Antimicrobial-Resistant Bacteria of Clinical Concern
World Small Animal Veterinary Association Congress Proceedings, 2017
Tim Nuttall
Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Campus, Roslin, UK

Risk Factors for Colonisation with Antimicrobial-Resistant Bacteria


Antimicrobial resistance (AMR) is an increasing threat to modern human and veterinary healthcare. This is a one-health problem - humans and animals are intimately associated, and we're exposed to the same drugs, bacteria and resistance genes. Medical staff, veterinary staff, the public and animal owners all have a role in limiting AMR and maintaining the efficacy of these drugs.

Antibiotics and Antibiotic Resistance

AMR genes are widespread as organisms use antibiotics to compete. These are probably in evolutionary balance. Therapeutic use, in contrast, results in abrupt exposure to high antibiotic concentrations and systemic use leads to whole microbiome exposure. This favours survival of AMR bacteria. Horizontal transfer allows AMR genes to spread within and between bacterial populations.

Antimicrobial-Resistant Organisms of Concern

Meticillin-resistant staphylococci (MRS) are of most concern. MRSP was first recognised in 2004, and has spread worldwide. In the UK, MRSA isolates have been relatively stable while MRSP isolates have increased. MRSP isolates tend to have a wider resistance spectrum than MRSA. Other bacteria with increasing AMR include multi-drug resistant (MDR) Pseudomonas, Salmonella and Streptococcus, and extended-spectrum beta-lactamase (ESBL) and AmpC producing E. coli and Klebsiella.

Antibiotic Resistance in Healthy Animals

AMR bacteria are readily isolated from healthy animals; nearly 40% of healthy horses and 18–29% of healthy dogs carry MDR E. coli, and 29% of horses and 6–40% of dogs carry MRS. Fortunately, more clinically significant AMR bacteria are less common. ESBL E. coli are only carried by 6.3% of horses and 4% of dogs, and AmpC E. coli, MRSA and MRSP by less than 1% of animals.

Does Antibiotic Use Select for Resistance?

Antibiotic use is the biggest factor driving the emergence and spread of resistance. Resistance to penicillins was seen shortly after their introduction; for example, meticillin was introduced in 1959 and MRSA first isolated in 1961. The first companion animal MRSA isolates were also reported in 1961, with multiple case series emerging in the 1990s. The same pattern has been seen with other antimicrobial classes. Levels of resistance correlate with antibiotic prescribing rates in human healthcare and reducing prescribing is associated with lower levels of resistance. Glycopeptides, cephalosporins, and fluoroquinolones are specifically associated with selection for MRSA in humans.

Animal data are not quite as clear, although multiple antibiotic courses are associated with AMR. Systemic treatment in dogs increases the prevalence of AMR among staphylococci and E. coli, and the effects last for three months. However, evidence that specific antimicrobials select for resistance is less clear. There is some evidence that cephalosporins, 3rd-generation cephalosporins and fluoroquinolones may select for resistance among staphylococci and E. coli, but clindamycin may have less effect on resistance.

Antimicrobial Stewardship

We need to reduce systemic antimicrobial use. Studies of prescribing behaviour in the UK have shown that 17–39% of vet-visiting dogs, cats and horses get antimicrobials. The most common problems are pruritus, diarrhoea and respiratory conditions, and it is likely that many cases did not require systemic broad-spectrum antimicrobials.

Drivers for Antimicrobial Use in Practice

Antimicrobial use can be influenced by practice expectations (real or imagined). The three most important drivers are vet prescribing behaviours, interactions with clients and practice norms.

Drivers for appropriate and inappropriate antimicrobial use


Client interaction

Effective communication
Trusting relationship
Poor knowledge of AMR in animals
Client demand for antimicrobials
Insufficient consultation time
Fear of losing the client and income
Public education
Engagement with Antibiotic Guardian campaigns

Veterinary interaction

Practice culture
Fear of missing an infection
Influence of senior colleagues
Lack of audit and comparative data
Engagement with Antibiotic Guardian campaigns

Professional regulation

Clear national guidance
Training voluntary versus mandatory; impact on uptake and clinical freedom?

Antimicrobial use guidelines

Practical Comprehensive Independent and evidence-based

Recognition of good practice

National or international accreditation of clinicians and practices
Public awareness

Antimicrobial Use Guidelines

A variety of guidelines are available. However, implementation varies from mandatory (e.g., in Denmark and Sweden) to professional responsibility (e.g., in the UK) or voluntary. The challenge is to improve engagement and adherence. Peer discussion of treatment protocols, but these should be evidence-based, follow current recommendations and take into account the resources available.

