Discussion

The differential diagnoses for pleural effusion in horses include pleuropneumonia. neoplastic effusion, congestive heart failure, thoracic haemorrhage, chylothorax and pulmonary hydatidosis. In the United States, approximately 70% of horses with pleural effusion have pleuropneumonia. In the United Kingdom, where lower respiratory tract infection is less common, approximately 30% of horses with pleural effusion have pleuropneumonia, and the relative incidence of neoplastic effusion is higher.

Pleuropneumonia:
Pleuropneumonia usually develops secondary to bacterial pneumonia, with extension of the infection into the pleural space. Penetrating thoracic wounds produce septic pleural effusion without pneumonia; however, extension of the infection to the pulmonary parenchyma can also occur, particularly if pulmonary contusions resulted from the traumatic event. Spontaneous pleuritis (without accompanying pneumonia) is uncommon. Viral respiratory infection, long-distance transportation, general anaesthesia, and strenuous exercise are common predisposing factors that impair pulmonary defence mechanisms allowing secondary bacterial invasion.

Clinical signs:
- Fever, depression, lethargy and inappetence
- Pleural pain, endotoxaemia and rapid, shallow respiration
- Moist, soft, productive cough
- Auscultation and percussion can identify regions of dullness consistent with pleural effusion, consolidation or abscessation
- Sternal oedema
- Nasal discharge is variably present, and ranges from mucopurulent to serosanguinous in character.

Diagnostic testing:
- Thoracic ultrasound
- Thoracocentesis
- Bacterial culture
- Thoracic radiography

Treatment:
- Broad-spectrum antimicrobial therapy
- Thoracic drainage - daily monitoring of the thoracic cavity via ultrasound is necessary to evaluate effective drainage, identify isolated fluid pockets and assess peripheral pulmonary disease
- Supportive care - intravenous fluid therapy, nutritional support and NSAID therapy
- Thoracostomy may be required in refractory cases

Complications:
The most common complications directly involving the thoracic cavity include pneumothorax, pleural adhesions, pulmonary abscess, pulmonary infarction, bronchopleural fistula and cranial mediastinal abscess. The most common complications involving other extrathoracic systems include laminitis, antibiotic-induced colitis, and jugular vein thrombosis at the catheter entry site.

Prognosis:
With owner commitment and clinician experience, the survival rate is reported to be as high as 90% by some investigators with a 60% chance to return to athletic performance.

Thoracic neoplasia:
Neoplasia of the thoracic cavity is rare, and is most commonly reported in older horses. Thoracic neoplasms are divided into mediastinal and intrapulmonary tumours. Intrapulmonary tumours are usually metastatic in origin, whereas primary pulmonary neoplasms are very rare. Mediastinal neoplasia may be metastatic or primary.
Lymphoma is the most common mediastinal tumour and accounts for approximately 50% of thoracic neoplasms. Multisystemic involvement is common in horses with mediastinal lymphoma, however, the most obvious clinical signs are associated with the mediastinal mass. A wide variety of other thoracic neoplasms have been recorded, however, squamous cell carcinoma, melanoma, fibrosarcoma and haemangiosarcoma appear to be the most common metastatic mediastinal/pleural cavity tumours. Mediastinal neoplasms often produce a large volume of malignant pleural effusion. Therefore, presenting clinical signs are likely to reflect large volume effusion such as rapid, shallow respiration, tachycardia, weight loss, pectoral oedema and distended jugular veins. Thoracic auscultation reveals muffled heart and lung sounds in the ventral lung fields, and ultrasonographic examination reveals a large volume of hypoechoic fluid with minimal cellularity and few fibrin tags. Pleural fluid recovered from horses with mediastinal neoplasia will appear straw-coloured and transluscent to slightly serosanguinous, with the exception of haemangiosarcoma, in which the pleural fluid is often haemorrhagic. Neoplastic effusions typically have low to moderate cellularity characterised by reactive mesothelial cells, lymphocytes and macrophages, and a high total protein concentration. Some mediastinal tumors may be exfoliative and neoplastic cells are readily apparent on cytological evaluation (melanoma, squamous cell carcinoma, haemangiosarcoma), however, the absence of neoplastic cells in malignant effusions is not uncommon.