Page 1226 - Small Animal Internal Medicine, 6th Edition

1198 PART X Joint Disorders

increase in the number (40,000-280,000/µL) of nucleated If septic arthritis is suspected and the animal has no
cells in the synovial fluid, with neutrophils predominating history of direct inoculation of the joint with bacteria, diag-
VetBooks.ir (usually >90%). In very acute or severe cases, it is common nostic evaluation for a source of bacteremia should be per-
formed, although often the cause cannot be identified.
to see bacteria within the cells, and the neutrophils may be
toxic, ruptured, or degranulated. Organisms that do not
cardiac and abdominal ultrasonography, are especially
cause rapid destruction of articular cartilage (i.e., strepto- Radiographs of the thorax, abdomen, and spine, as well as
cocci, Mycoplasma) may not cause remarkable toxic or helpful in identifying a focal site of infection. Culture of
degenerative changes in synovial fluid neutrophils. In chronic material from any suspected site of infection should be per-
infections or in animals that have received prior treatment formed if possible.
with antibiotics, bacteria may no longer be evident and neu-
trophils may appear healthy. Treatment
Synovial fluid should be cultured for aerobic and anaero- The goals of therapy are to rapidly resolve the bacterial infec-
bic bacteria. Direct bacterial culture of synovial fluid is posi- tion and remove intra-articular accumulations of enzymes
tive in less than half of all animals with septic arthritis; and fibrin debris. Identifiable systemic sources of infection
improved diagnostic yield may be obtained by inoculating should also be eliminated. In an animal suspected of having
synovial fluid into blood culture medium (9:1 ratio) and septic arthritis, antibiotics should be administered as soon
incubating it for 24 hours at 37° C before inoculation. Bac- as possible after all samples are collected. Until culture results
teria can also be recovered from cultures of synovial mem- are available, a broad-spectrum, β-lactamase-resistant anti-
brane biopsy, blood, or urine specimens. biotic such as a first-generation cephalosporin (e.g., cepha-
Radiographic changes of the involved joints in septic lexin, 20-40 mg/kg PO q8h) or Clavamox (Zoetis; 12-25 mg/
arthritis may initially be minimal and nonspecific, and kg PO q8h) is indicated. Initially the antibiotic can be
limited to thickening of the joint capsule, widening of the administered parenterally, followed by long-term oral
joint space, and irregular thickening of periarticular soft administration. Quinolones should be used if gram-negative
tissues (Fig. 69.2). In chronic infections, cartilage degenera- organisms are suspected; retinal toxicity is a potential
tion, periarticular new bone formation, a marked periosteal concern in cats receiving quinolones, but pradofloxacin has
reaction, and subchondral bone lysis may be observed an improved safety profile. Metronidazole should be added
(Fig. 69.3). if anaerobic infection is suspected. Animals with acute septic

A B

FIG 69.2
Lateral (A) and dorsopalmar (B) radiographs of the swollen left carpus of a 2-year-old
Bullmastiff with a 1-week history of lameness caused by septic arthritis. Surgical
exploration revealed two porcupine quills within the infected joint.