for colic were reviewed. Horses recovering from surgery and surviving >48 h were included. Data relating to case details, duration of surgery, post operative infection, peri‐ and post operative antimicrobial administration, presence, intensity and duration of pyrexia, were recorded. Data were analysed using standard statistical methods for simple comparisons between groups and by logistic regression for more complex comparisons. Results: One‐hundred‐and‐thirteen horses were included in the final analyses, 48 (43%) of which were diagnosed with a post operative infection. Duration of surgery and anaesthesia were associated with post operative infection. Eighty‐five percent of horses (n = 96) exhibited pyrexia (rectal temperature >38.3°C) post operatively. Peak temperature >39.2°C, time post surgery to peak temperature >48 h and duration of pyrexia >48 h were significantly associated with infection. In a combined model, time to first pyrexic >48 h post surgery, peak temperature and time to peak >48 h were equally weighted and the model's positive predictive value for post operative infection was 72%. Timing and dose rate of preoperative antimicrobials were not associated with infection but duration of post operative antimicrobial drug use was. Conclusion and clinical relevance: Slight to mild pyrexia (38–39.4°C) in the early post operative period is not necessarily associated with impending bacterial infection in colic patients and the use of antimicrobials in these patients may be costly and unnecessary.     

Antimicrobial‐associated diarrhoea in three equine referral practices

Reasons for performing study: Although antimicrobial‐associated diarrhoea (AAD) is the most frequently observed adverse effect of antimicrobial therapy in horses, few multicentred studies on the prevalence of AAD have been performed. Objectives: To determine the prevalence of AAD in horses that developed diarrhoea after antimicrobial treatment for nondiarrhoeic conditions and identify the antimicrobials used. Methods: The 2009 database of 3 referral hospitals was searched to identify nonhospitalised horses (weanling age or older) treated with antimicrobials for nongastrointestinal conditions. Horses with these criteria that presented with diarrhoea during 2009 were included in the study. Additional information, including antimicrobial administered and results of faecal pathogen testing, was gathered on each hospitalised case. Results: Of the 5251 horses treated with antimicrobials for nongastrointestinal signs, 32 were diagnosed with probable AAD, a prevalence of 0.6% (95% confidence interval: 0.43–0.86%). The AAD‐diagnosed horses had an 18.8% (6/32) mortality rate. Horses with AAD had been treated for an average of 4.2 days. The most frequently used antimicrobials in horses with AAD were gentamicin in combination with penicillin (n = 7), enrofloxacin (n = 7) and doxycycline (n = 4). Clostridium difficile was identified in faecal samples from 4 horses, 2 of which died and Salmonella from 3 horses. Conclusions: Results indicated that the prevalence of AAD is low. Any antimicrobial class commonly used in equine practice is a potential cause of equine AAD. Other risk factors, such as opportunistic enteropathogens, may play a part in the development of diarrhoea secondary to antimicrobial usage. Potential relevance: Although the risk of equine AAD is low, this sequela of antimicrobial treatment is possible especially when opportunistic enteropathogens or other risk factors are present. Because drugs from any antimicrobial class can be potentially involved in AAD, clinicians have additional incentive to ensure the judicious use of antimicrobial agents.     

Can the use of antimicrobials in adult equine patients with acute colitis be justified in the era of antimicrobial stewardship?

The use of antimicrobials in adult horses with acute colitis is controversial and clear antimicrobial guidelines are lacking. It is generally accepted that antimicrobials should be reserved for patients with specific bacterial enterocolitis such as Neorickettsia risticii or Clostridium difficile; however, in practice, horses show similar clinical signs independently of the underlying aetiology. The clinician is therefore often confronted with the dilemma of rapid initiation of treatment to avoid mortality vs. delayed and selective targeted antimicrobial treatment based on faecal diagnostic testing. The risk of antibiotic associated diarrhoea and the emergence of antimicrobial resistance are other important considerations.     

