Canine chronic bronchitis and pulmonary fibrosis (Proceedings)

A large number of disorders (infectious, non-infectious inflammatory, immune-mediated, neoplastic etc.) can affect the bronchopulmonary tree in dogs. A comprehensive review of each disorder is beyond the scope of this presentation. Rather we will review key clinical points about the diagnosis and management of canine chronic bronchitis and pulmonary fibrosis.

Chronic bronchitis in the dog

Definition:

Canine chronic bronchitis refers to persistent inflammation in the bronchial tree, that may involve lobar and segmental bronchi, or the smaller airways.

Etiology:

      - The vast majority of cases of chronic bronchitis in dogs are idiopathic

      - Environmental pollutants (including passive exposure to cigarette smoke) and hypersensitivity reactions are speculated but it is difficult to establish a clear link

      - Chronic or recurrent infection may contribute, and may suggest an abnormality of local innate or humoral immunity (e.g. IgA deficiency)

      - Ciliary dyskinesis is a rare congenital abnormality, that typically manifests as recurrent bacterial sinusitis, bronchitis and pneumonia

      - Proposed but unproven relationship of chronic respiratory disease to poor oral health, especially in older small breed dogs

Signalment:

      - Adult dogs

      - Small-medium sized breed

      - Cocker Spaniels predisposed

      - vs. infectious tracheobronchitis where larger breed dogs < 1 year old are over represented

Clinical Signs:

      - coughing

           o a hallmark of bronchitis regardless of etiology

           o usually progressive in frequency and severity

           o generally productive with expectoration (sputum swallowed) and gagging

           o often noted for years prior to presentation

           o worsens with activity / excitement and can often be elicited by tracheal palpation

      - wheezing

           o may be evident, especially on expiration

      - tachypnea

           o with advanced disease or pulmonary fibrosis

      - shortness of breath with exercise

           o severely affected dogs can become cyanotic with exertion

           o dyspnea predominantly expiratory, but can be both inspiratory and expiratory

      - cough-related syncope:

           o in severely affected dogs

      - appetite is generally unaffected and body condition score is usually normal to increased:

           o weight loss, depression and anorexia may suggest an alternative diagnosis (e.g. CHF) or secondary pneumonia (particularly in the presence of fever

Physical examination:

      - cough (especially with excitement)

      - expiratory (+/- inspiratory) dyspnea and wheezes

      - crackles in advanced cases

      - variable cardiac auscultation depending on the presence/absence of underlying heart disease (absence of a murmur and a pronounced sinus arrhythmia should allow you to rule out CHF)

Diagnosis:

The diagnosis of chronic bronchitis is based on history, physical examination, thoracic radiographs, airway examination and airway cytology. Definitive diagnosis requires demonstration of lower airway inflammation, while excluding other potential causes.

      - Thoracic radiographs:

           o Bronchointerstitial pattern (can be difficult to distinguish from 'old dog lungs')

           o Variable tracheobronchial collapse

           o Bronchiectasis (secular dilation) may be evident in some cases

           o Right middle lung lobe resorption atelectasis may be evident secondary to bronchial occlusion by mucus plugs

           o Alveolar infiltrates may suggest concurrent pneumonia

           o Pulmonary overinflation (emphysema) may be evident

      - Bronchoscopy:

           o Hyperemia and irregularity of the bronchial mucosa

           o Mucus hypersecretion

           o Exudates that may occlude smaller bronchi

           o Expiratory collapse of one or more bronchi

           o Bronchiectasis

      - Airway cytology and culture:

           o Samples can be obtained by bronchoscopy (blind or bronchoscopically) or tracheal wash (endotracheal or transtracheal)

           o Idiopathic bronchitis is associated with mucus hypersecretion and a mixed (although predominantly neutrophilic) inflammatory cell population (i.e. neutrophils along with reactive epithelial cells and varying numbers of eosinophils and mononuclear cells)

           o Eosinophilic inflammation suggests an allergic or parasitic etiology

           o Intracellular bacteria are consistent with pneumonia

Treatment:

Therapy of chronic bronchitis is determined on the basis of the severity of clinical signs, the results of the aforementioned diagnostic tests and modified based on response to treatment. Treatment options which can be considered are discussed below:

      - Corticosteroid therapy:

           o The most effective therapy for control of nonbacterial bronchitis in dogs

           o Ideally delivered as inhalant, however often cost prohibitive, esp. in large dogs

           o oral prednisone is the mainstay of therapy in most cases

           o Tapered following control of clinical signs to lowest effective maintenance dose (given once every 24-48 hours)

           o A poor response to Csts should prompt reconsideration of the diagnosis, and/or raise concerns about bacterial infection

           o May need to be discontinued or significantly dose reduced if infection is diagnosed

      - Bronchodilators:

           o Appear to provide symptomatic relief to a subset of dogs, 'responders' can be assessed by an albuterol trial while in the hospital

           o Two main classes of drugs are used:

               ■ Methylxanthine derivatives (e.g. theophylline and aminophylline)

