Professor, Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Roslin, Scotland, UK
Usually acute and chronic diseases will share the same clinical signs, such as coughing and sneezing, but other signs might not become apparent until the disease becomes more chronic and severe, dyspnoea being an example. In some acute diseases, dyspnoea might be the predominant sign noted.
1. Anatomical Diseases: Brachycephalic Airway Syndrome and Tracheal Collapse
1.1. Brachycephalic Airway Syndrome (BCAS)
Anatomical deformities affecting several (brachycephalic) breeds which result in varying degrees of upper airway obstruction and includes congenital anatomical deformities (singly or in combination), stenotic nares, extended/thickened soft palate, laryngeal deformities, laryngeal collapse, everted saccules, hypoplastic trachea. Surgical correction of some of the anatomical deformities can markedly improve clinical signs, but there is no effective treatment for laryngeal collapse and hypoplastic trachea.
1.2. Tracheal Collapse
Dorsoventral flattening of the trachea and flaccidity of the dorsal membrane are the major problems and the trachea may only develop a partial collapse. Clinical signs tend to develop with age. Obesity and other respiratory diseases can compromise tracheal mechanics in dogs where the trachea lacks structural rigidity and the dorsal membrane is flaccid and wide, and so precipitate clinical signs.
Conservative management should be attempted first and includes obesity control, use of a harness, controlled exercise, glucocorticosteroids to control airway inflammation, bronchodilators (minimal value in most cases) and selective use of antitussives. In situations where the collapse is life-threatening or the persistence of the clinical signs intolerable then surgical and interventional procedures can be considered, including extra luminal support rings (now rarely done) and intraluminal stents (a more widely used technique).
2. Acute Inflammatory Becoming Chronic: Chronic Rhinitis Chronic Bronchitis, Chronic Bronchopneumonia, Feline Asthma
2.1. Chronic Rhinitis and Sinorhinitis
With chronic rhinitis there are hyperplastic changes in the nasal mucosa, secondary bacterial infections, some of which can be deep-seated and involve the adjacent bony structures, while mycotic sinorhinitis (Aspergillus fumigatus) is itself a chronic condition. In dogs, chronic nasal cases tend to be dominated by nonspecific rhinitis (inflammatory), neoplasia and foreign bodies. In cats, FHV and B. bronchiseptica infection are the most important. Clinical signs can include nasal discharge (unilateral/bilateral, clear [serous] mucoid, mucopurulent or blood tinged), sneezing, epistaxis occasionally (neoplasia, foreign body), stertor and facial deformity and facial pain.
Therapy is dependent on the underlying cause and can vary from surgery and radiation therapy for tumours, antibiotics appropriate for osteomyelitis to direct antifungal agent instillation for nasal mycoses.
2.2. Chronic Bronchitis, Bronchomalacia and Bronchiectasis
The primary cause of chronic bronchitis (CB) is usually unknown but excessive production of mucus causes plugging of smaller airways and further damage by providing a suitable environment for secondary bacterial infections. Many CB dogs are likely to have concurrent bronchomalacia, simply because of their age. Some will progress to develop bronchiectasis (permanent dilation of larger bronchi). Secondary respiratory infections are common in CB, made worse if there is bronchiectasis. Coughing, tachypnoea, dyspnoea and exercise intolerance are common findings with intermitted pyrexia, anorexia or inappetence and lethargy when there is superimposed bacterial bronchopneumonia.
Glucocorticosteroids (prednisolone) are the drug of first choice and may be required long term. If feasible they should be administered by inhalation (fluticasone) once initial control has been achieved with oral administration. Bronchodilators are of questionable value. Mucolytics are worth trying with or without inhaled steam or nebulised hypertonic saline, chest percussion and physiotherapy. Antitussives are best avoided or at least should be reserved for control of excessive coughing. Antibacterial therapy is necessary if concurrent bacterial bronchopneumonia.
2.3. Chronic Bronchopneumonia
In many cases the underlying cause of pneumonia cannot be determined, but the effect is the same with neutrophil infiltration of the lung and airways and secondary proliferation of bacteria, most of which are part of the normal local flora (typically gram-negative aerobes). A major cause of pneumonia in the dog is aspiration. Acute pneumonia can become chronic and typically is where infection has become localised to a single lobe and while antibiotic responsive always recurs when treatment is stopped. In those cases, lung lobectomy is a reasonable treatment. In chronic cases, other noncommensal micro-aerophilic organisms (Bacteroides, Nocardia, Actinomyces spp.) are often involved, and with areas of necrosis in the affected lung lobe significantly impacting on the efficacy of antibiotics.
