Optimizing Antimicrobial Therapy
Dr. Jim McConnell

Concepts regarding the use of antimicrobial drugs in veterinary medicine
have undergone significant changes over the past few years. Increasing
incidences of foodborne infections and antimicrobial-resistant infections
have given rise to more sophisticated and effective methods of selection and
dosing these essential and powerful drugs.

At this year’s ACVIM Veterinary Medical Forum in Chicago, I attended an
important series of presentations reviewing the current status of
antimicrobial therapy. Pertinent clinical concepts presented included:

Fluoroquinolones

Fluoroquinolone resistance has increased over the last five years. It
appears the “honeymoon” era of this veterinary silver bullet is ending.
Resistance rates exceeding 25% for some forms of infection (e.g., nosocomial)
are now being reported from veterinary clinical pathology labs.

Fluoroquinolones exhibit concentration-dependent killing, such that
higher peak serum concentrations are significantly more effective and less
toxic. Also, fluoroquinolones exhibit a post-antibiotic effect, such that
even at concentrations below minimum bactericidal effect, surviving bacteria
suffer a period of impaired replication, which allows for less frequent
dosing. Pairing these drugs’ concentration-dependent killing and
post-antibiotic effect has given rise to the now well-recognized once daily
dosing schedule. Professional flexible dosing information is now provided.
Instead of a “fixed” dosing regimen on the drug label, information on several
dosing regimens and the drug’s various MIC’s, distribution data, and efficacy
data for different types of infections are provided. This gives
veterinarians vital information to eliminate bacterial infections in their
patients.

Cephalosporins

Extended spectrum cephalosporins are increasingly used to treat resistant
bacterial infections in small animals. In using these potent agents, it is
critical to select them based on culture and sensitivity data whenever
possible and realize that all cephalosporins are not created equal. Some
specific recommendations on their use include:

ceftiofur (Naxel) - this drug is useful for straightforward E. coli and
Proteus UTI’s, but has an inadequate spectrum for resistant gram negative
infections;

cefoxitin (Mefoxin) - this drug is excellent for anaerobic and mixed
infectious (eg., aspiration pneumonia, bowel performation);

cefotetan (Cefotan) - this drug has good activity against anaerobic
infections and an even better spectrum against gram negative infections than
cefoxitin. It also can be given less frequently than cefoxitin (BID sq vs.
TID sq);

ceftriaxone (Rocephin) - this drug achieves high CSF concentrations and
is very useful in treating bacterial meningitis. It is also very effective
in treating some forms of Lyme disease in people.

Azithromycin (Zithromax)

Azithromycin is a drug derived from erythromycin. It has better oral
absorption, longer half-life, broader spectrum of activity, and better
tolerance than its parent drug. It is effective against gram positive and
anaerobic infections and is effective against intracellular organisms like
Chlamydia, mycoplasma, toxoplasma, and some mycobacteria.

Azithromycin can elminiate susceptible infections in dogs and cats in
three to five days and can be given orally once a day.

Metronidazole

Metronidazole is an excellent antimicrobial for anaerobic infections. In
surveys of cultured anaerobic veterinary strains, efficacy is close to 100%.

Metronidazole toxicity has become more commonly diagnosed as it’s use in
veterinary patients rises. The recommended dose for metronidazole is 30
mg/kg/day. If this drug is used in a patient with impaired liver function,
the dose is 7.5-15 mg/kg/day.


Antimicrobial Formulary

*cost - average daily cost per 10 kg dog per day

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