Cough (non-cardiac) in the dog is most commonly associated with infectious, inflammatory, or degenerative airway disease. In some situations, more than one cause may be identified. Defining the components of disease present is most likely to result in successful therapy.
Canine chronic bronchitis is an inflammatory condition of the airways resulting in a daily cough for greater than 2 months of the year. Causes are unknown, but inflammatory damage to the airway results in epithelial cell hypertrophy and squamous metaplasia, goblet cell hypertrophy, submucosal gland hyperplasia, and mucosal/submucosal inflammation, edema, and fibrosis. These result in an increase in the amount and viscosity of airway mucus, narrowing of the airway lumen, and chronic irritation within the airway. Airway collapse may be caused by airway inflammation or may represent a degenerative change, with loss of airway support within the cartilage. Clinically, these disorders are manifest by chronic cough most commonly.
Chronic bronchitis and airway collapse most commonly affect middle-aged to older dogs (> 8 years of age) and any breed can be afflicted (although only small breed dogs have tracheal collapse). Cough may be harsh or productive, and concurrent airway collapse is often assumed when a honking cough is found. The physical examination may be relatively normal. Wheezing on expiration is considered a classic finding in chronic bronchitis, although many dogs only demonstrate increased or harsh lung sounds. Most dogs exhibit tracheal sensitivity on palpation, and dogs with airway collapse will often have an expiratory snap over the thorax during cough. With worsening disease, increased respiratory effort with an abdominal push may be noted, with or without cyanosis. Obesity is a common finding.
The diagnosis of chronic bronchitis or airway collapse requires exclusion of other causes of cough, particularly Mycoplasma or Bordetella infection, which can be found concurrently with bronchitis. A minimum database (CBC, chemistry, urinalysis) evaluates the dog's general health, but does not specifically address the diagnosis of chronic bronchitis. Thoracic radiographs are an important part of the work-up since they aid in the diagnosis of chronic bronchitis and help rule out other causes of cough. Classically, in a dog with chronic bronchitis, radiographs show a bronchial pattern or increased number and thickness of airway walls; however, radiographs can also be relatively unremarkable. Normal thoracic radiographs can also be found with airway collapse, although obtaining multiple views of the thorax and neck can enhance detection of changes in airway caliber during the phases of respiration. Intrathoracic airways will collapse on expiration.
Airway sampling is used to define the disease process in dogs with cough. A transoral or transtracheal wash can be useful for obtaining bronchial cytology to define inflammation and rule out infection; however, bronchoscopy is required to document airway collapse. Bronchoscopy is very useful in any dog with chronic cough, as it allows visual inspection of the airway to document mucosal hyperemia, increased mucus secretions, and irregular mucosa, as well as airway collapse. Cytologic specimens in chronic bronchitis usually show neutrophilic inflammation and samples in dogs with airway collapse may show neutrophilic or lymphocytic inflammation or be normal. Bacterial and mycoplasmal cultures are warranted in all animals with suspected bronchitis to rule out a significant bacterial infection.
Corticosteroids are used to reduce inflammatory damage to the mucosa and reduce excessive production of secretions in dogs with chronic bronchitis, but may or may not be useful in dogs with airway collapse. Prednisone or prednisolone can be used at relatively high doses initially (0.5–1.0 mg/kg BID for 5–7 days) and then tapered to once daily while maintaining control of cough. As the dose is lowered, a longer course of therapy may be needed and the goal is to attain alternate day therapy. Some dogs may be tapered completely off medications; however, exacerbation of disease is treated with an increase in prednisone to the dose that effectively controlled clinical signs. Animals that cannot be controlled on glucocorticoids or those that suffer excessively from side effects associated with steroid use can be treated with inhaled steroids. Oral steroids are generally continued during the first several weeks of inhaled therapy because of a delay in the onset of efficacy for inhaled medications. Oral steroids will also help reduce mucus in the airways and improve penetration of the topical drug.
