Cough (Proceedings)

Cough is a common presenting complaint for dogs and, to a lesser extent, cats. Cough is a sign of an underlying disorder, not a primary disease. Therefore, the cause of the cough should be identified and the underlying disease, not just the cough, should be treated. In some cases the cause of cough may be simple to identify and easy to correct. In others, diagnostic testing is unrewarding, and the cough remains unresponsive to therapeutic manipulations or the chronicity of the cough precludes successful management.

Definitions

Cough is defined as a sudden expiratory effort producing a noisy expulsion of air from the lungs. A cough usually signals an effort to clear the lungs of real or imagined foreign material. A variety of mechanical or chemical irritations of the pulmonary system may trigger a cough. The major causes of coughing in small animals may be divided into infectious/inflammatory, cardiac, neoplastic and parasitic. A cough can be classified as acute, or chronic. A cough is considered chronic if it lasts for two months or longer.

Pathophysiology

Most coughing in animals is presumed to be an involuntary response. Stimuli to cough include pressure on the outside of the airway, presence of foreign material, excessive secretions, or noxious gases in the airway. Cough serves as an important function both by aiding in the clearance of foreign debris and by enhancing the actions of the mucocilary elevator. The cough reflex is the primary defense mechanism to prevent aspiration of large particles or debris.

The cough pathway has been extensively investigated. The cough pathway includes the cough receptors/sensory nerves, the vagus nerve, the central cough center (brainstem, pons) and the effectors (glottis, expiratory muscles). Innervation for these receptors and the sites for triggering cough are supplied by the vagus nerve.

The first step in creation of a cough is stimulation of the cough receptors. Cough receptors are made up of sensory nerves. Species differences exist in sensory nerves. At least three different receptors are involved in stimulation of a cough response. These three cough receptors are 1) the rapidly adapting stretch receptors, 2) the pulmonary C-fibers and 3) the bronchial C-fibers.

The cough pathway may be stimulated by mechanical and/or chemical factors. Endogenous triggers to cough include the presence of airway secretions and airway inflammation. Exogenous agents include smoke or aspirated foreign materials such as food or water. Certain diseases can magnify the response to a specific agent, resulting in increased cough, such as with Bordetella infection. Anatomical differences also affect the resulting cough response as airways differ in their reaction to various stimuli. The more proximal airways (larynx and trachea) are very sensitive to mechanical stimuli, but are less sensitive to chemical stimulation. The more distal airways such as the bronchi and bronchioles are more sensitive to chemical stimulus and less responsive to mechanical stimulation. This is largely a reflection of the type of receptors present in each location. Direct stimulation of the larynx will result in the "expiratory reflex"; which is a cough without prior inspiration. Stimulation of receptors from distal airways typically results in an inspiratory phase prior to the cough. The prior inspiration serves to maximize the subsequent expiratory airflow rate. The former reflex may practically be appreciated in the initial cough triggered during attempts at endotracheal intubation in cats.

Chemical mediators, released by receptors in response to stimulation, serve to modulate the cough response. These mediators include substance P, calcitonin gene-related peptide (CGRP), neurokinin A (NKA) and other tachykinins. etc.

History

Historical information and a description of the cough may help to pinpoint the etiology. Cough may also be described as moist, dry, productive or honking. A moist cough suggests the presence of airway secretions. Animals may be observed to either swallow or produce sputum after a bout of coughing, and in these cases the cough is considered productive. Cough may also be classified as to a specific time of day (e.g., night, morning) or coupled with an event such as drinking, eating, running or pulling on a leash. In cats, cough may be confused with retching or attempts to vomit. For a variety of diseases, cough is often the primary presenting complaint. However, many small breed dogs cough frequently and owners may not interpret that as abnormal unless questioned. This is important because most therapeutic attempts to modify cough are more successful early in the course of the disease. Dogs in particular are likely to get aerosol infection (eg kennel cough) so close questioning as to exposure to other dogs is important. Additionally, it is wise to question the owners about any home therapy or prior treatments by other veterinarians.

Physical Examination

Pets that are presented for evaluation of cough should be immediately assessed as "stable" or "not stable". Doberman pinschers in particular are very likely to have severe heart failure associated with dilated cardiomyopathy and be presented for cough/gagging. These dogs frequently have a supraventricular tachycardia or atrial fibrillation and severe pulmonary edema despite looking relatively bright. Occasionally, they may be mistaken for kennel cough or pneumonia. Other "not stable" causes of cough include pneumonias and spontaneous pneumothoraces. Unstable pets should be rapidly administered supplemental oxygen and diagnostic testing should be pursued in a rapid, yet non-stressful pace.

In stable patients, a complete physical examination of the cardiopulmonary system is warranted, including examination of the oral cavity, palpation of the neck for masses or lymphadenopathy, auscultation of the upper and lower airway and heart, and abdominal palpation. Pets should be evaluated for weight loss or loss of muscle mass.

