Diagnostic Techniques for Respiratory Disease in Animals

A complete patient history and physical examination should help determine the possible cause and location of respiratory disease in animals.

Two-view cervical and thoracic radiographic images can be useful when obstructive upper airway disease or fixed airway obstruction is suspected (eg, tracheal foreign body, masses, foreign bodies, or stenosis). Thoracic radiographic imaging is recommended for any patient exhibiting clinical signs of lower respiratory disease (eg, cough, rapid and shallow breathing, dyspnea); however, its diagnostic value can be limited in patients with a large thorax (eg, adult horses or cattle).

Arterial blood gas analysis or pulse oximetry can help assess the level of hypoxia and the need for supplemental oxygen therapy in cases of severe respiratory distress.

Thoracic point-of-care ultrasound (TPOCUS) is helpful in patients with diseases of the pulmonary parenchyma and pleural space. TPOCUS is noninvasive, can be performed cageside, and can provide important information to drive additional diagnostics and treatment, especially in unstable patients.

The diagnostic procedure of choice for suspected obstructive upper airway disease is either examination of the mouth and oropharynx or endoscopic examination of the respiratory tract, preferably without sedation. Laryngeal function should be assessed, and obstructive lesions within the nasopharynx, oropharynx, larynx, trachea, or principal bronchi should be identified. Sedation is often required; the chosen sedative should not interfere with arytenoid function if laryngeal paralysis is suspected.

Diagnostic procedures for suspected diffuse or lobar lung disease include endotracheal or transtracheal wash, bronchoscopy with bronchoalveolar lavage or endobronchial biopsy, and transthoracic fine-needle aspiration of the lung or lung biopsy. For suspected bacterial pneumonia, bacterial culture and susceptibility testing of transtracheal wash samples is recommended. Cytological evaluation of transtracheal or bronchoalveolar lavage fluid can aid in diagnosing fungal, parasitic, or allergic lung diseases. Transthoracic fine-needle aspirates of the lung are often useful for diagnosing fungal pneumonia; however, they have lower yields in the definitive diagnosis of solitary pulmonary lesions, which often requires transthoracic lung biopsy or surgical excision.

Transthoracic ultrasonography is a sensitive diagnostic tool for pleural disease (eg, pleural effusion, pneumothorax) and for parenchymal lung disease when lesions are adjacent to the pleural surface.

In dogs or cats with pleural effusion, thoracocentesis should be performed to obtain samples for cytological and potentially microbiological evaluation of fluid. In cats, pleural effusion often occurs with cardiac disease; therefore, echocardiography is indicated. When chylous effusion is suspected, serum and chyle triglyceride concentrations should be determined. Chylous effusions are associated with triglyceride concentrations greater than those in serum.

Acute nasal discharge and/or sneezing are suggestive of infection (viral or bacterial) or nasal foreign body. Chronic nasal discharge warrants further investigation via radiography (nose, guttural pouches in horses), nasal CT, rhinoscopy, nasopharyngoscopy, or nasal biopsy. Rhinoscopy might be of limited value if copious thick discharge or hemorrhage is present.

Bacterial cultures of nasal tissue can be helpful if bacterial rhinitis is suspected; however, in some species (eg, dogs and cats) primary bacterial rhinitis is rare and typically occurs secondary to other nasal conditions.

Cytological evaluation of nasal tissue can help diagnose nasal fungal infections.

Serological testing for fungal respiratory infections may be considered; however, these findings should correlate with the patient's clinical signs and documented presence of fungal organisms, because false-positive and false-negative test results can occur.