This abstract will not discuss heart disease as a cause of cough in dogs. This topic will be addressed in the cardiology stream.
Dogs cough for lots of reasons. Sometimes dogs cough for trivial reasons, perhaps because there is dust in the air. This is a normal response to an airborne irritant; in this case the cough is not a reflection of a disease. In these 'non-disease' situations the cough (in general) does not occur every day, the cough is not getting worse, the patient is not exercise limited, there is no noisy breathing and the patient is otherwise in excellent health. However, if the cough occurs every day, if the frequency or intensity is increasing, if the cough is associated with fatigue, exercise intolerance, noisy breathing or ill health, then the cause of the cough should be thoroughly investigated.
For better or worse, there is only a limited number of tests available in general practice to determine the cause of canine cough. However, there are only a few disorders of the canine respiratory tract that cause significant cough. The author has found it helpful to try to make an anatomical diagnosis of the problem prior to identifying the specific disease state that causes the clinical sign.
History
The very first question to ask an owner of a dog with a cough is whether the patient is exposed to cigarette or cigar smoke. Passive smoking is clearly associated with nasal and lung cancer in dogs, and is probably associated with bronchitis. Perhaps most importantly, no matter what cause of cough is determined, a patient exposed to further smoke will not respond optimally to any form of treatment, and the severity and frequency of cough will be worse in the presence of smoke than if smoke is avoided.
The second question to ask owners of dogs with a cough is whether or not the patient gags, chokes or swallows at the end of coughing. Patients with productive cough cannot easily expectorate (spit). In general, the mucus is brought to the back of the throat and the patient gags, chokes and/or swallows. This is an important diagnostic consideration because mucus production in dogs is associated more with lower airway or lung disease (bronchitis, pneumonia etc.) than upper airway disorders (tracheal collapse, laryngeal paralysis). This is not a perfect criterion, but will often help the practitioner in making an anatomical diagnosis prior to making a definitive diagnosis.
The third question to ask owners is whether there has been a change in the bark or quality of sound the patient makes. Changes in the bark, for example to a higher pitch, localise the disorder to the laryngeal area. High-pitched goose-honking sounds are likely to be coming from the upper trachea, while lower pitched throaty noises are more likely to be the result of lower bronchi and small airway disease. A soft moist cough is characteristic of lung infection, i.e., pneumonia.
Physical Examination
The pattern of breathing is often the most important clue to the cause of coughing in dogs. Problems breathing in are due to upper airway obstruction (pharynx, larynx, trachea) or pleural space disease (air, fluid or tissue). Problems breathing out are likely to be due to bronchial disease. Panting may be due to fear, excitement, heat, fever and other non-respiratory related conditions. When panting is the result of primary respiratory disease, the disorder is likely to be causing significant hypoxaemia and should be attended to immediately.
Auscultation is not as helpful a diagnostic tool as we would wish, primarily because our patients perform tidal breathing; when our physicians examine us, they ask us to take in deep breaths. This is because most abnormal respiratory sounds are more obvious during exaggerated breathing; tidal breathing causes adventitious sounds only when the disease state is advanced. Noisy breathing which is loudest at the laryngeal area suggests laryngeal obstruction. Crackles are most commonly heard in dogs with chronic bronchitis or pneumonia.
The heart rate is usually less than 90 beats per minute in any dog with chronic lower airway disease. Higher rates are associated with systemic disorders such as pneumonia, heart failure etc. Increased vagal tone associated with bronchial disease often causes a sinus arrhythmia (increased rate on inspiration, decreased rate on expiration).
Chest Radiographs
Long and thick soft palates can cause choke/gag and cough. This may sometimes be appreciated by viewing a lateral film of the laryngeal area. The patient is positioned laterally on the table, and the author generally uses bone technique. The soft palate should not extend more than a few millimetres past the tip of the epiglottis.
Tracheal collapse can be appreciated in many dogs using a lateral radiographic technique. Sometimes the oesophagus can cast a shadow that silhouettes with the trachea, causing a 'false' tracheal collapse appearance. If in doubt, a left lateral position will cause the trachea and oesophagus to fall side-to-side rather than top to bottom, avoiding overlapping shadows.
Bronchial disease may result in thickened bronchial walls. These walls cause the 'doughnut' appearance when seen end on, and the 'tram line' appearance when viewed laterally. Sometimes the only significant change on a chest radiograph is narrowing of the carina due to bronchomalacia. Only two disorders will narrow the carina with any frequency: chronic bronchitis and left heart enlargement. If left heart enlargement is not suspected, a narrow carina can be a definitive finding to support the diagnosis of chronic bronchitis.
Lung disorders will cause some combination of an interstitial and/or alveolar pattern. Pneumonia may cause clinical signs 1-3 days prior to the development of alveolar consolidation on plain radiographs. Conversely, the patient may get clinically better days before the radiographs show improvement.
Pulse Oximetry
The pulse oximeter can be used in patients who are awake. Some dogs will allow placement of the sensor on their lip. The author usually uses a 'rectal probe' to determine oxygen haemoglobin saturation in awake dogs. This is useful in two general situations:
Panting dogs with a normal pulse oximetry reading are not hypoxaemic; therefore, they are likely to be panting for non-respiratory related conditions.
Patients with respiratory disease may need supplemental oxygen. The pulse oximeter can be used to determine whether an individual patient requires oxygen, and can also be used to determine when oxygen is no longer required. In general, a pulse oximetry reading of 95% and above reflects sufficient oxygenation and supplemental oxygen is not clearly required. A reading of 90% or less represents a PaO2 of 60 mmHg or less and is an indication for immediate oxygen supplementation.
Therapy
Treatment is clearly based on the disease that is present. A few comments are generally true:
If cough suppressants are indicated, the dose required is usually much higher than the standard doses recommended in most texts.
Standard doses of many antibiotics are not sufficient to achieve therapeutic levels in bronchial tissue. There is a 'blood-bronchus' barrier that limits penetration of many of these antibiotics, due to lipid solubility, pH etc. In general, penicillins and cephalosporins do not penetrate well.
Many older dogs with pneumonia have aspirated material from the upper airway. These infections are usually mixed and include an anaerobic component. Commonly used veterinary fluoroquinolones have no primary activity against anaerobes.
Chronic antibiotic therapy for chronic bacterial infections including nasal infections may facilitate the growth of Pseudomonas species. Marbofloxacin is probably more effective than enrofloxacin in this setting.
Pneumonia may cause a painful cough. Non-steroidal anti-inflammatory drugs (NSAIDs) such as carprofen may lessen the musculoskeletal discomfort associated with coughing, and in this way increase the productive nature of the cough in canine patients.
Respiratory endoscopy is indicated for a cough or noisy breathing that is not easily explained by radiographic or physical examination findings, or when the patient is not responding to treatment as expected.
There are no diseases that result in chronic, primary bacterial infections within the nasal cavity. If there is a nasal discharge that persists more than a few weeks, or if the nasal discharge resolves on antibiotics and returns when antibiotics are stopped, then there is a primary disorder that 'allows' secondary bacterial infection to occur. This may be tooth root abscesses, foreign bodies, fungal infections, tumours, non-malignant growths or lymphocytic-plasmacytic-neutrophilic infiltrates. This primary disorder must be diagnosed and treated for the discharge to be 'cured'.