The following tables provide recommendations
for specific diseases/syndromes and pathogens. The
tables have been drafted considering a combination
of factors, including expected pathogens, expected
susceptibility patterns and typical patient factors.
The recommended doses for antimicrobial agents
used in equine medicine are listed in Table 10.2.
The scientific quality of the literature on which these
tables are based is highly variable, as there is a general
paucity of well-controlled studies on antimicrobial
efficacy in horses. Many antimicrobial recommendations,
particularly multiple drug combinations, have
been passed down through the literature but are not
based on any objective data. A common example of
this is the combination of penicillin, gentamicin and
metronidazole, which is sometimes used for the treatment
of life-threatening conditions such as pleuropneumonia
and peritonitis. This triple antimicrobial
combination is considered the most broad-spectrum
coverage possible for equine pathogens, with the
exception of resistant organisms and Mycoplasma spp.
However, this triple combination tends to be employed
based on fears of missing a pathogen involved, economic
value of the horse or lack of knowledge about
the disease. The combination of a β-lactam with an
aminogylcoside is a very broad-spectrum combination
for sensitive organisms. However, some anaerobes,
notably some strains of Clostridium and Bacteroides
are not affected by β-lactams. Metronidazole treatment
improves anaerobic coverage with better
pharmacodynamic and pharmacokinetic characteristics
for long-acting penetration into difficult to reach
body sites. Nevertheless, most infections in horses are
caused by aerobic Gram-positive and Gram-negative
bacteria, and thus this triple antimicrobial combination
does not represent improved coverage. In fact,
it is possible that the more antimicrobial treatments
that disrupt the intestinal anaerobic population, the
more likely the horse could develop antimicrobialassociated
colitis. Thus, it would be more prudent to
put thought and effort into finding the cause(s) and
choose antimicrobials with better pharmacodynamic
and pharmacokinetic characteristics against Grampositive
and Gram-negative infections. Equine anaerobic
infections in most cases are more likely associated
with mixed chronic infections (e.g. >5 days) in body
sites that can develop into low oxygen tension sites
(e.g. pleura, peritoneum, deep wounds).