Chronic cough in the dog (Proceedings)
Chronic bronchial disease (CBD) is a general term used to describe a complex, progressive respiratory syndrome characterized by excessive mucous secretion within airways and thickening (hyperplasia of smooth muscle and epithelium) in the bronchial tree and frequent coughing. Cough is often described in the veterinary literature as one that persists at least "2 consecutive months" (cough duration is derived from the human literature and often extrapolated to veterinary medicine…not certain this is completely appropriate). The definition of chronic bronchitis implies that the coughing episodes occur exclusive of other bronchopulmonary disease, e.g., respiratory mycoses, neoplasia, and bacterial infection. In veterinary medicine, however, it is impossible to disregard the impact that secondary infections have on the progression and severity of clinical signs associated with chronic bronchial disease, particularly those associated with acquired bronchial and tracheal collapse. Interestingly, the literature on chronic bronchial disease in humans attributes the underlying cause to 3 factors: age, inhaled particulate material (especially smoke from tobacco), and bacteria. Clients willing to treat a pet with chronic bronchial disease must accept the premise that treatment is aimed at control, not cure.
Chronic coughing is the hallmark clinical sign in dogs with bronchial disease. However, CBD can induce severe, acute-onset paroxysmal coughing episodes for which the patient is subsequently presented in respiratory distress. Collapse/syncope are occasionally reported by clients in acute episodes. In our experience (NCSU), acute respiratory distress associated with CBD is likely to be accompanied by acquired airway (not necessarily tracheal) collapse. Neither age nor gender seems to be predisposing factors to the development of CBD in dogs. While the disease is most common in dogs over 5 years of age, younger dogs can be (albeit rarely) affected. Among dogs, clinical signs associated with CBD appear to be most prevalent in small and toy breeds, particularly toy poodles, Pekingese, Yorkshire terriers, Chihuahuas, and Pomeranians. At least one author has suggested a hereditary predisposition to CBD in dogs. It is perhaps more appropriate to consider these breeds (uniquely?) at risk of developing severe clinical signs of bronchial disease, since CBD clearly occurs in mixed breed and large breed dogs as well as smaller breeds. Compromised airway integrity of toy dog breeds (chondrodysplasia), possibly an inherited problem, may further complicate the clinical course of CBD in the older dog. Obesity and advanced dental/periodontal disease are common, independent findings among small and toy dog breeds with CBD and are regarded as additional complicating (contributing??) factors in the clinical patient.Detection of abnormal respiratory sounds during thoracic auscultation is not a consistently reliable indicator of CBD. Wheezing on expiration, if present, is considered a hallmark of sign of chronic bronchial disease. The ability to elicit coughing by simple manipulation of the cervical trachea is an inconsistent finding in dogs with CBD and an uncommon finding in affected cats. Crackles, when present, can be attributed to the presence of fluid, usually viscous respiratory secretions, in constricted airways.
Dogs with chronic small airway disease are predisposed to bronchial and intrathoracic tracheal collapse. Therefore, during coughing episodes, it is oftentimes possible to auscult airway collapse. Toward the end of expiration, particularly during cough, airway collapse is evident during thoracic auscultation as a loud, discrete thump, referred to as an end-expiratory click or "snap." The sound is generated as the main bronchi and intrathoracic trachea collapse abruptly. Tracheal collapse can culminate in respiratory distress and syncope in dogs during paroxysmal coughing episodes. It is possible for affected dogs to die subsequent to airway obstruction and respiratory arrest during an acute episode.
Conventional hematology and biochemistry profiles are not likely to contribute to the diagnosis of CBD but are still indicated to rule out other underlying disease.
Trans-tracheal wash/aspiration (see below) is less likely than broncho-alveolar lavage (BAL) in revealing diagnostic information. Cytological evaluation of fluid recovered during BAL is expected to consist of predominantly of degenerate neutrophils with minimal or no bacteria. However, acute bacterial infection is a possible cause of acute exacerbation.
In the early, non-obstructive stages of CBD, a generalized interstitial lung pattern is usually present, although bronchial changes predominate. Thickening of bronchial walls, indicated by the "doughnut" appearance of end-on bronchi, and "tram lines," the longitudinal shadows associated with thickened bronchi, can be seen. Bronchial calcification alone, commonly seen as a normal age-related change in old dogs, should not be interpreted as bronchitis.
As CBD progresses, there is a tendency for the small airways, bronchi and, eventually, the intrathoracic trachea to collapse during exhalation, particularly during the expiratory phase of cough. The prevalence and severity of tracheal collapse appears to be most severe in adult, miniature, and toy dog breeds. Although chondrodysplasia and trachealis muscle dysfunction have been implicated in the pathogenesis of tracheal collapse, the functional diameter of the small airways in dogs with chronic bronchitis is also an important cause of bronchial and tracheal collapse, particularly in older dogs. Acquired airway collapse is a significant and complicating factor in dogs (especially small breeds) with CBD. Acquired changes in intra-thoracic airway aerodynamics lead to lower intra-thoracic airway pressure during exhalation (cough) and can lead to rapid, intermittent, but total, collapse of the airway, especially at the level of the carina (tracheal bifurcation). These can be heard during auscultation as the expiratory phase of cough (end-expiratory 'snap') is abruptly interrupted. (video of acquired airway collapse will be shown during the presentation)