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9.4 Canine Arthropathies

9.4.1 Suppurative Arthropathies
An increased proportion of nondegenerate to degenerate neutrophils above what is suspected to be
due to blood contamination is indicative of suppurative inflammation. Specifically, neutrophil per-
centages are considered increased when above 10% of the total nucleated cells. However, in many
cases, they are even higher, often between 20 and 99% (MacWilliams and Friedrichs 2003).
Hemorrhage may accompany suppurative arthropathies, making the source contributing to neu-
trophil numbers difficult to discern. The two forms of suppurative inflammation include immune‐
mediated and infectious arthropathies. Differentiating these two forms can be difficult as cytologic
features can be similar (Table 9.1). The presence of degenerate neutrophils is suspicious for an
underlying infectious etiology. However, caution should be used when relying on the presence of
degenerate neutrophils in synovial fluid since neutrophils can deteriorate rapidly once in fluid or
tissue due to their short life span. As neutrophils decline, they can acquire cytologic features, such
as swollen nuclei and basophilic cytoplasm, that overlap with the appearance of true degenerate
neutrophils.

9.4.1.1 Immune-mediated
Immune‐mediated joint disease is a common cause for suppurative polyarthritis in canine patients.
Since immune‐mediated polyarthritis (IMPA) generally affects multiple joints, samples from sev-
eral joints, particularly the carpi and tarsi, should be collected and evaluated. Cytologic evaluation
typically reveals a predominance of nondegenerate neutrophils. Ragocytes and lupus erythema-
tous (LE) cells are indicative of IMPA (Figure 9.4) but are not commonly observed and should not
be relied on for diagnosis of an immune‐mediated process.

9.4.1.2 Infectious
Microscopic interpretation of infected joints reveals mildly to markedly increased TNCCs that are
primarily composed of nondegenerate and degenerate neutrophils. While degenerate neutrophils
are one feature of septic arthritis, some patients may not display many of these cells and septic
arthritis should not be ruled out in these instances (Marchevsky and Read 1999; Mielke et al. 2018).
Similarly, infectious agents are uncommonly identified on cytology (Figure 9.5C, D) and cultures
are also only positive in about 50% of the cases (Scharf et al. 2015; Mielke et al. 2018). Therefore,
infectious arthritis can mimic immune‐mediated polyarthritis on cytology and correlation with
clinical signs and careful classification of neutrophils as degenerate or nondegenerate is recom-
mended before initiating treatment.
Obtaining a culture in cases of suspected infection is crucial for establishing a diagnosis and
enabling selection of appropriate antibiotics. Therefore, every attempt should be made to facilitate
a successful culture result in these scenarios. The use of blood culture media is recommended to
increase sensitivity and the likelihood of obtaining a positive result (Montgomery et al. 1989;
Miller et al. 2018). Unfortunately, the smallest blood culture medium flasks available (pediatric
blood culture flasks, which are generally used in veterinary medicine) still require at least 0.5 ml of
fluid. If too little synovial fluid is available for culture via direct ejection into the blood culture
flask, yield can be maximized by sterilely flushing the culture media back and forth between the
collection needle and culture bottle to remove as much of the sample from the needle/hub as pos-
sible. Alternatively, the hub of a capped needle can be submitted to some diagnostic laboratories,
with instructions to flush the sample. However, submission in the blood culture bottle is preferred