Administration of drugs via the inhalational route is valuable because it delivers a potent amount of drug at the site of disease. The respiratory tract has a large surface area for topical delivery, and drugs that can be administered locally avoid potentially harmful systemic side effects. Inhaled therapy can be highly efficacious but training is essential. Because animals will not inhale on command, a facemask and spacing chamber are needed for delivery of medication from a metered dose inhaler (MDI). The Aerodawg® spacing chamber (Trudell Medical) has a round facemask suitable for use in most dogs, and the FlowVu indicator that allows owners to monitor respirations easily.
The most commonly recommended steroid for use in controlling signs of chronic bronchitis is Flovent® (fluticasone propionate inhalation powder), which is available in a MDI containing 120 doses to deliver 44, 110, or 220 µg/puff (US). Initial therapy with the 110 mcg/puff MDI with BID dosing appears to be used most commonly. The MDI must be shaken well prior to actuation and should be attached to the spacer before the dose is ejected. If the MDI is not used for a week or more, the unit must be primed prior to use, meaning that the canister must be actuated several times before drug enters the delivery phase.
Some dogs that fail to respond to anti-inflammatory therapy may benefit from the addition of a bronchodilator to improve expiratory airflow or to support a reduction in the dose of steroid required. Bronchodilators seem to have a role in managing bronchial collapse also, although they do not physically open the large, collapsed airways. Bronchodilators commonly used include the methylxanthines (extended-release theophylline at 10 mg/kg BID) or beta agonists such as terbutaline (0.625–5 mg/dog BID). Theophylline has variable metabolism across individuals, and side effects include vomiting, diarrhea and agitation. Often, these can be avoided by starting initially at a dose of 5 mg/kg PO BID for a few days, then increasing the dose to 10 mg/kg PO BID if the dog tolerates it. Extended-release properties of theophylline are maintained when the drug is split in half, but the pill cannot be quartered or ground up and remain effective.
When inflammation has been controlled but cough persists, a narcotic cough suppressant may be required. This occurs most often in dogs with concurrent airway collapse. Success can generally be obtained with hydrocodone (0.22 mg/kg PO QID–BID) or butorphanol (0.55–1.1 mg/kg PO QID–BID). I start with frequent administration and decrease the dosing frequency as the dog responds.
For dogs with excessive mucus production, saline nebulization can be helpful. Nebulization can be achieved using an ultrasonic nebulizer, vibrating mesh nebulizer, or compressed air nebulizer. These machines are designed to convert liquid (sterile water or saline) into small droplets (< 5 microns) that will deposit in the lower airways. The specifications for each type of nebulizer or humidifier should be evaluated for particle size and to determine whether saline or water is required.
A variety of nebulizers are available for purchase. Mesh nebulizers are the smallest machines but tend to be the most expensive (~US$200). Ultrasonic nebulizers are often the most quiet and range in price from US$50–150. Nebulizers are usually sold in a package containing a power source, nebulizer cup, extension hoses and/or mask, and a measuring device for adding medication to the cup. To administer drugs, a facemask is essential; however, for purposes of airway hydration, an aquarium, sealed cage, or animal carrier covered in plastic can be used as a holding chamber to trap the mist. In large dogs, the nebulizer hose can be inserted into an Elizabethan collar covered in plastic.
Finally, management of chronic respiratory disease generally requires weight loss, because this can result in improvement in gas exchange and reduction in cough. This is best achieved by obtaining a careful diet history and calculating current caloric intake, paying particular attention to the type and number of treats that the dog is given. The owner can start feeding 80% of the current dietary intake and monitoring to ensure a 1–2% weight loss per week. Often, it is helpful to feed the majority of calories in meals but to reserve 10% of the dog's calories for use as treats. This can improve owner compliance. If current caloric intake cannot be determined, the resting energy requirement can be calculated using the formula: RER = 70 (body weight in kg)0.75. Prescription diets may be required when major weight loss is needed to ensure that adequate nutrition is provided during the weight loss program.
Owners should be aware that the prognosis for abolishing cough in dogs with bronchitis or airway collapse is guarded and many dogs require lifelong medication. A reasonable goal is to reduce clinical signs by at least 50%. Worsening of disease can lead to bronchiectasis or cor pulmonale. Visualization of bronchitic nodules or airway collapse during bronchoscopy likely indicates the irreversibility of the process.
References
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