A cough is a protective reflex action, which expels secretions from the airways. It can sometimes be caused by inflammation or compression, or a cough can have a trivial cause, such as a dusty environment. However, a cough that is persistent, or that can be linked to other clinical signs such as exercise intolerance, noisy or laboured breathing, should be investigated. While dogs may not die from a cough alone, the accompanying disease process can be very serious.

The first major differentiation to be made is whether the cough is cardiac or respiratory in origin, because treatment for the two varies significantly. Clinical findings that can be associated with a cardiac cause include a heart murmur, gallop rhythm or arrhythmia. Age and breed can also provide clues because heart disease often occurs in middle-aged to old dogs, and certain breeds have a high predisposition to heart disease, such as Cavalier King Charles Spaniels. Smaller dogs are more likely to cough with heart disease when an enlarged heart compresses the mainstem bronchi. However, small-breed dogs are also more likely to suffer from tracheal collapse and bronchomalacia, so it is important to remember that dogs can have a heart murmur and respiratory disease at the same time.

If the cause for the cough is identified as respiratory in origin, then the next step is to localise the cough to either the airways or the lungs (Figure 1).

Figure 1. Causes of cough divided into areas of the respiratory tract.

 Airways

 Laryngeal paralysis

 Tracheal collapse

 Canine infectious tracheobronchitis

 Chronic bronchitis

 Neoplasia

 Foreign body

 Parasites (Oslerus osleri and heartworm)

 Bronchial compression (cardiomegaly, mass lesion, bronchomalacia)

 Pulmonary parenchyma and vasculature (lungs)

 Infectious (viral pneumonias)

 Bacterial pneumonias

 Protozoal pneumonias (toxoplasmosis)

 Fungal pneumonias

 Parasitic disease (Angiostrongylus vasorum)

 Aspiration pneumonia

 Idiopathic pulmonary fibrosis

 Pulmonary neoplasia

 Pulmonary contusions

 Pulmonary thromboembolism

 Pulmonary oedema

A cough can originate from any part of the respiratory system, and localising it can give a lot of information about the likely cause. History can also help with forming a diagnosis. For example, a dog that has recently been in kennels and has just started to cough should make veterinary staff suspicious of acute tracheobronchitis (kennel cough) and appropriate nursing precautions should be undertaken immediately to minimise spread to other patients.

Intensity of the cough also provides clues as to its origin. A loud, harsh cough can be associated with airway inflammation, such as chronic bronchitis, or large airway collapse, such as tracheal collapse (the classic 'goose honk' sound). A soft cough is often linked to pneumonia or pulmonary oedema. Timing of the cough can also be significant. If pulling on the collar exacerbates it, then it may be tracheal in origin. Coughs that are related to cardiac disease can be heard more at night or after sleeping. However, the timing of coughs may be biased by when the owner is actually with the dog (i.e., in the evenings or when exercising). It is also significant to note whether or not the cough is productive, or if there is a swallow or retch accompanying it, as this may also provide some clues as to the disease process. For example, haemoptysis (coughing blood) would be more likely to occur from a foreign body or neoplasia.

Whilst diagnostic tests may vary depending upon the type and origin of the cough, thoracic radiographs are an essential part for the diagnosis of all cough-related diseases. For example, a radiograph showing a large cardiac silhouette, displaced trachea and perihilar lung pattern, would be highly suggestive of heart failure. Radiographs that show a normal cardiac silhouette and ventral alveolar pattern would be suggestive of aspiration pneumonia. Good-quality radiographs are very important for diagnosis, but they still might not be sensitive enough to identify subtle lung pattern changes. Inflated views under general anaesthesia provide better-quality radiographs for assessing lung patterns. For veterinary centres that have access to computed tomography (CT), this has become a valuable tool for diagnosing upper and lower respiratory tract diseases and pulmonary vascular disorders. The advantage that CT provides is that it allows a three-dimensional (3D) image of the thorax with greatly enhanced contrast resolution. However, both CT and good-quality inspirational radiographs require general anaesthesia, so the stability of the patient needs to be assessed against the necessity of the test. Fluoroscopy is another useful diagnostic tool. It allows visualisation of the trachea in 'real time' as a cough is induced, which can help diagnose tracheal collapse and swallowing disorders non-invasively.

Other diagnostic techniques that may be used include transtracheal wash, or bronchoscopy with bronchoalveolar lavage (BAL). Both may be indicated when bronchial or alveolar disease is suspected. Transtracheal wash does not require sedation or anaesthesia, whereas BAL does require general anaesthesia. However, if bronchoscopy is performed at the same time, visualisation of the airways is an added benefit. The samples that are taken from these techniques are then analysed, and can be useful to diagnose inflammatory diseases, neoplasia, infection and parasites.

There are multiple reasons why a dog may cough. Differentiation between cardiac and respiratory disease is the first step, which will be guided by physical examination and history. Breed, age and the type of cough will also help decide what type and level of nursing care is required.

References

1.  Luis Fuentes V, Johnson LR, et al. BSAVA Manual of Canine and Feline Cardiorespiratory Medicine. 2nd ed. Quedgeley: British Small Animal Veterinary Association, 2010.

2.  Nelson RW, Couto CG. Small Animal Medicine. 4th ed. Philadelphia: Mosby Elsevier, 2009.