Lynelle R. Johnson,     Davis, California

Clinical Findings

Infectious bronchitis can be found in young and mature dogs, whereas the remaining bronchial disorders are diseases of middle-aged to older dogs. Both large- and small-breed dogs are affected, and a harsh cough is generally reported, although it can be moist or productive when copious respiratory secretions are produced. Gagging, retching, regurgitation, or voice change can suggest laryngeal dysfunction or gastroesophageal reflux disease and subsequent aspiration injury as a cause of cough. Paroxysms of coughing or a “goose-honk” cough are more often encountered in dogs with airway collapse. Dogs are systemically healthy, although in the later stages of disease, exercise intolerance or labored respirations can be reported and activity becomes limited by exercise-induced coughing. Severely affected animals can demonstrate cyanosis or collapse.

On physical examination, dogs with airway disease are often overweight. Some dogs are tachypneic or pant incessantly, whereas dogs severely affected by bronchitis or airway collapse may have prolonged expiration or an expiratory push. Thoracic auscultation can reveal coarse, diffuse crackles or expiratory wheezes, but findings may also be normal. Tracheal sensitivity is usually present because of nonspecific airway inflammation. Airway collapse should be suspected when a snapping sound is auscultated over the thoracic cage as intrathoracic airways collapse on expiration. In small breeds a murmur of mitral regurgitation is a common coincident finding in dogs with bronchitis or bronchomalacia. When present, hepatomegaly is likely caused by obesity.

Diagnosis

The diagnosis of chronic bronchitis is one of exclusion because airway infection (with Mycoplasma, Bordetella, parasites), bronchomalacia, or an airway foreign body can cause similar clinical signs. Therefore the diagnosis optimally is based on the history, clinical findings, thoracic radiographs, fluoroscopy, endoscopy, and airway sampling for bacterial culture and cytologic examination.

Clinicopathologic abnormalities typically are absent, even in dogs with infectious bronchitis. Thoracic radiography is an important part of the diagnostic workup, and a generalized increase in interstitial or bronchial infiltrates can be found in dogs with airway inflammation. However, radiography is relatively insensitive for chronic bronchitis, and in a case-controlled evaluation only increased thickness of airway walls and increased numbers of visible airway walls differentiated dogs with bronchitis from normal dogs (Mantis et al, 1998). Dogs with long-standing disease can develop irreversible bronchiectasis, which, when severe, will be visible radiographically (Figure 160-1). Normal-appearing chest radiographs are found relatively often in dogs with chronic bronchitis. Radiographs are also highly insensitive for documentation of airway collapse (Johnson and Pollard, 2010; Macready et al, 2007). Fluoroscopy might be considered the gold standard for identification of tracheal and bronchial collapse, although bronchoscopy is often required to document more subtle degrees of bronchomalacia or small airway collapse.

Collecting airway samples by tracheal wash or bronchoscopy is recommended to characterize the cellular infiltrate in the airway and to rule out infectious causes of cough. Bronchoscopy is particularly useful when typical radiographic findings of bronchitis are lacking or when bronchomalacia is suspected. Dogs with chronic bronchitis have airway hyperemia, the airway mucosa has a cobblestone or irregular appearance, and many have increased mucus lining the airway. In animals with long-standing bronchitis fibrous nodules can be seen protruding into the bronchial lumen. Bronchomalacia is characterized by a static circumferential narrowing of the bronchial lumen (Figure 160-2) or by dynamic changes in airway caliber during respiration.

Cytologically, chronic bronchitis is characterized by the presence of nondegenerate neutrophils. Some dogs have a predominance of eosinophils in airway washings or mixed inflammation, and this might indicate parasitic bronchitis or a form of eosinophilic lung disease (see Chapter 163). Increased mucus is often present, and Curschmann’s spirals (bronchial casts of airway mucus) are sometimes noted. Epithelial cells and squamous metaplasia can also be seen. Dogs with bronchomalacia can have normal findings on cytologic examination of bronchoalveolar lavage fluid, can have hypercellular lavage fluid, or can have cytologic evidence of concurrent bronchitis. Whether inflammation is a cause or a consequence of cough has not been determined.

Infectious bronchitis is characterized by the presence of septic suppurative inflammation. Organisms are generally recognizable on cytologic examination; however, Mycoplasma spp. are uncommonly seen cytologically and may be isolated only through culture on specialized media or by polymerase chain reaction testing. Caution is warranted when interpreting results of airway specimen culture because the trachea and carina of dogs are not sterile and various species of commensal bacteria and oral flora can be found in tracheal wash or bronchoalveolar lavage samples despite careful attention to technique. True bacterial infections of the lower respiratory tract are characterized by quantitative cultures of bronchoalveolar lavage fluid yielding more than 1.7 × 10³ colony-forming units/ml, lack of squamous cells or the oropharyngeal Simonsiella spp. on cytologic examination, and a variably increased percentage of neutrophils (Peeters et al, 2000).

Pulmonary function tests or advanced imaging can be performed to assess the severity of gas exchange or gas flow abnormalities. Arterial blood gas analysis generally shows only nonspecific, mild to moderate hypoxemia. Computed tomography is useful for detecting early signs of bronchiectasis.