Antimicrobials are amongst the most commonly prescribed medications by veterinarians. Understanding how to properly employ
these powerful drugs is the key to a successful outcome. Choosing the appropriate antimicrobial can be a daunting task, particularly
when faced with an unusual pathogen or one which is resistant to multiple antimicrobials. Making the appropriate choice is
a six step process:
1. Does the patient have a treatable infection?
2. On the surface, this question appears relatively simple to answer, however, in real-life situations it can be quite
difficult. It is important to avoid the temptation to treat with an antimicrobial "just in case". Doing so makes choosing
an appropriate drug a matter of guess work.
3. Where is the infection located?
4. By determining where the infection is located, appropriate cultures can be collected and the most likely pathogens
5. What are the likely pathogen(s)?
6. Once the location of the infection has been identified a list of the most common pathogens can be generated. This
allows empirical therapy to be implemented pending the culture and sensitivity results.
7. Which antimicrobial(s) are the most likely to work?
8. It is important to remember that we use in vitro sensitivity to predict in vivo efficacy. Properly interpreting microbial culture and sensitivity requires more knowledge that just reading which drugs
are sensitive and which are resistant. A more in depth discussion of this is found below. Numerous factors can lead to treatment
failure even when an "appropriate" antimicrobial is selected. These can include low drug concentration and the site of infection
(i.e. CNS or prostate), poor bioavailability (oral ampicillin), and poor owner compliance with the prescribed dosing regimen.
9. What dose and route are necessary to achieve effective drug concentrations at the site of infection?
10. It is important to determine whether the drug being administered is a concentration dependent or time dependent antimicrobial
as well as the half life since this will affect dosing frequency. Many commonly used drugs have a dosing range. The choice
of the appropriate dose within the range will depend upon taking into account the relative sensitivity of the organism and
the drug concentration at the site of infection.
11. What is the appropriate length of treatment and treatment protocol?
12. Length of treatment depends upon the chronicity and location of the infection. As a rule of thumb, acute infections
require short courses, while chronic and/or inaccessible infections require longer courses of therapy.
• Acute infections (i.e. simple UTI): 7-10 days
• Chronic infections (i.e. deep pyoderma, osteomyelitis): 3-6 weeks