2.4. Feline Asthma
Asthma is reversible bronchoconstriction with inflammation that results in coughing, wheezing and dyspnoea. Clinical signs are variable and can show an acute onset, but many cats move into a chronic intermittent presentation with coughing, wheezing, crackles, dyspnoea, tachypnoea and orthopnoea.
Treatment depends on the severity and the persistence of clinical signs and exclusion from the owner's bedroom (human dander and house dust mite) may help. Bronchodilators may be of some benefit with terbutaline (0.625–1.25 mg per cat q8–12h PO) being the most commonly used or salbutamol or albuterol by inhalation (AeroKat spacer device). Glucocorticosteroids, however, are the primary method of control as this is an inflammatory disease. Oral prednisolone at 1–2 mg/kg q12h for 7–10 days then slowly reducing to 0.2 mg/kg q48h is used first and then inhalation of fluticasone at 125 mg q12h or beclometasone propionate 100 mg q12 can be trialled.
3. Slowly Developing Diseases: Laryngeal Paralysis, Idiopathic Pulmonary Fibrosis, Pulmonary Neoplasia
3.1. Laryngeal Paralysis
Laryngeal paralysis is a failure to abduct the arytenoid cartilages during inspiration and is very common in geriatric dogs, particularly Labrador retrievers. The cause is unknown in the majority of cases, but polyneuropathies and myopathies, neuromuscular disease and congenital forms and trauma/damage to the recurrent laryngeal nerve need to be considered. Clinical signs can vary and may not be apparent in milder cases until respiratory work increases. Hyperthermia, exercise, stress and excitement often exacerbate dyspnoea, which is typically inspiratory (stridor). There is progressive appearance over months or years which eventually results in severe respiratory distress and exercise intolerance, and in very severe cases, cyanosis and collapse. Dysphonia (change in tone of bark) can be found as well as coughing, gagging, choking and dysphagia.
Sedation if distressed and oxygen supplementation with cage rest and cooling can overcome the initial emergency. Glucocorticosteroids can relieve laryngeal swelling, but in many case laryngoplasty is needed. This is successful in about 90% of dogs, but surgery may eventually fail and may predispose to aspiration pneumonia. It is not worth undertaking this procedure in dogs that have concurrent dysphagia or chronic bronchitis or evidence of previous aspiration pneumonia.
3.2. Idiopathic Pulmonary Fibrosis
Pulmonary fibrosis is best regarded as an interstitial lung disease and the most convincing fibrotic disease in the dog and cat is idiopathic pulmonary fibrosis (IPF). Chronic interstitial lung changes are also identified on thoracic radiography that can be attributed to systemic illnesses such as metastatic mineralization with hyperadrenocorticism, acute and chronic renal failure, or may even be a consequence of natural ageing changes, and it is presumed that these also reflect a spectrum of the fibrotic response.
Dogs and cats affected with IPF will show the typical signs of respiratory disease, and the timing and severity will be dependent on the extent of the disease and its rate of progression. Coughing, tachypnoea and dyspnoea are common findings, coupled with varying degrees of exercise intolerance, cyanosis and syncope. In dogs the condition is seen mainly in middle to old-aged West Highland white terriers, and about 50% will have concurrent pulmonary hypertension. Survival from diagnosis can vary markedly, but median survival of 9 months is likely. Diagnosis is best achieved with high resolution computed tomography.
Therapy is of little benefit in IPF, but they do tend to be trialled on prednisolone and may show a response if chronic bronchitis or active inflammation is implicated, but not if the presentation is exclusively caused by fibrosis. Because many of these dogs have pulmonary hypertension, sildenafil is often trialled in many of these patients and response monitored.
3.3. Pulmonary Neoplasia
Lung parenchymal tumours can be either primary or metastatic, with the latter more common since the lung is the main site of metastases for malignant tumours. Lung neoplasia is typically found in middle to old-aged dogs and cats and results in clinical signs typical of respiratory disease. Coughing is common with dyspnoea, exercise intolerance and paraneoplastic signs such as cachexia and hypertrophic pulmonary osteopathy appearing as the disease progresses.
The most common primary tumours are carcinomas, with adenocarcinoma and bronchoalveolar carcinoma making up the majority, and are easily identified on routine survey thoracic radiographs. Confirmation can be achieved with FNA biopsy.
For primary lung cancer, resection of the affected lung lobe is the preferred treatment for primary lung neoplasia and prognosis is greatly improved if there is no lymph node involvement. Survival can be up to 24 months, but much less in those dogs that have negative prognostic indicators such as involvement of several lobes and/or lymph node, pleural effusion and hilar localisation. Chemotherapy has limited value in pulmonary neoplasia, but can be considered for palliative treatment of extensive primary lung tumours where resection has not been complete.
Standard clinical textbooks