Canine Chronic Bronchitis Lisa Shearer DVM, DVSc, Diplomate ACVIM (internal medicine)

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1 Canine Chronic Bronchitis Lisa Shearer DVM, DVSc, Diplomate ACVIM (internal medicine) Chronic bronchitis (CB) is defined as a daily cough for greater than two months in which other causes of cough, such as congestive heart failure, pneumonia, bronchopulmonary neoplasia and heartworm infestation, have been excluded. The inflammatory etiology in CB is often unknown; however, environmental pollutants or other inhaled irritants, smoke and low-grade, chronic aspiration may play a role. Historical and physical examination findings CB affects middle-aged to older dogs of both small and large breeds. The most common presentation is a non-productive chronic cough in an otherwise healthy dog. In some cases, excessive airway secretions result in a productive cough and post-tussive retching episodes are often misinterpreted as vomiting by owners. Other historical findings can include interruption of play or exercise due to coughing, exercise intolerance and cough syncope in severe cases. Physical examination of dogs with CB is often unremarkable, but varies with disease severity. Tachypnea with increased expiratory effort may be noted in more severe cases and is often historically exacerbated by exercise or excitement. Tracheal palpation may elicit a cough, but this is an inconsistent finding. Thoracic auscultation may reveal inspiratory crackles and expiratory wheezes. Auscultation is also important to help differentiate cardiac from bronchial etiologies for coughing. Valvular insufficiency, suspected by auscultation of a heart murmur, is a common finding in senior dogs. However, the presence of a heart murmur does not confirm congestive heart failure (CHF) as the cause of coughing. Assessment of the resting heart rate can be useful in distinguishing between CHF and CB. CHF, due to sympathetic nervous system activation, is typically accompanied by an increased heart rate. Conversely, due to increased vagal tone with respiratory disease, dogs with CB typically display a normal heart rate or mild bradycardia. Commonly, dogs with CB are documented to be overweight. Pathophysiology Bronchial wall infiltration with inflammatory cells, such as neutrophils or eosinophils, leads to oxidative injury and release of inflammatory enzymes. Repair of the damaged mucosa is attempted through epithelial and smooth muscle proliferation and mucous gland hyperplasia and hypertrophy. As the inciting cause is often not identified, this cycle of injury and repair is perpetuated. The mucosal hyperplasia, edema and excessive mucous production that ensues leads to airway narrowing, increased airway resistance and the clinical manifestation of coughing. These histopathologic changes reflect the chronicity of the condition and are typically non-reversible and often progressive. The damaged airway epithelium in dogs with CB allows invasion of inhaled pathogenic bacteria or irritants resulting in secondary pneumonia and perpetuation of the inflammatory response.

2 -2- Diagnostic Approach Canine chronic bronchitis is a diagnosis of exclusion. Dogs with chronic coughing should undergo a thorough diagnostic work-up to exclude other differentials and to evaluate disease severity. Clinicopathologic Testing Complete blood count and serum biochemical profile are often non-contributory. Occasionally, eosinophilia may be noted, suggestive of allergic bronchitis, heartworm infestation or other parasitic disease. Eosinophilia is an inconsistent finding and has been documented in less than 50% of confirmed cases of allergic bronchitis. Fecal testing and heartworm antigen ELISA should also be performed. Arterial Blood Gas Analysis Arterial blood gas analysis allows assessment of pulmonary function and monitoring of disease progression. Mild to moderate hypoxemia may be noted but hypercarbia is uncommon and present only when respiratory failure arises. Other pulmonary function tests, such as tidal breathing flow volume loops, are not readily available. Radiographic Evaluation Radiographs are useful to exclude other causes of coughing and are supportive of CB if a bronchial or bronchointerstitial pattern is noted. Conversely, normal radiographs do not exclude a diagnosis of CB. It has been documented that thoracic radiographs have a sensitivity of ~50-60% for the diagnosis of CB in dogs. In severe cases, radiographs may reveal evidence of bronchiectasis, a sequelae of severe CB. Bronchoscopy Bronchoscopy is a useful diagnostic test to allow visual confirmation of airway changes and to obtain airway samples for cytology and culture. Bronchoscopic examination in dogs with CB reveals hyperemic airways, excessive airway mucus and in some dogs, a rough and thickened airway mucosa. In chronic cases, fibrotic nodules can be occasionally visualized extending into the lumen of the bronchi. Laryngeal examination and direct airway visualization by bronchoscopy also help exclude other differentials or concurrent conditions such as laryngeal paralysis, tracheal collapse, foreign bodies, bronchiectasis and mass lesions. Airway Cytology and Culture Sample collection can be obtained by bronchoalveolar lavage or a good trans-tracheal wash if the patient is not stable for general anesthesia. The characteristic cytologic change associated with CB is an increased number of non-degenerate neutrophils. Other cytologic findings include increased amounts of mucus or Curshmann s spirals (bronchial casts of airway mucus) and increased numbers of epithelial cells. In some cases, increased eosinophils are noted which may represent an allergic etiology, systemic hypersensitivity (e.g. gastrointestinal parasites or ectoparasite infestation) or, as in humans, may represent an acute exacerbation of the chronic condition.

