Coughing is a common presentation in small animal practice, which can be caused by either heart or respiratory tract disease.
In order to optimally manage the coughing patient, clinicians need to decide whether cardiac or airway disease is the cause.
Coughing is a non-specific response to inflammation or stretch of the airways following a variety of insults, including viral,
bacterial or other infections, allergic or hypersensitivity responses, foreign material, external compression, accumulation
of edema fluid, structural abnormalities, or neoplasia. When the airways become inflamed, the clinical manifestations include
erythema and hyperemia, mucosal edema, increased mucus production with proliferation of goblet and Clara cells, and infiltration
of inflammatory cells. Normally, the mucociliary escalator, the alveolar macrophages, and the bronchus associated lymphoid
tissue are the most important protective mechanisms of the lower airways. The cough reflex comes into play when these responses
have been overwhelmed by an increased volume of edema, exudate or mucus, or by the presence of foreign material. The cough
reflex may also be triggered by repeated local trauma or stretch, such as might occur in dogs with structural abnormalities
such as collapsing trachea or compression of the left mainstem bronchus as a result of left atrial enlargement.
The cough reflex is triggered locally in the airways, and controlled by cough centers in the brainstem. A cough begins as
a maximal inspiration, followed by initial forced exhalation against a closed glottis. Sudden opening of the glottis results
in rapid expulsion of air under considerable pressure, which assists in removal of debris, foreign material, and mucus from
the respiratory tract. This is further assisted by simultaneous contraction of the bronchial smooth muscle, which narrows
the airways, further increasing the force with which material is expelled. Coughing may be defined as productive or non-productive.
A productive cough occurs when material is expectorated from the trachea into the pharynx. In dogs and cats this material
is usually swallowed, but it can occasionally be expectorated to the exterior. Clinically, a productive cough sounds moist
and low-pitched, and the animal often swallows immediately afterwards. In contrast, non-productive coughing is usually harsh,
high-pitched or even honking. Expectoration of mucus may occur occasionally, but is usually not a feature.
Dogs with heart disease may cough because of the presence of pulmonary edema fluid within the pulmonary airspaces if they
are in congestive heart failure. Alternatively, they may not be in congestive heart failure, but may cough because an enlarged
left atrium is pressing upwards against the left mainstem bronchus, directly compressing and irritating the airway.
Common causes of coughing in dogs
- Left-sided congestive heart failure, often mitral regurgitation
- Collapsing trachea
- Chronic bronchitis
- Compression of the left mainstem bronchus
- Chronic bronchopneumonia
- Tracheal or bronchial foreign bodies
- Tracheal or bronchial masses or neoplasms
- Pulmonary neoplasia such as bronchogenic carcinoma
- Parasitic infestations
- Heartworms (Dirofilaria immitis)
- Upper airway obstruction such as laryngeal paralysis or brachycephalic airway syndrome
- Fungal infections such as blastomycosis or aspergillosis
- Pulmonary inflammatory disorders eg pulmonary infiltrate with eosinophils