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T. Nuttall1
1University of Edinburgh, Royal dick School of Veterinary Studies, Roslin, United Kingdom
Risk factors for colonisation with antimicrobial resistant bacteria
Dr Tim Nuttall
Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Campus, Roslin, UK.
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 ef cacy 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  rst 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 signi cant 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  rst isolated in 1961. The  rst 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  uoroquinolones are speci cally 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 speci c antimicrobials select for resistance is less clear. There is some evidence that cephalosporins, 3rd generation cephalosporins and  uoroquinolones 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 in uenced by practice expectations (real or imagined). The three most important drivers are vet prescribing behaviours, interactions with clients and practice norms.
An Urban Experience

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