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. Interestingly, the literature on chronic bronchial disease in humans attributes clinical signs to 3 underlying factors: age, inhaled particulate material, 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. 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.
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.
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.
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.)
Bronchoalveolar Lavage and Culture
In our hands, the diagnostic value of cytologic examination of tracheobronchial washings collected during tracheal aspiration or bronchoalveolar lavage is limited by the experience of the clinician and the size of the patient.
In the dog, direct visualization of the trachea and right and left main bronchi using a flexible endoscope is a valuable, although optional, diagnostic procedure. Among affected dogs, bronchi have an irregular contour, are mottled white and pink in color, and usually contain accumulations of thick mucous that cling to the bronchial walls and trachea. Oftentimes thin strands of tenacious mucous can be seen traversing the bronchial lumen.
Treatment Of Chronic Bronchial Disease
The Acute Exacerbation with Distress
It is possible for dogs with CBD to present with respiratory distress, cyanosis, and syncope following a severe, acute-onset coughing episode. Affected dogs characteristically have complete bronchial and intra-thoracic tracheal collapse. Oxygen administered by face mask should be administered immediately, and an intravenous catheter is placed in any available vein. Sedation with morphine (dogs only - 0.5 mg/kg, SQ or IM) or diazepam (dogs @ 5 to 20 mg IV or cats @ 5 mg maximum, IV) is indicated in the conscious, anxious patient. The patient is given a single dose of methylprednisolone (1–2 mg/kg, IV). It may be safer to actually anesthetize particularly anxious patients with an ultrashort-acting barbiturate, intubate, then administer oxygen through an endotracheal tube. When the patient has been stabilized, thoracic radiographs should be obtained as soon as possible to determine the integrity of the lungs and airways.
Dogs with CBD derive significant benefit from the short-term administration of anti-inflammatory doses of corticosteroids. Orally administered corticosteroids not only have a rapid, anti-inflammatory effect, they are a potent antitussive. Rapid resolution of cough is expected following onset of corticosteroids. This is true in patients with acute and chronic disease. Even when evidence of tracheobronchial collapse and/or pneumonia exists, short-term corticosteroids (up to 5–7 days) have an important role in managing the affected patient. Oral prednisolone is given at doses ranging from 0.2 to 0.5 mg/kg, twice daily in both dogs and cats. Once stable, most dogs can be effectively managed with a single dose given on alternate days.
The goal of corticosteroid therapy is not long-term, daily treatment. The clinician should strive to administer the smallest effective dose possible for the shortest period of time needed to control the clinical signs. Exacerbation of cough is expected in the future. Therefore, it is preferred that steroids be administered selectively in these patients. I prefer to use steroid therapy in these patients as short-term rescue treatment.
The role of long-term antimicrobial therapy is underappreciated in managing patients with CBD. Although bacterial infection (pneumonia) is seldom recognized as a co-factor in dogs with CBD, many pulmonologists do consider "low-grade" bacterial colonization within the small airways to be a key factor in CBD. Although relatively uncommon, opportunistic infections (pneumonia) involving normal respiratory flora can become life-threatening in dogs with significantly compromised respiratory defense mechanisms, particularly tracheobronchial collapse and diminished mucociliary transport. The role of B. bronchiseptica as a complicating factor in the pathogenesis of CBD must not be underestimated. When in vitro culture and sensitivity results are not immediately available, the clinician is justified in prescribing antimicrobials. In the author's experience, antimicrobial therapy plays a critical role in the long-term management of CBD in dogs.
Several antimicrobial agents are available for use. Those most commonly prescribed are listed below:
Doxycycline (3 to 5 mg/kg, orally, q 12 h)
Azithromycin (5 mg/kg, orally, once daily, recommended for compliance)
Enrofloxacin (2.5 to 5.0 mg/kg, orally, once daily) caution when using with methylxanthine (aminophylline or theophylline) bronchodilators. See below.
Administration: For example, azithromycin would be prescribed for 14 to 21 days (5.0 mg/kg, once daily) for a patient with CBD. Following treatment, it is not uncommon for the patient's clinical signs to resolve for several weeks, or even months, followed by a gradual redevelopment of cough. In this case, the treatment regimen with azithromycin can be repeated with similar results expected. If an individual patient does become less responsive to therapy, another antimicrobial can be selected and administered in the same way.
The methylxanthine bronchodilators, theophylline and aminophylline (theophylline ethylenediamine), are often described as the preferred treatment for long-term management in dogs (extended-release theophylline, initially 5.0 mg/kg, orally, q 12 h; with gradual increase up to 10 mg/kg). Note: Use of theophylline with enrofloxacin can culminate in toxic accumulation of theophylline. It is therefore recommended to reduce the theophylline dose by 30% if used concurrently with a fluoroquinolone. Alternatively, beta-adrenergic bronchodilators (terbutaline and albuterol) can be used (small dogs: 0.625 to 1.25 mg [total dose] orally, q 12 h) (larger dogs: up to 2.5 to 5.0 mg/kg, orally, q 12 h). In my experience, the long-term benefit derived from bronchodilator therapy varies considerably among individual patients.
The greatest benefits to aerosol therapy are derived in patients with acute onset signs, an excessive accumulation of bronchial and tracheal secretions, and those with secondary bronchial infections. Treatment, if needed, entails aerosolization of 5 to 7 mL of sterile saline (with or without antibiotics added to the solution) at least 3 to 4 times daily. Each treatment requires 15 to 20 minutes.
The objective of therapy is to delay progression of the underlying respiratory disease...not cure the patient. Treatment is life-long. However, the results can be rewarding and excellent quality of life can be achieved for years following the diagnosis.
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