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Transudates are a type of effusion that has a low cellularity and a low protein content. Transudates occur due to imbalances in the Starling forces, of which the two principal examples are hydrostatic pressure and plasma oncotic pressure (or colloid osmotic pressure). The following causes of transudate effusions are therefore recognised:

  • Reduced plasma oncotic pressure may occur if plasma proteins are lost. Albumin contributes 75% of the total oncotic pressure so hypoalbuminaemia is the major cause of transudate effusions.
  • Increased hydrostatic pressure may occur with inappropriate activation of the renin-angiotensin-aldosterone system (RAAS), as in congestive heart failure and portal hypertension. Both diseases commonly result in high-volume ascites, although this is more often composed of a modified transudate.


Clinical Signs

Effusions may occur in any of the major body cavities, causing ascites, hydrothorax or pericardial effusion. Ascites is often of a high volume. An abdominal fluid thrill will often be palpable and the abdomen may appear to be grossly swollen.

Hydrothorax will cause coughing, tachypnoea and dyspnoea if severe. Dullness will be evident on thoracic percussion if a pleural effusion has developed and the heart sounds will be muffled on auscultation.

Pericardial effusion may be sufficiently severe to cause cardiac tamponade and right-sided heart failure. The heart sounds will be muffled on auscultation and there may be hepatojugular reflux, a jugular pulse or signs of left-sided forward failure. In horses and cattle, it is much more likely that ventral oedema will be observed in animals with liver failure (causing hypoalbuminaemia) or congestive heart failure.

Diagnostic Imaging

Effusions are easily diagnosed by ultrasonography and this modality may also be used to guide fine needle aspiration to obtain a sample of the fluid. Effusions also produce a distinctive pattern on plain radiographs:

  • With pericardial effusion, the heart may appear to be generally enlarged with a globular shape. There may be a crisp cardiac silhouette (as the heart is moving within a stationary bag of fluid) and a hypovascular lung pattern due to pulmonary underperfusion.
  • With ascites, there is a loss of serosal detail due to the presence of fluid in the abdominal cavity. This appearance may also occur with large abdominal masses and in emaciated animals.
  • With pleural effusions, the lung lobes are contracted and lobulation is evident. Areas of peripheral radio-opacity should be evident, especially peripherally in the chest.


Definitive diagnosis of the type of effusion relies on collection of a sample and subsequent cytological analysis. A refractometer is frequently used to measure the specific gravity of the fluid. The following findings would be expected for a transudate:

Appearance Clear, colourless fluid
Specific gravity <1.018
Total protein < 25g/l
Nucleated cells <0.5 x 10e9/L, of which the most common are mesothelial cells, macrophages and low numbers of non-degenerate neutrophils.


In general, it is not advisable to drain effusions unless they are causing clinical signs as the procedures are invasive and drainage of large volumes of fluid will further deplete body protein reserves. Neverthless, pleural and pericardial effusions should be drained to stabilise an animal with dyspnoea or signs of heart failure.

The management of hypoalbuminaemia is discussed here.

Inappropriate activation of the RAAS is best treated with the diuretic spironolactone (a potassium sparing diuretic) because this prevents aldosterone from acting on Na/K pumps in the distal convoluted tubules and collecting ducts of the renal nephrons. Frusemide (a loop diuretic) may be used later if aldosterone does not produce satisfactory results. The underlying cause of the increased hydrostatic pressure should also be addressed.

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