Longitudinal study of antimicrobial-resistant commensal Escherichia coli in the faeces of horses in an equine hospital

The increasing prevalence of antimicrobial resistance in bacteria represents a considerable problem for human and veterinary medicine, causing complications in the treatment of infections. Resistance in Escherichia coli from horses has been documented in commensal and pathogenic strains, but little information exists regarding the prevalence of such bacteria in hospitalised horses or associated risk factors. A longitudinal cohort study was conducted of 103 horses admitted to a referral equine hospital for more than 48 h, with faecal samples collected on hospital admission and subsequently every two days until discharge. Horses undergoing radioactive gamma scintigraphic examination, un-weaned foals and mares with un-weaned foals were excluded. Data were collected from enrolled animals, including antimicrobial treatment history and hospitalisation details. Samples were cultured for resistant E. coli; isolates had their antimicrobial resistance profile determined. High sample prevalence for resistant E. coli was identified for all antimicrobials examined except co-amoxiclav. The prevalence of resistance was consistently lower at admission, rising to a peak 4 days post-admission. Risk factors were analysed using multilevel, multivariable modelling, which identified significant clustering of resistance outcomes within horses. For all outcomes except trimethoprim resistance, the day the sample was obtained was significant, with increased risk of resistance for samples taken on day 2 or later. Antimicrobial treatment in the previous seven days and increased total daily dosages of cotrimoxazole prescribed in the hospital in the previous 24-48 h were associated with increased risk. Location within the hospital and admission reason were significant risk factors for some resistance outcomes. High levels of multidrug-resistant E. coli (47.7% of samples) and extended spectrum β-lactamase-producing E. coli (27.3% of samples) were recovered; such bacteria could significantly complicate treatment if they were the cause of infection and may represent a risk to personnel in close contact with hospitalised horses.     

Associations among antimicrobial drug treatments and antimicrobial resistance of fecal Escherichia coli of swine on 34 farrow-to-finish farms in Ontario, Canada

Logistic regression was used to model associations between antimicrobial treatment and resistance among fecal Escherichia coli of finisher pigs at the farm level. Four sets of potential risk factors representing different levels of refinement of antimicrobial use on farms were modelled on resistance to antimicrobials. Final models for each antimicrobial were constructed from treatment and management variables significant on initial screening, and corrections for overdispersion were made. In general, in-feed antimicrobial treatment of pigs was more consistently associated with an increased risk of resistance than individual-animal treatment. Antimicrobial treatment in starter rations was significant in final models of resistance to ampicillin, carbadox, nitrofurantoin, sulfisoxizole, and tetracycline. Treatment in grower–finisher rations was significantly associated with resistance to ampicillin, spectinomycin, sulfisoxizole, and tetracycline. There was little evidence that in-feed antimicrobials increased the risk of resistance to gentamicin, which is a drug used only for individual-pig treatment in this study population. These results suggest that antimicrobial medication of rations of post-weaning pigs selects for and maintains antimicrobial resistance among E. coli of finisher pigs. Although resistance was common on farms that did not medicate rations of post-weaning pigs, the results indicate that antimicrobial use does increase the risk of resistance to the antimicrobials studied.     

Bacterial isolates and antimicrobial susceptibilities in equine bacterial ulcerative keratitis (1993--2004)

