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5. Antimicrobial use and antimicrobial resistance.

companion05.jpgDon says, How does antimicrobial use affect antimicrobial resistance?

Sue says, Antimicrobial use is an important contributor to the occurrence of antimicrobial resistance in both animals and humans. Several epidemiological studies in dogs have demonstrated that prior antimicrobial exposure was associated with antimicrobial resistance in a number of bacterial species including E. coli isolates from feces (2,9,10), and opportunistic pathogens (11,12), including methicillin-resistant Staphylococcus aureus (13). An experimental study demonstrated that dogs treated with enrofloxacin were more effectively colonized with multidrug-resistant E. coli (14). 

Recently, associations were observed between cephalexin use (a first-generation cephalosporin) in dogs and antimicrobial resistance to a B-lactam inhibitor combination (amoxicillin-clavulanic acid), third generation cephalosporins (ceftriaxone, cefoxitin) and cephamycins (ceftiofur). Dogs that were treated with cephalexin had a higher risk of resistance to these antimicrobials, from 3-8 times higher risk, when compared to dogs not treated with antimicrobials. The development of antimicrobial resistance was faster in dogs treated with cephalexin than in dogs not treated with antimicrobials. These are extremely important findings since cephalexin is the most commonly prescribed antimicrobial in dogs and these antimicrobials are critically important in the treatment of human infections. It suggests that the use of cephalexin may provide significant selection pressure for the occurrence of antimicrobial resistance.
Figure 1 A comparison of the times to loss of susceptibility (or the occurrence of antimicrobial resistance) in fecal E. coli isolates in dogs treated with cephalexin (red line) compared to dogs untreated with antimicrobials (black line).

Susceptibility to Ceftiofur


Susceptibility to Amoxicillin-clavulanic Acid


Susceptibility to Ceftriaxone


Legend: Cephalexin Treated Dogs Dogs Untreated With Antimicrobials

At any given time, the percentage of fecal E. coli isolates that were susceptible to the above antimicrobials was significantly lower in dogs treated with cephalexin (red line) than in dogs that were not treated with antimicrobials (black line). In other words, antimicrobial resistant fecal E.coli isolates from dogs treated with cephalexin were isolated more frequently and at a faster rate than in dogs that were not treated with antimicrobials.



  1. Cephalexin treatment was started 0-12 hours after the Day 0 sample was collected and duration of cephalexin treatment ranged from 5 to 14 days.
  2. The study population were dogs (n=33 cephalexin treated, n=8 untreated) from primary-care (i.e. non-referral) small animal veterinary hospitals (n=21) in southern Ontario and were treated with cephalexin for a disease condition that was diagnosed and managed by the attending veterinarian



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