Clinical history and physical examination should help to determine the possible cause and site of respiratory disease. Lateral cervical and thoracic radiographs may be helpful when obstructive upper airway disease or fixed airway obstruction is suspected (eg, tracheal foreign body, masses, foreign bodies, or stenosis). Thoracic radiographs are essential in any animal exhibiting lower respiratory signs (eg, cough, rapid shallow breathing, dyspnea), but diagnostic value may be limited in animals that have a large thorax (eg, adult horses or cattle). Arterial blood gas analysis or pulse oximetry may help assess the need for oxygen therapy in an animal with severe dyspnea.
When obstructive upper airway disease is suspected, the diagnostic procedure of choice is endoscopy of the respiratory tract, preferably without sedation. Laryngeal function should be assessed, and the presence of obstructive lesions within the nasopharynx, oropharynx, larynx, trachea, or principal bronchi should be identified.
With diffuse or lobar lung disease, diagnostic procedures include transtracheal wash, bronchoscopy with bronchoalveolar lavage or endobronchial biopsy, and transthoracic fine-needle aspirates of lung or lung biopsy. When bacterial pneumonia is suspected, bacterial culture of transtracheal wash is recommended. Cytologic evaluation of transtracheal or bronchoalveolar lavage fluid may aid in the diagnosis of fungal, parasitic, or allergic lung diseases. Transthoracic fine-needle aspirates of lung often are useful in the diagnosis of fungal pneumonia but have lower yields in the definitive diagnosis of solitary pulmonary lesions. Solitary pulmonary masses often require transthoracic lung biopsy or surgical excision for definitive diagnosis. 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 effusions, thoracocentesis should be performed for cytologic and potentially microbiologic evaluation of fluid. In cats, pleural effusions often occur with cardiac disease, so an echocardiography should be performed. In animals suspected to have a chylous effusion, serum and fluid triglyceride levels should be determined. Chylous effusions are associated with fluid triglyceride levels greater than that in serum.
Acute nasal discharge, sneezing, or both, may suggest the presence of infection (viral or bacterial) or a nasal foreign body. Chronic nasal discharge warrants further investigation via radiography (nose, guttural pouches in horses), nasal CT, rhinoscopy, nasopharyngoscopy, or nasal biopsy. Rhinoscopy may be of limited value if copious thick discharge or hemorrhage is present. Bacterial cultures of nasal tissue may be of value 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. Cytologic evaluation of nasal tissue may help diagnose nasal fungal infections. Serologic testing for fungal respiratory infections may be considered, but these findings should correlate with the animal's clinical signs and documentation of the presence of fungal organisms because false-positive and false-negative tests can occur.