Definition

• A sudden and often repetitively occurring defense reflex that helps clear large airways of excess secretions, irritants, foreign particles, and microbes, or clear foreign material from upper airways.
• The cough reflex consists of three phases: inhalation, forced exhalation against a closed glottis, and violent expulsion of air from the lungs following opening of the glottis, usually accompanied by a sudden noise. Coughing can happen voluntarily as well as involuntarily, although in dogs and cats it is presumed to be essentially involuntary.

Pathophysiology

• A physiologic reflex in healthy animals that protects the lower airways from inhalation of foreign particles and helps clear particles that have been entrapped in the mucus; acts in conjunction with the mucociliary clearance mechanism.
• The cough pathway includes cough receptors, (sensory nerves in the airways), the vagus nerve, the central cough center, and effector muscles.
• The cough pathway can be stimulated by mechanical or chemical factors. Endogenous triggers include airway secretions and inflammation. Exogenous triggers include smoke and aspirated foreign material.
• Cough receptors include rapidly adapting stretch receptors (sensitive to mechanical stimuli) that are located within the mucosa of the tracheobronchial tree (especially larynx and trachea), and pulmonary/bronchial C-fibers, which are more sensitive to chemical stimulation. Coughing mechanisms and pathways are very complex and are not fully understood, even in humans.

Systems Affected

• Respiratory-cough of any origin can be an inciting factor for inflammation that aggravates and perpetuates further cough.
• Cardiovascular-enlargement or impaired function of the right ventricle can result from a respiratory disorder (cor pulmonale); cough syncope can occur from a vasovagal response.

Signalment

• Dogs and cats of all ages and breeds.
• Much more common clinical sign in dogs than in cats.
• Cough of tracheal origin is less common in cats than in dogs.
• Age, breed, and sex predispositions vary with inciting cause.

Signs

• Cough must be differentiated from similar signs such as reverse sneezing, gagging, retching, and vomiting.
• Description of the cough can be helpful in identification of the anatomic structures involved in dogs (i.e., honking cough is typical of tracheal or airway collapse, harsh sonorous cough followed by terminal retch characterizes cough of tracheal or bronchial origin, faint moist cough is heard in moderate to severe pneumonia).
• Cough can be described as dry or moist, productive, honking, short or harsh, faint or sonorous, followed by gagging or retching.
• Cough can be elicited by traction on the collar (laryngeal or tracheal origin), aggravated by exercise or excitation (tracheal collapse), or can occur after a period of rest (cough due to heart failure).
• Can be accompanied by stertor or stridor (laryngeal, tracheal origin) or labored breathing (many areas).

Causes

Risk Factors

Differential Diagnosis

• Many conditions result in similar signs.
• Coughing can be confused with other signs such as sneezing, reverse sneezing, gagging, panting, retching, and vomiting. The presence of terminal retch is often misinterpreted as vomiting.

CBC/Biochemistry/Urinalysis

Minimum database may suggest acute bacterial infection (leukocytosis with left shift) or eosinophilic airway disease (peripheral eosinophilia).

Other Laboratory Tests

• Filter test for microfilaria and/or heartworm antigen serology-for heartworm disease.
• Serum antibody titer-toxoplasmosis, Angiostrongylus vasorum.
• Coagulation profile-for any patient that presents with cough associated with either epistaxis or hemoptysis.
• Fecal examination (Baermann test: identification of Angiostrongylus (dogs), Aelurostrongylus (cat) or other parasites (Filaroides, Crenosoma).
• PCR diagnosis available for several micro-organisms.
• Tests for evaluation of systemic diseases (e.g., hyperadrenocorticism, potentially predisposing to pneumonia or causing pulmonary thromboembolism).

Imaging

• Thoracic radiographs are the first step prior to any additional testing; provide essential information about intrathoracic airways, lung parenchyma, pleural space, mediastinum and cardiovascular system.
• Fluoroscopy-helpful to investigate diseases in which dynamic obstruction is suspected (tracheal collapse, bronchial collapse, bronchomalacia).
• Echocardiography-helpful when heart failure or dysfunction is suspected.
• Thoracic ultrasonography-in case of pleural effusion or when a pulmonary or mediastinal mass is suspected.

Diagnostic Procedures

• Endoscopy allows visualization of both static (tumor, granuloma, abnormal mucosa, excessive secretions) and dynamic (laryngeal paralysis, dynamic airway collapse) airway abnormalities.
• When bronchial and/or alveolar infiltrates are present-samples from lower airways can be obtained for diagnostic purpose (cytology, bacterial/mycologic cultures) by broncho-alveolar lavage or tracheal wash.
• Transthoracic (fine-needle aspiration) biopsy or thoracoscopy-allow biopsy sample when interstitial infiltration is prominent.
• Thoracocentesis-allows sampling of pleural fluid, can be performed under ultrasonographic guidance.
• Pulse oximetry and blood gas determination.
• Pulmonary function tests-require sophisticated material and/or experienced technicians, not readily available in private practice.

Drug(s) Of Choice

Contraindications

Antitussive agents are strictly contraindicated when cough is needed to clean the airways, i.e., in infectious or inflammatory airway disease, bronchiectasis.

Possible Interactions

Theophylline-clearance is inhibited by other drugs such as fluoroquinolones, increasing the risk of theophylline toxicity.

Patient Monitoring

• Acute cough must be adequately treated in order to avoid chronic cough, which can lead to possibly irreversible lesions.
• Conditions leading to chronic cough sometimes can only be alleviated but not cured; communicate with client to ensure successful management of cough.

Possible Complications

• Aggravation of tracheal collapse.
• Progression toward chronic bronchitis, chronic obstructive pulmonary disease, lung emphysema, irreversible bronchial and parenchymal remodeling, bronchiectasis.
• Acute severe cough might lead to syncope, rib fracture, or pneumothorax.
• Right heart dysfunction.

Age-Related Factors

• In dogs with anatomic disorders of inherited (e.g., primary ciliary dyskinesia) or congenital origin, signs might start early in life.
• Puppies and kittens are more likely to suffer from infectious disease.
• Inflammatory disorders affect middle-aged adults.
• Heart failure and tumors are more common in older animals.

Pregnancy/Fertility/Breeding

• Dogs affected with primary ciliary dyskinesia more than cats.
• Possible decreased fertility (in male and female dogs) as cilia from the urogenital tract and flagellated cells can be affected.
• Proven hereditary in some breeds (Old English sheepdog, carrier test detection exists).

See Also

• Asthma, Bronchitis-Cats
• Bronchitis, Chronic
• Canine Infectious Respiratory Disease
• Congestive Heart Failure, Left-Sided
• Hypoxemia
• Nasal Discharge
• Pneumonia, Bacterial
• Pneumonia, Eosinophilic
• Respiratory Parasites
• Tracheal and Airway Collapse

Abbreviation

• CNS = central nervous system

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