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Empirical therapy is appropriate:
• Topical therapy
• Life-threatening infections that need immediate treatment
• A  rst episode of infection
• Animals with no systemic antibiotic therapy within the last 3 months
• Surface or super cial pyodermas
• Cytology consistent with staphylococci
• Antibiotic resistance is unlikely
Culture and antimicrobial sensitivity testing is necessary:
• A life-threatening infection (although immediate therapy is necessary while waiting for results)
• Deep pyoderma
• Inconsistent clinical signs and cytology
• Rod bacteria
• If empirical treatment fails
• If resistance is more likely (e.g. after multiple antibiotic courses, non-healing wounds, or post-operative and other nosocomial infections)
Cytology can be used to determine the most important bacteria if culture detects multiple species with differing antimicrobial sensitivities.
Material for culture can be obtained by a variety of means depending on the type and depth of lesions. Primary lesions should be selected where possible. Alcohol can be used to reduce surface contamination, but this should be allowed to evaporate before the sample is collected.
If antibiotic withdrawal isn’t possible prolonged and/or enriched cultures may be necessary and recent therapy should be noted on the submission form.
Understanding susceptibility and resistance breakpoints
Kirby-Bauer tests use antibiotic impregnated paper discs. The zone of inhibition around a disc is compared to agreed susceptibility/resistance breakpoints. The minimum inhibitory concentration (MIC; lowest antibiotic concentration that inhibits growth) is determined in broth culture or using E-strips® and compared to agreed breakpoints to determine susceptibility or resistance.
If the isolate is susceptible then it is likely that antibiotic levels following systemic administration will exceed the MIC in the target tissue and the infection should respond to treatment. In contrast, if the isolate is resistant it is unlikely that the antibiotic will achieve a therapeutic concentration and the infection is not likely to resolve. Knowing the actual MIC is useful, as antibiotics with MICs close to the break point may need to be given at higher doses to ensure that they achieve therapeutic concentrations in the target tissues.
Breakpoints based on MICs, pharmacokinetics and tissue penetration. They are species speci c, and
An Urban Experience
breakpoints established for one species may not be accurate in others. They are also affected by variation
in dosing, bioavailability, pharmacokinetics and tissue penetration. Finally, breakpoints are only relevant for systemic therapy and should not be used with topical therapy. The breakpoint does not imply that the bacteria will never respond to the antibiotic, only that systemic treatment will not attain a suf cient concentration at the site of infection. Topical therapy with mg/ml antibiotic concentrations, for example, can overcome apparent resistance.
Why doesn’t the clinical response match the in vitro test?
In vitro susceptibility tests don’t necessarily predict
the clinical outcome. Disc diffusion tests may give misleading results – for example beta-lactam and cephalosporin susceptibility or resistance in vitro can
be poorly predictive of the presence of meticillin- resistant staphylococci (MRSA/MRSP) or extended spectrum beta-lactamase (ESBL) E. coli and further tests may be necessary. Many MRSA and MRSP isolates exhibit inducible clindamycin resistance in vivo despite apparent in vitro sensitivity. This can be tested for using D-zone tests with erythromycin or PCR. Enro oxacin is metabolised into cipro oxacin, and tests based on one drug alone may not accurately predict the outcome. In addition to the MIC, understanding the nature of the infection, the pharmacokinetics of the antibiotic and patient factors will help achieve a successful outcome.
Selecting an appropriate antibiotic
It is important to consider whether the infection is deep, severe and/or generalised enough to warrant systemic antibiotics. Topical antimicrobials or topical antibiotics can be effective in surface and super cial infections
and some focal deep infections. Effective topical antimicrobials include chlorhexidine, Manuka honey, hypochlorous acid, silver sulfadiazine, mupirocin and fusidic acid. Other antibiotics are also available in topical formulation for eye and ear infections, and injectable preparations can be added to creams and ointments.
First line antibiotics are no less potent than other drugs in the correct circumstances and are appropriate for empirical treatment. They include cefadroxil, cefalexin, clavulanate-amoxicillin, clindamycin and lincomycin. Cefovecin can be considered where administration or compliance may be dif cult.
Second line antibiotics should only be used when there is culture evidence that  rst line drugs will not be effective. These include cefovecin and  uoroquinolones.
Third line antibiotics must only be used where no  rst or second line antibiotics are effective, and topical antimicrobial therapy is not feasible or effective. These include aminoglycosides, azithromycin, ceftazidime, chloramphenicol, clarithromycin,  orphenicol,

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