Key concepts for responsible antimicrobial use


You should know or strongly suspect there is a bacterial infection

Don 't use antimicrobials to treat non-specific clinical signs
Understand the clinical signs associated with bacterial infection
Use cytology to confirm bacterial involvement
Know when to culture

Are systemic antimicrobials needed?

Consider topical antimicrobials
Manage the underlying condition

Does the animal require immediate treatment?

How serious is the infection? Can you wait for the culture results?

Choose an appropriate antimicrobial

Aim for the lowest tier most narrow spectrum drug
Consider penetration to the target tissue
Consider topical treatment

Use the correct dose

Always weigh animals and round the dose up

Treatment duration

Treat to clinical cure and avoid overly long courses of treatment

Improve compliance

Explain treatment, using good communication and follow-up

Veterinary Contact and Antimicrobial-Resistant Bacteria

Colonisation with AMR bacteria increases with veterinary contact. For example, MRS have been isolated from 0.5% to 10% of vet-visiting animals and clinical samples in Europe and Canada. The prevalence was 46% among canine in-patients in Japan, and in the US they have been found in 15–38% of dogs with pyoderma and up to 20% of clinical samples. Some 7–13% of veterinary staff are colonised with MRS, which reflect their area of work. MRS can also be isolated from up to 10% of veterinary practice samples, particularly hand touch sites. Fluoroquinolone-resistant, ESBL and AmpC E. coli have been found in 5–10% of faecal and environmental samples from UK veterinary hospitals, particularly ward floors, tables and keyboards.

Specific risk factors include:

  • Multiple antibiotic courses
  • Postoperative infections
  • Nosocomial (healthcare-acquired) infections
  • Prolonged hospitalisation (especially in ICUs)
  • Surgical or nursing implants (e.g., catheters and feeding tubes)

Key steps in effective infection control


Hand hygiene

Clean and disinfect hands before and after touching animals or material
Wash hands if visibly soiled
Avoid watches, jewelry and nails that interfere with cleaning
Wear gloves if necessary, but these are not a substitute for hand hygiene

Protective clothing

Wear clean appropriate protective clothing
Change out of protective clothing when leaving the premises

Barrier nursing

Use extra protection for high risk cases; change between patients


Use a high standard of preparation, cleanliness and surgical skill
Protect wounds from licking and trauma


Use effective products
Follow clear guidelines
Don 't rely on visual assessments of cleanliness
Launder regularly, separating clean and soiled items


Clinical audit and review of infections and resistance patterns
Discuss results with your microbiology laboratory
Join passive surveillance programmes


Train and encourage all staff
Adopt written protocols
Appoint an infection control champion (or team)

Zoonotic Colonisation and Infection

AMR bacteria and genes can be transferred between humans and animals in both directions. This varies — MRSP is fairly animal specific but MRSA seem to move between species more readily, and gene transfer is more frequent among Gram-negative bacteria. The risk of infection is very low as these are mostly commensal opportunists. Nevertheless, owners of animals with AMR colonisation or infections should be asked about risk factors for infections and be given appropriate hygiene advice. Whether animals belonging to owners with recent hospital contact are at greater risk of AMR carriage is less clear, but it would be prudent to check.

Raw Food Diets

These diets are more often contaminated with potential pathogens (e.g., Salmonella, Campylobacter or Listeria) or AMR bacteria compared to dry foods. Raw meat is a risk factor for carriage of AMR bacteria (e.g., ESBL-producing E. coli) and shedding of pathogens.

Environmental Exposure

Early studies have suggested that contamination of the environment by antibiotic-treated dogs and farm animals may facilitate dissemination of AMR in communities. Other sources of environmental contamination include sewage and effluent from farms, hospitals and veterinary premises.


We have clear responsibilities in reducing antimicrobial use and infection control. We must encourage owners to expect less antibiotic treatment and to treat effectively. Finally, we can work with policy makers to develop effective guidelines and regulation.

Further Resources

1.  British Veterinary Association -

2.  British Small Animal Veterinary Association -


4.  British Equine Veterinary Association - (VIN editor: link was modified on 5-18-18)

5.  Responsible Use of Medicines in Agriculture (RUMA) -

6.  Federation of European Companion Animal Veterinary Associations (FECAVA) - (VIN editor: link was modified on 12-22-17)

7.  International Society for Companion Animal Infectious Diseases (ISCAID) -

8.  Bella Moss Foundation - (VIN editor: link was modified on 12-22-17)

9.  Antibiotic Action and Guardian campaigns - &


Speaker Information
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Tim Nuttall
Royal (Dick) School of Veterinary Studies
University of Edinburgh
Easter Bush Campus, Roslin, UK

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