Introduction
When I was sitting in class last century, it was questioned whether there was such a thing as naturally occurring chronic bronchitis (CB) in dogs (cats were not even mentioned!). I think we all now realize that it is a common disease in dogs as well as in cats with varying degrees of chronic morbidity.
CB is the term initially applied by Wheeldon in 1974 to describe the pathology documented in dogs when coughing has occurred for two or more consecutive months during the preceding year and which was not attributable to another cause (e.g., neoplasia, CHF). It also implies a non-reversible (indeed it is often a slowly progressive) condition. Both dogs and cats develop CB, and the 2 month time course identified in dogs has generally been applied to cats as well.
'Feline asthma' is a term that has been used since at least the 1960s to apply to cats who have been chronically coughing. In human medicine, asthma is usually reserved for those with chronic cough, airway eosinophilia, reversible airflow obstruction and bronchial hyperactivity. I prefer not to use this term in cats as confirmation of airway hyperreactivity and airway eosinophilia are usually unknown or absent.
A specific etiology for CB is rarely determined. Chronic airway inflammation leads to chronic coughing. The primary effects on the respiratory system are hypoxemia, exercise intolerance, coughing and respiratory distress. Recurrent airway inflammation is the suspected etiology. Acute exacerbations are commonly superimposed on a chronic course. Persistent tracheobronchial irritation results in changes in the epithelium and wall of the tracheobronchial tree. Mucus production is increased due to changes in glandular structures as well as goblet cells. Other commonly reported changes include airway inflammation, epithelial edema, thickening and metaplasia. Airway narrowing (with the associated increase in resistance and decreased expiratory air flow rates) is the net effect of these changes. In severe cases the work of breathing increases and is detected as respiratory distress (an increase in breathing rate/effort, disproportionate to the patient's level of exertion).
Siamese cats have been reported to be more frequently and more severely affected by chronic bronchitis than other breeds of cats. CB most often affects middle aged and older animals. A female sex predilection for feline CB has been suggested by some authors but has not been confirmed in our studies.
History
Coughing is the hallmark of lower airway disease. Chronic bronchitis typically causes a dry, hacking, non-productive cough; post-tussive gagging is especially common in cats and owners often misinterpret this as either hair balls or vomiting. Bacterial pneumonia, typically associated with a moist productive cough in dogs, is uncommon in cats. Coughing may occur at any time during the day but is common following exertion (exercise intolerance), at night (nocturnal coughing) as secretions accumulate, or when if the trachea is manipulated. Cats also present with tachypnea as the primary complaint.
Physical Examination
With pre-existing tracheal irritation/inflammation, any additional irritation (by palpation or manipulation) of the trachea normally results in coughing; this 'increased tracheal sensitivity' is a non-specific indicator of existing inflammation or irritation. An expiratory abdominal push (increased effort during quiet/resting breathing) and/or end-expiratory wheezing are characteristics encountered in patients with severe small airway disease and often air trapping (hyperinflation). Most CB animals are bright, alert and afebrile. Bronchovesicular lung sounds and crackles are commonly heard. Wheezing may be noted, especially when airflow initially moves through airways obstructed by secretions.
In cats with CB, lung sounds may be normal at rest but (post-tussive) crackles become prominent after coughing is induced as secretions are loosened. Tracheal sensitivity should be evaluated in all patients. Tachypnea is a more frequent primary complaint in cats than in dogs with CB. Murmurs and gallop rhythms are not uncommon in older cats but (unlike with dogs) heart disease does not cause coughing in cats.
Diagnosis
Differential Diagnoses
Differential diagnoses for a cat presented in respiratory distress and with a history of coughing, crackles/wheezing, tachypnea include: chronic bronchitis, cardiomyopathy, bronchiectasis, laryngeal dysfunction (paralysis, neoplasia), foreign bodies, parasitic disease, pulmonary fibrosis and neoplasia.