REASONS FOR PERFORMING STUDY: Bacterial ulcerative keratitis is a common and often vision-threatening problem in horses. Emerging bacterial resistance to commonly used topical antibiotics has been demonstrated. Previous antibiotic use may alter the antimicrobial susceptibility of bacterial isolates. OBJECTIVES: To document aerobic bacterial isolates and associated bacterial susceptibilities from horses with ulcerative keratitis treated at the University of Tennessee between January 1993 and May 2004 and determine whether prior antibiotic therapy affected antimicrobial susceptibility of the isolates. METHODS: Medical records from horses with ulcerative keratitis and positive aerobic bacterial cultures and antimicrobial susceptibility were evaluated. Clinical history regarding antibiotic therapy prior to culture was documented. RESULTS: Fifty-one aerobic bacterial isolates from 43 horses were identified. Streptococcus equi subspecies zooepidemicus was the most commonly isolated organism, accounting for 33.3% of all isolates, followed by Pseudomonas aeruginosa (11.8%), Staphylococcus spp. (11.8%) and Gram-negative nonfermenting rods (7.8 %). No resistance was noted amongst S. equi ssp. zooepidemicus to cephalothin, chloramphenicol or ciprofloxacin. Only 64 % of S. equi ssp. zooepidemicus isolates were sensitive to bacitracin. No resistance was noted among P. aeruginosa to gentamicin, tobramycin or ciprofloxacin. Antibiotic therapy with neomycin-polymixin B-bacitracin prior to presentation and culture was documented in 11/17 horses in which S. equi ssp. zooepidemicus was isolated and in 4/6 horses in which P. aeruginosa was isolated. Three horses received topical corticosteroids prior to culture, of which 2 had polymicrobial infections. CONCLUSIONS: S. equi ssp. zooepidemicus and P. aeruginosa were the most frequently isolated bacterial organisms in equine ulcerative keratitis. No significant trends in aminoglycoside or fluoroquinolone resistance were noted among these organisms. However, the resistance of S. equi ssp. zooepidemicus to bacitracin with common use of this antibiotic suggests that previous antibiotic therapy probably affects antimicrobial resistance. POTENTIAL RELEVANCE: Therapy prior to culture may play an important role in antimicrobial susceptibility of corneal bacterial isolates. Corticosteroid use may increase the risk of polymicrobial infections of corneal ulcers, leading to a worse prognosis. Although significant fluoroquinolone resistance has not been documented in the veterinary literature, these antimicrobials should be reserved for known infected corneal ulcers and not used for prophylaxis. Empirical antibiotic therapy should not only be guided by clinical signs, but also take into consideration previous antimicrobial and corticosteroid therapy.     

The treatment of salmonellosis in Equidae.

Among 23 Equidae in which various treatment or management regimens for salmonellosis were observed and evaluated, 9 horses were infected with Salmonella typhimurium and 8 with S anatum; an aged mule harbored S newport, and 1 isolant was not identified. Four dual sero-type infections occurred: 3 typhimurium/anatum and 1 anatum/newington. The results obtained with various antimicrobials in therapy of peracute, acute, and chronic infections were discouraging or questionable with regard to efficacy against salmonellae per se. The antimicrobial resistance patterns of the salmonellae and coliforms from the same fecal sample of the patient were determined. Salmonellae cultured from feces prior to, during, and following treatment with a variety of drugs were often sensitive to the antimicrobial administered during these periods. Drug sensitivity or resistance appeared to have little, if any, bearing on eliminating salmonellae from the feces or organs of infected Equidae.     

Dosing equine antimicrobials: Ensuring clinical success and avoiding antimicrobial resistance

Underdosing of equine antimicrobials is a global issue and may be contributing to antimicrobial resistance in horses. Recent research in Australia has identified that most antimicrobial use in peer-reviewed literature is at a higher dose than that currently on the label for many common equine antimicrobials in the United States, the UK and Australia. These dosing regimens used are now considered the most appropriate doses to be using in horses and veterinarians treating equine patients should be aware of the contemporary dosing regimens.     

Enrofloxacin use in a long-distance transport model of equine respiratory disease

Successful clinical management of bacterial pneumonia in horses depends on the administration of an appropriate antimicrobial agent at an adequate dosage and frequency, given by the correct route of administration for an adequate duration. Empiric antimicrobial therapy should be based on the clinician's experience and current veterinary literature. Based on the frequency of Gram-positive bacterial pathogens isolated from horses with bacterial pneumonia, every effort should be made to provide antimicrobial coverage for this class of pathogens. Gram-negative pathogens may co-exist; therefore, broad spectrum antimicrobial coverage may be necessary. Antimicrobial agents posing potential drug related risks (e.g. nephrotoxicity in dehydrated patients, peracute colitis in racehorses) may not be appropriate for empiric use and enrofloxacin offers an enhanced spectrum of antimicrobial activity with reduced risk of complication. Our findings indicate that in cases of exclusive Gram-positive respiratory disease, solo therapy with enrofloxacin is not indicated.     