3 -3- Given the presence of suppurative inflammation, culture of airway fluid (including Mycoplasma culture) is recommended to rule out concurrent infection prior to initiating therapy. However, in most dogs with CB, bacterial infection is not a significant problem. As normal canine airways are not sterile, interpretation of culture results can be challenging. It has been documented that true lower respiratory tract infections, in addition to demonstrating suppurative inflammation cytologically, should result in a quantitative culture exceeding 1.7x103 CFU/mL or >3 intracellular bacteria visualized on 50 high-power fields on gram stain of a cytospin preparation. In dogs receiving therapy for CB, an airway culture is also recommended if an acute exacerbation of clinical signs is noted. Dogs with CB can aspirate oral bacteria during coughing, which, given their compromised airway defenses and excessive mucus in which to trap bacteria, can lead to secondary infections warranting specific antimicrobial therapy. Treatment Anti-inflammatory Agents The mainstay of treatment is control of the inflammatory infiltrate using glucocorticoids. Glucocorticoid therapy should be tailored to the individual patient based on duration of the disease, severity of clinical signs and the presence of other systemic conditions. The goal is to commence treatment at a dosage sufficient to induce clinical remission and then taper the dosage by approximately 25-50% every 7-10 days to achieve the minimum effective dosage. To help normalize the pituitary-adrenal axis, an alternate-day protocol of glucocorticoid administration is preferred. Short acting steroids such as prednisone are often effective in cases of uncomplicated CB and are initially required at dosages of 1-2 mg/kg/day. It should be conveyed to owners that life-long treatment with anti-inflammatory dosages is often required. In some cases, discontinuation of therapy may be possible; however, if clinical relapse is documented, the higher dose glucocorticoid therapy that controlled clinical symptoms should be re-instituted. Long acting glucocorticoids such as dexamethasone are generally not required unless a rapid acting, intravenous preparation is required in situations of respiratory distress. An alternative route of anti-inflammatory administration is an inhaled glucocorticoid (e.g. fluticasone) via metered-dose inhaler. This route minimizes the associated systemic side effects of the glucocorticoid; however, inhalant therapy is not tolerated by all dogs and requires an appropriately fitted mask for the breed to ensure consistent administration. Bronchodilators Increased airway resistance in CB is likely due to mucosal hyperplasia and excessive airway mucus, rather than airway hyper-reactivity and bronchoconstriction. Despite this information, bronchodilators have been beneficial in reducing clinical signs in dogs with CB and minimizing glucocorticoid requirements. A bronchodilator trial is a reasonable option for dogs that exhibit marked expiratory effort and persistent clinical signs despite appropriate glucocorticoid therapy. Additional benefits include improved diaphragmatic contractility, improved mucociliary clearance and reduced respiratory muscle fatigue. Methylxanthine derivatives (e.g. theophylline) and β2-agonists (e.g. terbutaline and albuterol) are the two classes of bronchodilators available for veterinary

4 patients. Given the possible side effects such as hyperexcitability and tachycardia, a low dosage should be used initially and increased as required and tolerated. -4- Antitussives Antitussives (e.g. hydrocodone, butorphanol) are indicated only if clinical signs persist following treatment of the inflammatory process, as coughing can perpetuate airway injury. Dogs with CB are reliant upon the cough reflex to clear airways of mucus accumulation. Suppression of the cough reflex leads to mucus trapping within the airways and perpetuation of inflammation and clinical signs. Antibiotics Antibiotics are indicated only with confirmed infection through culture or cytological techniques. Antibiotic selection should have a broad spectrum of activity against bacteria commonly isolated from the lung and should be based on culture and sensitivity results when available. Duration of treatment will depend on whether the infection represents bronchial colonization (5-10 days of treatment) or true pneumonia (treat for 4-6 weeks of treatment). If Bordetella is isolated, treatment with aerosolized gentamicin is preferred; however, long term oral doxycycline has shown efficacy. In patients with CB and secondary bronchiectasis, long-term antibiotics may be required to manage recurrent pneumonia. Ancillary Therapy Obesity often exacerbates clinical signs as this leads to decreased thoracic wall compliance and increased work of breathing. Owners of overweight dogs should be advised on a weight loss protocol. Dogs with CB, in particular those with concurrent tracheal collapse, will benefit from the use of a body harness instead of a neck collar. Owners should be advised to minimize exposure to airway irritants, smoke and humidity. Lastly, some dogs will benefit from airway humidification or nebulization and gentle coupage to clearance of airway secretions. This can be achieved by taking their pet into the bathroom while a hot shower is run or more efficiently, through the use of an ultrasonic nebulizer. Prognosis Due to the non-reversible and progressive nature of CB, owners should be advised that the long-term prognosis is guarded. Complete recovery is uncommon; however, CB can often be controlled. A good response is considered to be a 70-80% reduction in the severity of clinical signs. Life-long treatment is often required; however, therapy should be tapered to the lowest effective dosage to minimize treatment side effects. The goal of therapy is to minimize airway inflammation to prevent worsening of airway changes that can ultimately lead to complications such as bronchiectasis or cor pulmonale.

5 -5- References Bemis DA, Appel MJG: Aerosol, parenteral, and oral antibiotic treatment for Bordetella bronchiseptica infections in dogs. J Am Vet Med Assoc 170:1082, Cormier Y et al: Effect of inflammation on peripheral airway reactivity in dogs. Clin Sci 84:73, Mantis P, Lamb CR, Boswood A: Assessment of the accuracy of thoracic radiograph in the diagnosis of canine chronic bronchitis. J Small Anim Pract 39:518, McKiernan BC: Diagnosis and treatment of canine chronic bronchitis. Twenty years of experience. Vet Clin North Am Small Anim Pract 30:1267, Padrid PA et al: Canine chronic bronchitis: a pathophysiologic evaluation of 18 cases. J Vet Intern Med 4:172, Peeters DE et al: Quantitative bacterial cultures and cytological examination of bronchoalveolar lavage specimens in dogs. J Vet Intern Med 14:534, 2000.