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9. Choosing an Antibiotic

In order for an antibiotic to be effective at killing the bacteria present, it needs to reach adequate concentrations in the body organ of interest after oral dosing.  Moreover, the antibiotic has to be efficacious against the organism(s) causing the infection.  The table below provides examples of some antibiotics that are good empiric choices for treating staphylococcal skin infections in dogs.  You should remember two important things:

  1. The most common bacterium that causes skin infection in dogs is Staphylococcus intermedius, and most strains of this bacterium produce β-lactamase, which will inhibit the efficacy of many β-lactam antibiotics.
  2. The antibiotics that cannot be used empirically to treat a staphylococcus infection include: penicillins, ampicillin, and amoxicillin, which are all β-lactam antibiotics.  Some synthetic penicillins (e.g. oxacillin, amoxicillin-clavulanic acid) and cephalosporins are β-lactamase resistant and may be used to treat infections empirically.  In addition, you should not use tetracycline or streptomycin to treat staphylococcus infections empirically because most strains will be resistant to them.

The length of treatment of any episode of infection is also important to consider.  Not treating an infection for an adequate period of time will result in non-resolution (despite possible clinical improvement) or frequent recurrence (or what may appear to be true recurrence but is actually non-resolution of initial infection).  The rule of thumb regarding the treatment duration of superficial pyoderma is to treat each infection episode for 7 days beyond the resolution of clinical signs.  Total treatment duration often takes at least 7 weeks.  If Junior has been treated for this condition previously with antibiotics and the skin lesions never completely resolved, it is possible that the duration of treatment was inadequate or the bacteria may have developed resistance.  The length of treatment may also affect owner compliance – you must emphasize the importance to your client of completing antibiotic therapy as prescribed until the recheck appointment.

Although rare, adverse effects of antibiotic therapy can occur.  Most oral antibiotics will cause nausea and sometimes vomiting and diarrhea.  Owners should be aware of these potential side effects and notify you if one or more occur.  You should clearly explain to the owner any possible side effects that may occur due to the medication you prescribe and possible ways to lessen these effects (e.g., for some antibiotics giving them with food may help).

As mentioned above, with pyoderma it is very important that you recheck the patient to ensure that the skin lesions have completely resolved.  This will help you decide whether or not at this subsequent visit you are going to perform a culture and susceptibility test.  For example, if Junior had received antibiotic therapy multiple times in the past and the correct dose was used, you should consider culturing the lesions. In addition, if the response to therapy has not been adequate, you could either switch to another antibiotic if there has been only one episode of infection in the past, or, ideally, perform culture and susceptibility testing.
You explain to the owner that you will need to recheck Junior in 3-5 weeks.  Let the owner know that if minimal to no improvement has been noticed at the recheck visit, you will perform culture and susceptibility testing before choosing another antibiotic empirically. 

Table – 09a. Systemic antibiotics recommended for Staphylococcal pyodermas


Recommended Oral Dose



10-15 mg/kg q8h

Good empiric choice for first episode of infection. Often causes GI upset.


22 mg/kg q12h

Good empiric choice for first episode of infection.


27.5 mg/kg q24h

Good empiric choice for first episode of infection. Potentiated sulfa drugs can cause keratoconjunctivitis sicca or hypothyroidism when used long-term (> 3-4 weeks).  Drug reaction can occur at any time during therapy.


15-30 mg/kg q12h

See comments above for ormetoprim-sulfadimethoxine.


22-33 mg/kq q8h to q12h

Good empiric choice for first episode of infection. (A β-lactam antibiotic usually effective against β-lactamase enzymes)


22 mg/kq q12h

Good empiric choice for first episode of infection. (A β-lactam antibiotic usually effective against β-lactamase enzymes)


5 mg/kq q24h or 10 mg/kg q12h

Only recommended in instances of recurrent pyodermas when C&S testing indicates its required use.  Better choice for deep pyodermas.


22 mg/kg q8h

Good choice for first episode of infection. (A β-lactam antibiotic usually effective against β-lactamase enzymes)

Amoxicillin-clavulanic acid

12.5-22 mg/kg q12h or q8

Good empiric choice for first episode of infection, although its limited efficacy may not warrant expense. (β-lactam - β-lactamase inhibitor combination)


5.5 mg/kq q12h or 11 mg/kg q24h

Selection should be based on C&S since Staphylococcus intermedius develops resistance to this antibiotic very easily.


Table – 09b. Systemic antibiotics NOT recommended as empiric choice for Staphylococcal pyodermas



Penicillins, ampicillin, amoxicillin

These are all β-lactam antibiotics


Rarely effective for Staphylococcal pyodermas


Rarely effective for Staphylococcal pyodermas


Keeping all of these considerations in mind, you choose to begin antibiotic therapy for Junior with oral cephalexin at 22 mg/kg twice daily and topical Chlorhexiderm shampoo (4% chlorhexidine).  You send home enough antibiotic to cover Junior to his recheck appointment (scheduled for three weeks from now) plus two weeks.



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