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Retrospective analysis of pleural effusion in cats.

König A, Hartmann K, Mueller RS, Wess G, Schulz BS. Retrospective analysis of pleural effusion in cats. J Feline Med Surg. 2018 Dec 17;:1098612X18816489. 

Pleural effusion is the abnormal presence of fluid between the surface of the lungs and the chest wall. Fluid may accumulate for a variety of reasons, often related to imbalances in Starling forces. Fluids may be classified as pure or modified transudates, or exudates. Pleural effusion is a common finding in cats with respiratory distress. Determining the cause of pleural effusion is essential in formulating a treatment plan and prognosis. The purpose of this study was to determine the relationship between etiology of effusion and clinical and laboratory findings, as well as determining the prevalence of various causes of effusion.

The study was designed as a retrospective observational analysis involving records of 306 cats where a definitive cause of pleural effusion was determined.  Cats were divided into 6 major groups: cardiac disease; neoplasia; pyothorax; FIP; chylothorax; and miscellaneous. Disease etiology was confirmed by testing considered gold standard for that condition (ie, echocardiography for cardiac disease; immunofluorescence for coronavirus; cytologic or histopathologic exam for neoplasia, etc). Signalment and presenting complaints, PE findings, CBC, and serum studies were recorded for most patients. At a minimum, specific gravity and total cell count were measured on effusion fluids.

The ten-day outcome was recorded for all cats. The underlying etiology of effusion for the population of cats is recorded below:


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For cats with cardiac disease, HCM was most common, followed in decreasing order by RCM, UCM, DCM, and others.

Several trends were noted in causes of pleural effusion. Cats with FIP tended to be younger than cats with either cardiac disease or neoplasia. Cats with cardiac disease had a lower rectal temperature, higher ALT and ALP, and lower effusion TP and cell count than other causes of effusion. Glucose concentration of effusions was lower for cats with pyothorax than other groups.

The authors conclude that neoplasia and cardiac disease are the most common causes of pleural effusion in cats, followed by pyothorax and FIP. 8.5% of cats had multiple concurrent conditions leading to effusion. Patient factors determining of the cause of effusion included age, core temperature, and liver enzyme values. Fluid factors included total protein, nucleated cell count, and glucose. 55.8% of cats were euthanized within 10 days of diagnosis.

Limitation to this study included the retrospective nature and lack of consistency between patients. The use of speciality hospital records may also have biased the patient population. Further stratification into types of effusion and potentially including other causes of effusion may have allowed further detail to be elucidated. Despite these limitations, this paper provides an informative breakdown of the prevalence of difference causes of effusion and techniques to differentiate them within private practice. (MRK)

See Also

Dempsey SM and Ewing PJ. A review of the pathophysiology, classification, and analysis of canine and feline cavitary effusions. J Am Anim Hosp Assoc 2011; 47: 1–11.


Davies C and Forrester SD. Pleural effusion in cats: 82 cases (1987 to 1995). J Small Anim Pract 1996; 37: 217–224.

Hirschberger J, DeNicola DB, Hermanns W, et al. Sensitivity and specificity of cytologic evaluation in the diagnosis of neoplasia in body fluids from dogs and cats. Vet Clin Pathol 1999; 28: 142–146