               ■ B2-agonists (e.g. terbutaline and albuterol)

           o Side effects include anxiety, restlessness, tachycardia, PU and emesis

           o Doses adjusted for individual patient response

      - Antitussives:

           o Breaking the cough cycle is an essential part of treatment

           o Contraindicated in cases of lobar pneumonia

           o Hydrocodone and butorphanol are used most commonly

           o Sedation is a common side effect, however this may be beneficial if coughing is triggered by excitement

      - Antibiotics:

           o Indicated in cases of primary infectious bronchitis, or chronic bronchitis complicated by secondary infection (often manifests clinically as an acute exacerbation of cough in a previously stable chronic bronchitis patient)

           o Should be based on results of culture and susceptibility of samples appropriately obtained from the lower airways

           o Pending culture results should be based on Gram stain of airway cytology

           o Broad spectrum antibiotics indicated for empirical therapy

               ■ PO Amoxicillin-clavulanate, doxycycline

               ■ Nebulized gentamicin

      - Supportive care:

           o Weight management

           o Harness, rather than neck collar/leash

           o Avoid environmental irritants (e.g. house dust, vapors, chemical fumes, tobacco smoke etc.)

           o Nebulization, followed by light exercise to loosen secretions

Prognosis:

      - The response to treatment is variable

      - Some dogs make near-complete recoveries, while others require aggressive multimodal medical management throughout their lives

      - Patients with advanced disease (e.g. bronchiectasis or lobar atelectasis) generally respond poorly to medical management

Pulmonary Fibrosis

Pulmonary fibrosis (PF) is an interstitial lung disease that involves gradual replacement of the lung parenchyma with fibrotic tissue, resulting in diffusion impairment. It can be secondary to an underlying disease, or primary (idiopathic). Causes of secondary PF (elucidated in human medicine) include inhalation of environmental pollutants, radiation therapy to the chest, certain medications (e.g. Bleomycin), pulmonary infections and connective tissue disorders.

Pulmonary fibrosis is becoming increasingly recognized in veterinary medicine associated with increased awareness of the disease and the availability of more advanced respiratory diagnostics.

Signalment:

      - PF is best characterized in West Highland White terriers

      - also recognized in a variety of other dog breeds (mostly terriers) and cats

History:

      - similar to severe chronic bronchitis

      - includes cough, tachypnea, exercise intolerance or overt dyspnea

      - a lengthy course may exist before veterinary care is sought, due to the misperception that exercise intolerance is accepted part of aging

      - weight loss is not common

Physical examination:

      - in contrast to chronic bronchitis, cough is slight or absent (unless concurrent bronchitis)

      - marked tachypnea

      - loud pulmonary crackles (most evident ventrally on inspiration)

      - loud S2 or other signs of pulmonary hypertension (occasionally present)

      - sinus arrhythmia is common vs. sinus tachycardia associated with heart failure

Diagnosis:

The major area of importance in the diagnosis of PF is the realization that the disease exists and it not uncommon! Older dogs, particularly Terriers, with signs of exercise intolerance or rapid breathing should be considered candidates for further evaluation of PF. Fibrosis is commonly mistaken for chronic bronchitis, and CHF. Diagnostic testing in dogs is usually limited to thoracic radiographs given limited availability of advanced modalities, and the extent of patient dyspnea which precludes excessive patient handling, however consider......

      - Thoracic radiographs:

           o Generalized, bilateral, 'heavy' interstitial pattern (without consolidation or mass)

           o Right sided cardiomegaly (secondary to pulmonary hypertension), occasionally

           o difficult to differentiate from chronic bronchitis (and the two can occur together)

           o it should be noted that about 50% of people with symptomatic PF have near normal thoracic radiographs; this may apply to dogs as well

      - Computed tomography (CT):

           o High resolution CT is the standard of care for evaluation of people with suspected interstitial disease

           o Much better correlation with physiologic dysfunction than thoracic radiography

      - Pulmonary function testing:

           o used to support a diagnosis of pulmonary impairment,

           o increased A-a gradient and moderate to severe hypoxemia (however collection of arterial blood gases is challenging, especially in small, dyspneic dogs)

           o decreased diffusing capacity (DLCO) and a restrictive defect.

      - Echocardiography:

           o useful to document pulmonary hypertension which may represent an additional therapeutic target

      - Bronchoscopy and BAL:

           o Unlikely to be helpful, except in excluding other diseases such as chronic bronchitis

           o PF patients have neutrophilic inflammation

      - Open lung biopsy:

           o Required for definitive diagnosis, however rarely performed in veterinary medicine due to associated costs, inherent risks and the inability of histopathologic findings to facilitate more effective therapy at this time

Treatment:

      - limited to supportive care with or without prednisone therapy

      - no controlled studies have been performed in dogs, hence no evidence based recommendations are available, therefore consider ...

      - Glucocorticoids; either PO prednisone or inhaled glucocorticoids

      - Antitussives

      - Therapy for pulmonary hypertension; primarily sildenafil, also pimobendan

      - Antibiotics; ideally guided by C&S

      - Diuretics? Tiny doses

      - Supportive care as above (avoid heat, weight management, avoid environmental pollutants)

Prognosis:

      - guarded; many dogs die or are euthanized due to progressive pulmonary failure and associated respiratory distress within 12-18 months

      - grave for dogs that are oxygen dependent at first diagnosis

      - lesser affected dogs may have a markedly prolonged survival.

References available on request.