Overview of the use of antimicrobials for the treatment of bacterial infections in horses

Use of antimicrobial drugs is central to the treatment of primary and secondary bacterial infection in horses. When selecting an antimicrobial to treat confirmed or suspected bacterial infection multiple factors should be considered, including: the likely infectious agent; distribution and dosage of selected drugs; mechanisms of action; and potential side effects. Many of these issues will be covered in subsequent articles in this series. The aim of this paper is to aid the clinician in the rational selection of antimicrobials by reviewing the mode of action, spectrum of activity, pharmacokinetics, pharmacodynamics, indications and potential side effects of the main classes of antimicrobial drugs. Extralabel use of drugs is common in veterinary medicine due to a lack of licensed products. This increases the importance of a thorough understanding of antimicrobials and their possible adverse effects.     

Effects of Perioperative Granulocyte Colony-Stimulating Factor on Horses With Ascending Colonic Ischemia

Granulocyte colony-stimulating factor (G-CSF) is a glycoprotein that regulates the proliferation and maturation of hematopoietic progenitor cells and modulates the function of mature neu-trophils. The responses to administration of G-CSF alone, and in combination with antimicrobials, were studied in an equine model of ascending colon ischemia. Complete segmental colonic ischemia (3.75 hours) with pelvic flexure enterotomy was created in four treatment groups. Group 1 horses received recombinant canine G-CSF (10 μg/kg, every 24 hours, intramuscularly), gentamicin sulfate (2.2 mg/kg, every 8 hours, intravenously), and potassium penicillin G (40,000 lU/kg, every 6 hours, intravenously). Group 2 horses were treated with the G-CSF vehicle and antimicrobials as for group 1. Group 3 horses received G-CSF and the antimicrobial drug vehicles, and group 4 horses served as the untreated control receiving G-CSF vehicle and antimicrobial vehicles. The results for 20 horses, five horses in each group, were compared. Treatment with G-CSF was associated with an increased concentration of white blood cells, band neutrophils, neutrophils, lymphocytes, and monocytes in the peripheral blood after surgery. Antimicrobial administration had no detectable effect on cell concentrations after surgery. Administration of G-CSF was associated with an increased concentration of nucleated cells in the peritoneal fluid including neutrophils, small mononuclear cells and large mononuclear cells. Horses that developed incisional infections had lower neutrophil concentrations in the peripheral blood on postoperative day 2 than horses without infected incisions. These results suggested that the prophylactic administration of G-CSF may be useful in the treatment of patients at risk for developing neutropenia after surgery.     

Borrelia burgdorferi Infection and Lyme Disease in North American Horses: A Consensus Statement

Borrelia burgdorferi infection is common in horses living in Lyme endemic areas and the geographic range for exposure is increasing. Morbidity after B. burgdorferi infection in horses is unknown. Documented, naturally occurring syndromes attributed to B. burgdorferi infection in horses include neuroborreliosis, uveitis, and cutaneous pseudolymphoma. Although other clinical signs such as lameness and stiffness are reported in horses, these are often not well documented. Diagnosis of Lyme disease is based on exposure to B. burgdorferi, cytology or histopathology of infected fluid or tissue and antigen detection. Treatment of Lyme disease in horses is similar to treatment of humans or small animals but treatment success might not be the same because of species differences in antimicrobial bioavailability and duration of infection before initiation of treatment. There are no approved equine label Lyme vaccines but there is strong evidence that proper vaccination could prevent infection in horses.     

