Pleural Diseases in Horses

Pleural Diseases in Horses

• Pleural diseases affect the pleura, a membrane lining the thoracic cavity.
• The pleura consists of parietal pleura (lining the thoracic cavity) & visceral pleura (covering lungs and other thoracic structures).
• The pleural cavity is the space between these layers, typically containing a small amount of lubricating fluid.
• Pleural diseases manifest as expansion of the pleural cavity, restricting lung capacity.
• Common causes of pleural space expansion include pleuropneumonia, wounds, neoplasia, hemothorax, and pneumothorax.
• Risk factors for pleuropneumonia in horses include long distance transportation, head-tied up position (reducing mucociliary clearance), and exposure to infectious agents, such as bacteria.
• Diagnostic methods for pleural diseases include ultrasonography, radiography, and pleurocentesis.
• Treatment for pleural diseases varies based on cause.

Pleural Effusion Types

• Pleural effusion is an abnormal buildup of fluid in the pleural space.
• Fluid can be transudate (increased volume of normal fluid, low cells, low protein), modified transudate, or exudate (pus).
• These can be further categorized into hydrothorax (transudative), pyothorax (exudative), hemothorax (blood effusion), and chylothorax (chyle effusion).
• Pneumothorax is another type, characterized by air filling the pleural space.

Pleural Disease with Transudate

• Transudate is an uncommon cause of pleural effusion in horses.
• Potential causes include increased venous pressure (e.g., congestive heart failure) or decreased plasma colloidal pressure (e.g., hypoproteinemia).

Pleural Disease with Chyle

• Chyle effusions are extremely rare in horses.
• They generally result from blunt trauma or neoplasia.

Causes of Pleural Effusions in Horses

• Neoplasia (tumors) and infection are the most frequent causes.
• Specific data gathered from a study in the UK showed 38% of horses with pleural effusions were related to neoplasia; and 62% involved infection.
• Characteristics, such as mean age, fluid volume, temperature, and levels of plasma fibrinogen, serum amyloid A, pleural cell counts, and pleural protein levels, were recorded for the study.

Pleural Disease with Modified Transudate

• Modified transudate is the second most common cause of pleural effusions in horses.
• This condition involves a mild increase in cells (mostly neutrophils less than 90%) and proteins (20-30g/L) typically in horses with thoracic neoplasms.
• Potential causes also include blockage of lymphatic drainage, and inflammation caused by the neoplasm.
• Lymphoma, melanoma (specifically affecting grey horses), and mesothelioma (from pleural cells) are examples of thoracic neoplasias.

Pleural Disease with Exudate

• Exudate in pleural fluid is a sign of inflammation accompanied by a moderate to marked increase in neutrophils and protein.
• Most common causes of exudate include bacterial infections, such as pneumonia, penetrating thoracic trauma, and metastatic abscesses.

Pleuropneumonia: Description and Progression

• Pleuropneumonia originates as a bacterial infection in the airways (septic bronchitis).
• Infection and inflammation spread from the airways to the pulmonary parenchyma, resulting in bronchopneumonia.
• Eventually, the infection spreads to the visceral pleura, resulting in pleuropneumonia.
• An exudative stage follows, where fluid accumulation occurs in the pleural cavity.

Pleuropneumonia: Clinical Signs

• Clinical presentation can vary because pleuropneumonia is not always obvious.
• Common symptoms include fever, dullness, shallow rapid breathing, chest pain, and a possible soft, or coughing sound.

Pleuropneumonia: Diagnosis

• Diagnosis involves a clinical examination, blood sampling (looking for inflammation markers like increased neutrophils, fibrinogen, serum amyloid A, and serum iron), as well as diagnostic imaging techniques.
• Endoscopy, examining airway for pus and blood, ultrasonography (where the lung might appear consolidated and like liver tissue, with roughening of the pleura and fluid buildup), and pleurocentesis (assessing fluid for cell count, neutrophil percentage, protein, lactate, and glucose).

Pleuropneumonia: Endoscopy

• Endoscopy may reveal a white-creamy-yellow exudate with a high percentage of neutrophils (suggesting a bacterial process), or sometimes orange or bloody exudate.

Pleuropneumonia: Ultrasonography

• Normal lung appears as a gliding white line on ultrasound.
• Pleuropneumonia is indicated by points of ultrasound penetration seen as "comet tails," hinting at pleural roughening and inflammation.
• More significant lung consolidation, indicating pneumonia, might also be visible.
• With exudative pleuropneumonia, pleural effusion would also be evident.

Pleuropneumonia: Pleural Drainage

• Pleural drainage involves collecting fluid samples to determine the cause (neutrophil % and protein).
• Teat cannula or needle with a three-way tap is used.
• For large effusion cases, more invasive techniques like the use of a thoracic trocar to drain the effusion.

Pleuropneumonia: Medical Therapy

• Antimicrobial therapy is typically based on culture results from tracheal and pleural samples to avoid incorrect treatment of the infection.
• Broad-spectrum antibiotics that cover anaerobes, along with NSAIDs for pain control (e.g., flunixin meglumine), are also used.
• Fibrinolytic therapy may be considered in certain cases.

Other Causes of Pleural Exudate

• Penetrating trauma (often from stake wounds).
• Aspiration of food materials (oesophageal obstruction, "choke").
• Spontaneous rupture of oesophagus (particularly in Friesian horses).
• Iatrogenic causes.

Pleural Disease with Hemothorax

• Hemothorax is commonly caused by blunt trauma.
• The blood in the pleural cavity often shows up as swirling, “smoke” on imaging.
• Hemothorax typically resolves as the blood is reabsorbed into the circulation.
• Drainage of hemothorax is only warranted when it compromises breathing.

Pneumothorax: Description and Diagnosis

• Pneumothorax is characterized by air filling the pleural cavity.
• A penetrating trauma from a stake wound, pleuropneumonia, and tumors are frequent causes of pneumothorax.
• Diagnosis is usually based on radiographic findings showing the dorsal lung borders not visible.
• Ultrasounds show air as a "still" reflection rather than a gliding white line, particularly in the dorsal part of the thorax.

Pneumothorax: Treatment

• In mild cases, pneumothorax may resolve naturally.
• More severe cases may require aspiration of the air from the affected side, particularly if breathing is impacted.