Musculoskeletal infections
The musculoskeletal system can present a variety of
challenges for antimicrobial therapy (Table 10.5). It
is difficult to reach therapeutic antimicrobial levels
in many tissues, such as bone, tendon, tendon sheath
and joints following oral or parenteral antimicrobial
administration. Alternative approaches such as intraarticular
therapy, intra-osseous infusion and regional
perfusion may be useful in many situations to provide
very high local antimicrobial levels. The tendency
for abscess formation in soft tissues presents further
challenges that inhibit antimicrobial penetration and
activity.
Septic arthritis is a major problem in equine medicine.
Direct culture is often unrewarding. The use of
enrichment culture is important, and optimally synovial
fluid samples should be inoculated into blood
culture broth shortly after collection. Samples in
blood culture broth are incubated for up to one week,
thereby greatly increasing sensitivity. Sterile technique
is critical because this enrichment process can
detect very low numbers of bacteria, including contaminants.
Empirical therapy is important because
of the low sensitivity of direct culture, the time delay
associated with enrichment culture and possible
consequences of disease. Gram staining of synovial
fluid should be performed to provide basic information
about the likely pathogen. Prompt treatment
is required, including antimicrobials and ancillary
procedures such as joint lavage, to reduce the risk of
performance- or life-threatening damage within the
joint. A distinct advantage in the treatment of septic
arthritis is the ease of local (intra-articular) therapy
in most situations. Intra-articular injection of antimicrobials
is a very common practice when treating
septic arthritis because of the ability to provide very
high drug levels at the infected site. This is an easy
procedure for most joints, and is often combined
with joint lavage. Concerns regarding the potential
for development of chemical arthritis following
injection of antimicrobials have been addressed;
however, this has not been demonstrated to be a
clinically relevant concern, particularly considering
the severe potential sequelae associated with septic
arthritis. Some antimicrobials are irritating and may
produce chemical synovitis, so only drugs known to
be safe and effective (e.g. amikacin, gentamicin, ceftiofur,
cefazolin, sodium/potassium penicillin) should
be injected into joints. Since antimicrobials injected