There are several criteria to assess therapeutic success:

Is the antimicrobial effective against the causal organism?

• Veterinary surgeons must have a working knowledge of the spectrum of activity of the main antimicrobials used in veterinary practice.
• The veterinary surgeon may have a good idea of the likely organism(s) involved. In addition, some organisms have:
  – Predictable/stable resistance patterns (e.g. b-haemolytic streptococci, Erysipelothrix rhusiopathiae, Actinomyces pyogenes)
  –  Variable resistance patterns (e.g. Staphylococcus aureus, Pasteurella species)
  –  Unpredictable resistance patterns (e.g. Escherichia coli, Pseudomonas species, Salmonella species).

Culture and sensitivity testing should be considered if the causal organism is suspected to have variable or unpredictable resistance patterns. However, it should be noted that antimicrobial treatment will usually need to be started prior to obtaining the culture and sensitivity test results.

Will the antimicrobial reach the site of infection in sufficient concentration?

• Veterinary surgeons must have a working knowledge of the pharmacokinetics of the antimicrobial, in particular a knowledge of its distribution.
• Most veterinary antimicrobials are well distributed (see below).
• Subtleties exist within this broad categorization:
  – Fluoroquinolones and macrolides achieve particularly high concentrations within cells
  – Certain antimicrobials, such as penicillins, cephalosporins and fluoroquinolones, achieve high concentrations in the urinary tract
  – Potentiated sulphonamides and fluoroquinolones are more likely to cross the blood–prostate barrier
  – Beta-lactams struggle to attain reasonable concentrations in pulmonary secretions.

 Distribution of veterinary antimicrobials