Discussion

Cytology of effusions can contribute to rapid diagnosis and to the ranking of differential diagnoses when used in conjunction with ultrasonography and CT imaging. Abdominal effusions are frequently associated with colic in horses. Normal peritoneal fluid is an ultrafiltrate of plasma and is of low volume. Increased volume (ascites) is related to reduced oncotic pressure, ruptured vessels and vasculitis. Normal peritoneal fluid is a low volume modified transudate containing approximately 50% neutrophils and 50% monocyte/macrophages, a few mesothelial cells, and is clear to pale yellow.

With acute inflammation an increased volume exudate will occur. Early acute, containing a high percentage of neutrophils which may be degenerate and contain bacteria in the case of sepsis. This can usually be differentiated from enterocentesis by the presence of intestinal bacteria (mixed population), protozoa, plant material. Dark green fluid is suggestive of bile contamination. High volume clear fluid indicates a transudate and likely liver disease, or in young foals a possible ruptured bladder. Repeated abdominocentesis can be helpful in monitoring the course of disease.

Tumours are uncommon in horses, but lymphoma, squamous cell carcinoma of the stomach, malignant melanoma may be diagnosed occasionally as well as rare tumours, e.g. haemangiosarcoma, adenocarcinoma and mesothelioma.
Pleural effusions can be associated with respiratory disease and heart disease and related conditions. Most horses have a fenestrated mediastinum so fluid on both sides should be the same. Normal pleural fluid is a dialysate of plasma of low cell and

protein content (transudate). Volume increases when the rate of formation is greater than that of removal mainly by the lymphatic system. Exudates are most common and usually relate to septic inflammation emanating from the respiratory system. They can be suppurative (high % degenerate neutrophils) or granulomatous (increased % activated macrophages multinucleated giant cells and chronic). High lymphocyte % may be seen in chronic inflammation, resolving sepsis and rarely lymphoma.

Synovial fluid is a dialysate of plasma modified by hyaluronic acid and gylcosaminoglycans. It lubricates the articular cartilage and provides nutrients. Normal fluid is viscous clear light yellow and low volume. Inflammation causes low viscosity, increased volume and cellularity. Sepsis is difficult to determine in this fluid as microorganisms are believed to be retained in the synovium and may not appear in the fluid, culture may be negative and neutrophils in good morphological condition. Immune mediated conditions with increased neutrophils and protein are much less common. Degenerative joint disease fluid frequently contains increased activated macrophages, synovial cells and occasionally cartilage fragments.

Cytological evaluation of CSF may be of value in conjunction with imaging in neurological disease. Normal CSF is of low cellularity and protein content. High protein and lymphocytes can indicate viral disease, or rarely lymphoma. With inflammation, increased neutrophils, eosinophils and protein can be seen, and bacteria, fungi or protozoa may be identified.