Diagnostic Tests
Diagnostic tests for patients with suspected lower airway disease/respiratory distress have been outlined in the talk titled "Diagnostic Approach to Respiratory Disease". A few specific comments more pertinent to cats:
CBC: Less than 40% of confirmed allergic airway cases (those with eosinophils on airway cytology) have an absolute peripheral eosinophilia. We typically think of cats and allergic lung disease ('feline asthma') but in reality, finding eosinophils on airway cytology or on the CBC is not common.
Radiography: Thoracic radiography is the most common method for evaluating the lower respiratory tract in cats. I prefer to obtain 3 views of the chest, both right and left laterals and the VD view for the evaluation of my respiratory cases. Bronchial disease classically presents with thickened bronchi ('donuts', 'tram lines'). Cats differ from dogs a bit in that there may be lung consolidation (presumably secondary to mucus plugging of a bronchus with subsequent resorption atelectasis) as well as lung hyperinflation (also referred to as 'air trapping'). Remember however that functional changes and visible structural changes do not always parallel each other; it is possible for cats to be in severe distress with relatively mild radiographic changes.
Airway culture and cytology are necessary to determine an etiologic diagnosis and in order to recommend the most appropriate therapy. Samples should be examined both cytologically as well as by Mycoplasma culture and aerobic bacterial culture/sensitivity determination. Feline bronchial disease is rarely bacterial in origin. Cytologically there may be up to ~25% eosinophils in healthy cat airways so I feel it is important that you always get both total and differential cell counts to properly interpret feline airway cytology. In my experience the most common infiltrate in bronchitic cats is neutrophilic, not eosinophilic.
Echocardiography (Echo): One of the major differential diagnoses for the acute respiratory distress (tachypneic) cat is primary heart disease (cardiomyopathy). The overall heart (chamber) size may not be accurately reflected on chest radiographs and an echo will be needed to evaluate the heart (specifically the left atrial size, LVFW thickness etc.).
Pulmonary fibrosis: We are relatively familiar with the pulmonary fibrosis that seems to be common in West Highland white terriers, but may be less familiar with the fact that cats may also develop pulmonary fibrosis. Lung biopsy is needed to confirm this diagnosis. Treatment is typically the same as for CB cases. Neoplasia has been identified in about 25% of these cases as a concomitant histologic finding.
Neoplasia: We encounter primary and metastatic tumors on a regular basis. Three view chest radiographs are indicated in order to fully evaluate all lung fields for involvement. Diagnosis is based on cytology or histopathology from a tissue biopsy. Abdominal ultrasound is indicated to ensure that there are no distant primary sites before surgery is considered. Surgical intervention for suspected primary lung tumors is recommended as early in the disease process as possible. The median survival time for primary lung tumors varies significantly based on whether the hilar lymph node is involved or not--always have a hilar node biopsied as part of the surgery.
Treatment
Treatment of chronic bronchial disease in cats is directed towards suppressing airway inflammation. Although infection is an uncommon cause of this inflammation, airway cultures should be obtained to look for this possibility. Careful history taking will help identify possible trigger events that might be present (examples include smoking, use of perfumes or aerosols, a dusty environment etc.).
Medical therapy involves the use of an anti-inflammatory (glucocorticoids or cyclosporine) and a bronchodilator (beta agonists or extended release theophylline). Systemic corticosteroids have been the mainstay of therapy for years. I prefer to use oral rather than long acting injectable products (e.g., prednisolone). Recently inhaled steroids (e.g., fluticasone HFA inhaler, Flovent®) have become popular and have been useful in selected cases although studies now show that there is systemic absorption following inhaled steroid administration and the pituitary-adrenal axis is affected.
Bronchodilators are indicated on at least a trial basis but particularly when there is air trapping or an increased expiratory effort detected on examination. Product choice is critical with a theophylline product as the pharmacokinetics vary considerably between products; in the U.S. only one brand is currently available which has demonstrated suitable kinetics. Terbutaline (injectable or oral) and albuterol (as the metered dose inhaler) are the beta agonists that are commonly used.