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.
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.