Antimicrobial resistance in faecal Escherichia coli isolates from horses treated with antimicrobials: A longitudinal study in hospitalised and non-hospitalised horses

The objective of this study was to examine the emergence and persistence of antimicrobial resistant faecal Escherichia coli in horses treated with antimicrobial drugs in a hospital and community setting. Faecal samples were collected from hospitalised (n=56) and non-hospitalised (n=14) horses treated with antimicrobials, and 10 non-treated hospitalised controls. Samples were obtained pre-treatment and 5 days later in all horses, and 2 weeks and 2 months after treatment in treated horses. Susceptibility to 15 antimicrobials was tested via disc diffusion on up to 3 E. coli isolates per sample. Phenotypic extended spectrum beta-lactamase (ESBL) production was identified via a combination disc method, and ESBL-encoding sequences identified by PCR. A resistant E. coli isolate was identified in 138/228 (60.5%) samples. The proportion of resistant samples was not significantly different between hospitalised and non-hospitalised treated horses. The odds of a sample containing a resistant isolate increased significantly at day 5 in treated horses, but not in controls. Two weeks following treatment, the odds of resistance in non-hospitalised horses returned to pre-treatment levels, but remained significantly above pre-treatment levels in hospital-treated horses, returning to base-line 2 months after treatment. Seven samples (17 isolates) were positive for ESBL production. The genes bla(CTX-M) and bla(TEM) were identified in 12/17 isolates, with bla(SHV) in 4/17. Antimicrobial administration to horses in hospital and community settings is associated with an increased but transient risk of faecal shedding of antimicrobial-resistant E. coli. The high prevalence of resistant isolates suggests that methods to minimise their potential spread should be considered.     

Factors associated with antimicrobial-resistant Escherichia coli in zoo animals

Factors associated with the carriage of antimicrobial-resistant Escherichia coli isolates were analysed among zoo animals. An association was observed between selection of amoxicillin as the first-line therapy and a significantly higher percentage of resistance to ampicillin (54.5%) from 11 animals treated with antimicrobials, compared with isolates from 32 untreated animals (9.4%). In addition, the percentage resistance to kanamycin (36.4%), gentamicin (27.3%), trimethoprim (27.3%) and tetracycline (63.6%) from 11 treated animals was significantly higher than those from 32 untreated animals (3.1%, 3.1%, 3.1% and 25%, respectively), although these antimicrobials were rarely used. All kanamycin-, gentamicin- and trimethoprim-resistant isolates and more than half of the tetracycline-resistant isolates from treated animals were also resistant to ampicillin. Co-resistance to other antimicrobials with ampicillin was suggested to contribute to an increasing of resistance towards antimicrobials that were rarely administered. The present investigation revealed an association of antimicrobial treatment with the spread of antimicrobial-resistant bacteria among zoo animals.     

Antimicrobial resistance in bacteria from Swiss veal calves at slaughter

Bacteria with antimicrobial resistance can be transferred from animals to humans and may compromise antimicrobial treatment in case of infection. To determine the antimicrobial resistance situation in bacteria from Swiss veal calves, faecal samples from 500 randomly selected calves originating from 129 farms were collected at four big slaughterhouses. Samples were cultured for Escherichia coli, Enterococcus sp. and Campylobacter sp. and isolated strains were tested for antimicrobial susceptibility to selected antimicrobial agents by the minimal inhibitory concentration technique using the broth microdilution method. From 100 farms, data on farm management, animal husbandry and antimicrobial treatments of the calves were collected by questionnaire. Risk factors associated with antimicrobial resistance were identified by logistic regression. In total, 467 E. coli, 413 Enterococcus sp. and 202 Campylobacter sp. were isolated. Of those, 68.7%, 98.7% and 67.8%, respectively, were resistant to at least one of the tested antimicrobial agents. Resistance was mainly observed to antimicrobials frequently used in farm animals. Prevalence of resistance to antimicrobials important for human treatment was generally low. However, a rather high number of quinupristin/dalfopristin-resistant Enterococcus faecium and ciprofloxacin-resistant Campylobacter sp. were detected. External calf purchase, large finishing groups, feeding of milk by-products and administration of antimicrobials through feed upon arrival of the animals on the farm significantly increased the risk of antimicrobial resistance at farm level. Participation in a quality assurance programme and injection of a macrolide upon arrival of the animals on the farm had a protective effect. The present study showed that veal calves may serve as a reservoir for resistant bacteria. To ensure food safety, veal calves should be included in the national monitoring programme for antimicrobial resistance in farm animals. By improving farm management and calf husbandry the prevalence of resistance may be reduced.