Differential diagnoses

The differential diagnosis for cough are multiple. Clearly, review of history, signalment and progression are exceeding helpful in narrowing the list. In grouping together possible causes, it is may be helpful to consider the following categories.

Allergic/Inflammatory

Feline asthma, chronic bronchitis, chronic obstructive pulmonary disease, pulmonary infiltrates with eosinophila, eosinophilic pneumonitis.

Cardiac

Pulmonary edema, Left atrial enlargement Pulmonary embolism (less likely)

Infectious

Tracheobronchitis Pneumonia including Bacterial, Viral, Fungal or Protozoal

Neoplastic

Primary lung, tracheal or larynx. Metastatic, enlarged lymph nodes

Parasites

Filaroides, Aelurostrongylus, Paragonimus, Capillaria, Dirofiliaria

Trauma and physical abnormalities

Foreign body, collapsing trachea, trachal stenosis or hypoplasia. Smoke inhalation

Diagnostic testing

Diagnostic tests for coughing depend on the severity of signs. A healthy dog with a cough after having been kenneled probably requires no diagnostic tests while an older dog with severe cough and respiratory distress may require extensive testing. Thoracic radiographs are very useful in determining the cause of the cough. Thoracic radiographs should be evaluated for signs of pulmonary infiltrates or masses, cardiomegaly, and tracheal/bronchial anatomy. Echocardiography may be performed in dogs with suspected cardiac cough. A transtracheal wash may be performed. Cytologic evaluation of the airway fluid may be very useful to document inflammation, infection or allergy. Aerobic culture may document bacterial organisms. A complete blood count (CBC) may show eosinophilia or neutrophilia.Heartworm testing in indicted in any dog not receiving prophylaxis. Examination for other parasites should be performed as needed (i.e. Baermann fecal examination)

Management

The most successful management of cough involves treatment and resolution of the underlying cause. Specific disease-oriented therapy often results in near complete resolution of the cough. If cough is associated with bronchoconstriction, bronchodilator therapy can result in partial resolution of the cough. For animals with infectious etiologies and productive cough, nebulization and coupage can improve clearance of airway secretions and debris and, as a result, improve cough effectiveness.

Cough Suppressants (Anti-tussives)

In general, the cough reflex is essential to help clear secretions or trapped foreign particles. Treatment of cough should always be directed towards treatment of the underlying cause. However, in some cases, the underlying cough can not be adequately controlled. Chronic cough may be very tiring for both the dog and for the family. In some cases, particularly those dogs with severe tracheal collapse or chronic bronchitis, bouts of cough may actually trigger more inflammation and subsequently, more cough. Thus in some cases, anti-tussive therapy is warranted.

     Anti-tussives may be divided into centrally-acting and locally-acting substances. Centrally-acting drugs act at the cough center in the brain by depressing the sensitivity of the cough centers to the various stimuli. Opoids are effective centrally-acting anti-tussives. Opoids may have additional properties, such as sedation or constipation. In some animals, one opoid may be clearly more effective than another. Thus, if significant coughing is still occurring, it may be useful to try another medication.

     Centrally-acting cough suppressants used in veterinary medicine:

          a) Codeine (many generics)                     0.1-0.3 mg/kg orally q 4- 6 hours

          b) Butorphanal (torbutrol, torbugesic)      0.25-0.5 mg/kg orally q 6-8 hours

          c) Hydrocodone (hycodan)                     0.2-0.3 mg/kg orally q 4-8 hours

          d) Dextromethorphan (OTC)                  0.2 mg/kg orally q 8 hours

Locally-acting cough suppressants used in veterinary medicine include bronchodilators, mucolytics and expectorants.

Selected readings

Padrid P.Feline asthma. Diagnosis and treatment. Vet Clin North Am Small Anim Pract. 2000 Nov;30(6):1279-93.

Johnson L.Tracheal collapse. Diagnosis and medical and surgical treatment. Vet Clin North Am Small Anim Pract. 2000 Nov;30(6):1253-66.

Peeters DE, McKiernan BC, Weisiger RM, Schaeffer DJ, Clercx C.Quantitative bacterial cultures and cytological examination of bronchoalveolar lavage specimens in dogs. J Vet Intern Med. 2000 Sep-Oct;14(5):534-41.

Clercx C, Peeters D, Snaps F, Hansen P, McEntee K, Detilleux J, Henroteaux M, Day MJ. Eosinophilic bronchopneumopathy in dogs. J Vet Intern Med. 2000 May-Jun;14(3):282-91.

Hardie EM, Ramirez O 3rd, Clary EM, Kornegay JN, Correa MT, Feimster RA, Robertson ER Abnormalities of the thoracic bellows: stress fractures of the ribs and hiatal hernia. J Vet Intern Med. 1998 Jul-Aug;12(4):279-87.

McKiernan BC. Diagnosis and treatment of canine chronic bronchitis. Twenty years of experience. Vet Clin North Am Small Anim Pract. 2000 Nov;30(6):1267-78