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An exudate is a type of effusion that has a high cell count and protein content. It is often associated with inflammation in the body cavities because changes to the endothelium during this process allow protein-rich fluid to escape from the vasculature and because leucocytes migrate out to the site of disease. As with any type of inflammation, the exudate may occur in response to a bacterial infection (septic) or it may be sterile (non-septic). The presence of infection and purulent material in the chest cavity is termed pyothorax. Commons causes of exudates are: Septic exudates

  • Pyothorax may be caused by:
    • Penetrating wounds to the thorax, including bites, bullet wounds and stick injuries.
    • Rupture of the Oesophagus.
    • Migrating foreign bodies, particularly grass awns in hunting dogs.
    • Severe forms of bacterial pneumonia that break into the pleural cavity.
  • Septic peritonitis may be caused by:
    • Penetrating wounds to the abdomen.
    • Rupture of the intestine, spilling luminal contents into the peritoneal cavity. This may be caused by trauma, strangulation, obstruction, ulceration or deeply infiltrative neoplasia.
    • Rupture of an infected uterus (pyometra) or pyosalpinx with discharge of pus from the fimbriae of the oviducts into the abdomen.
    • Rupture of an infected urinary tract.
  • Septic pericarditis, which is much more common in farm animals and leads to the production of a purulent pericardial fluid.

Non-septic exudates

  • In the abdomen, chemical peritonitis induced by the prolonged presence of urine or bile will lead to the conversion of a modified transudate into an exudate. Bile peritonitis is characterised by a distinctive green discolouration of the parietal peritoneum and abdominal organs.
  • Acute necrotic pancreatitis is associated with necrosis of the peripancreatic fat and discharge of enzymes and other factors into the abdomen.
  • Neoplasia may produce exudates if they have large necrotic portions.
  • Infection with Feline Infectious Peritonitis Virus may cause the development of either an exudate or a modified transudate.


Clinical Signs

Exudates may occur in any of the major body cavities. In the abdomen, there may be signs of abdominal pain (due to the underlying cause of the effusion), an abdominal fluid thrill or a palpable mass.
In the chest, a pleural effusion (including pyothorax) may cause 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 effusions 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. The exudate of a chronic septic pericarditis undergoes organisation and replacement with fibrous tissue which bridges the visceral and parietal pericardia. This results in a restrictive pericarditis with clinical signs similar to cardiac tamponade. Infections which penetrate deeply into the cardiac muscle may cause myocarditis with disruptions of the normal conduction pathways and resultant dysryhthmias.
Septic processes may be accompanied by more general signs of infection, including pyrexia, depression, lethargy and anorexia. Highly inflammatory exudates may cause severe pain when they damage the parietal pleura or peritoneum. Affected animals may be reluctant to walk and will stand with a typical 'tucked-up' posture.

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. Pneumoperitoneum may occur if the gut has ruptured or, in the case of pancreatitis, there may be an area of localised peritonitis (resembling 'ground glass') in the cranial abdomen.

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.

Pyothorax may be diagnosed and treated by thoracoscopy, the passage of an endoscope into the pleural space. This technique is especially useful in the detection of loculation, the formation of septa of fibrous tissue that divided the effusion into pockets of fluid.


Definitive diagnosis of any effusion relies on collection of a sample and cytological analysis. A refractometer can be used to measure the specific gravity of the fluid. The following features are typical of an exudate:

Appearance Turbid or flocculent red, yellow or white (purulent) fluid
Specific gravity > 1.018
Total protein > 30g/L
Nucleated cells >3 x 10e9/L, mainly comprising non-degenerate or degenerate neutrophils, macrophages, lymphocytes or eosinophils (with parasites)

With a septic exudate, the neutrophils are more likely to be present and intracellular (phagocytosed) bacteria may be visible.

In horses, enterocentesis (or paracentesis) is often performed as part of a colic work-up. The following findings are considered to be abnormal and may lend support to a decision to manage the case surgically:

Appearance *Yellow/green fluid containing plant fibres suggests that the gut has ruptured, giving the horse a hopeless prognosis as endotoxic shock will develop very rapidly.
*Blood-tinged fluid suggests that an area of the gut wall is compromised, probably due to ischaemia. Since it is a much more acute process, the appearance of the peritoneal fluid deteriorates more rapidly in horses with strangulations than in those with simple obstructions.
Total protein content >20 g/l
Total cell count >20 x 10e9, especially if degenerate neutrophils or bacteria are present.


Specific treatment is dependent on the cause of the effusion. For further details, see the following sections:

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