Anjop J. Venker-van Haagen, DVM, PhD, DECVS
Diseases of the trachea and bronchi are usually manifested by specific problems such as coughing, dyspnea, or stridorous breathing. Since all three of these signs may originate from parts of the airway other than the trachea and bronchi, as well as from the circulatory system, additional questions are asked about the animal's general condition, appetite, drinking, activity, and endurance, and about changes in its habits. The answers to these questions together with the results of a general physical examination will point the way to the diagnosis.
Coughing. Coughing is a reflex for protection of the respiratory mucosa. The reflex is activated by stimulation of the "cough receptors" or is evoked voluntarily. Cough receptors are sensory receptors located in the mucosa of the larynx, trachea, carina, and bronchi. They respond to airway pressure, vocal fold motion, tactile stimuli, and chemical stimuli. The threshold for each of these stimuli is variable, according to the variable condition of the mucosa. For example, in laryngitis coughing is evoked by barking. The afferents of the receptors run with the cranial laryngeal and vagus nerves to the solitary tract and nucleus. The interneuronal network of the reticular formation activates the respiratory center and the motor nucleus of the vagus nerve and the nucleus ambiguus, inducing the cough reflex. The cough reflex begins with a deep inspiration with the glottis wide open. This is followed by closure of the glottis and strong contraction of the expiratory muscles, which builds up air pressure against the closed glottis. Then sudden opening of the glottis allows the air and any material that is present to be expelled forcefully. This basic pattern can have many variations. It may be repeated to produce a paroxysm of coughing. It may be fragmented by the repeated opening and closing of the glottis during a single expiratory effort. The relative contributions of inspiration and expiration may vary.
Dyspnea. Dyspnea can be caused by obstruction of the trachea, as occurs in tracheal collapse in small breeds of dogs or tracheal neoplasia in both dogs and cats. In bronchitis and bronchopneumonia, mucopurulent exudate may obstruct the airway during both inspiration and expiration. In asthma-like diseases in cats, constriction of the smaller bronchi and bronchioles may obstruct communication with the alveoli. These diseases must be distinguished from diseases of the lung parenchyma, but in both diseases of the trachea and bronchi and those of the lung parenchyma, the dyspnea is predominantly expiratory.
Stridorous breathing. Partial obstruction of the trachea may result in a particular extra sound during the respiratory cycle. It occurs during expiration and differs from coughing in being a softer, wheezing sound. It must be distinguished from the various stridors caused by upper airway disease and from the soft vocalization that is sometimes heard when there is pain in the thorax.
Special Diagnostic Techniques
Special techniques are indispensable for diagnosis of most diseases of the tracheobronchial tree. Although kennel cough can be diagnosed by the association of the history and the clinical signs, chronic coughing and dyspnea require further investigation. Radiographs of the cervical trachea and the thorax can reveal changes in the cartilages of the tracheobronchial tree or the mucosa when the lumen of the trachea and bronchi is altered by the disease process. Radiographs always precede inspection of the lumen of the tracheobronchial tree by bronchoscopy. Bronchoscopy provides visual confirmation of the changes in the tracheobronchial tree, such as anomalies in the shape of the tracheal or bronchial cartilaginous rings or obstruction of the lumen by foreign bodies or masses. When the mucosal lining is reddened by hypervascularization or when mucopurulent material is found covering the mucosa, bronchial and bronchoalveolar lavage with subsequent culture and cytological examination of the material obtained will often lead to the diagnosis or at least an indication of the underlying disease.
Radiography is a very important technique for the diagnosis of tracheal and bronchial diseases. Radiographic imaging of the changes in the pulmonary parenchyma is of additional importance because bronchial diseases may be associated with bronchopneumonia. Radiographic sensitivity can be improved by the use of high-quality equipment--including screens, grids, etc.--and correct positioning, effective restraint, appropriate exposure, and good development procedures.
The lateral view is the most important in radiographic examination of the cervical trachea. The dorsoventral view is wanted occasionally, especially to evaluate dislocation of this part of the trachea. The lateral radiographic examination must be performed with careful positioning of the head and neck in relation to the thorax. Excessive flexion of the occipitoatlantal joint or the neck may cause an undesirable change in the lumen of the trachea, which could be mistakenly interpreted as an abnormality. The thoracic part of the trachea should be examined on radiographs of the thorax. The tracheal images should be surveyed for abnormalities in the diameter of the luminal air column, the continuity of the mucosa, and the contrast of the tracheal rings. In order to detect possible external changes affecting the trachea, the position of the trachea relative to the cervical and thoracic surroundings should be examined. The normal diameter of the trachea is difficult to define, for the diameter decreases slightly from cranial to caudal, but as a rule of thumb the diameter of the trachea at the level the third rib should be approximately three times the width of the rib at the level of the trachea.
On radiographs of the normal lungs the bronchial tree is mainly visible in the central area. When the lung parenchyma is adequately inflated only a slight density indicates the lung contours. The bronchi are more apparent in older dogs due to calcification of the bronchial walls. Visible thickening of bronchi, which suggests pathologic changes, gives them the appearance of ring-like structures like doughnuts and parallel lines like tram lines. The presence of air bronchograms indicates that there is consolidation of the lung tissue adjacent to the bronchi.
The usefulness of bronchoscopy in the diagnosis of diseases of the tracheobronchial tree in the dog and the cat is well recognized. The equipment has been described and bronchoscopic findings in the dog and cat have been illustrated. Although the cost of good-quality equipment is substantial, it is a worthwhile investment for the more specialized small animal practitioner. Videoendoscopy is ideal for teaching, since a group of students can follow the bronchoscopic examination in real time. Bronchoscopic technique differs according to whether a flexible or rigid bronchoscope is used. Rigid systems are less expensive and much more durable. Several sizes of high-quality rigid bronchoscopes can be purchased for the price of just one size of a good-quality flexible bronchoscope. The advantage of the flexible bronchoscope in human medicine is that it makes bronchoscopy possible without anesthesia, but this is in any case never possible in dogs and cats. On the other hand, the thorax is narrower laterally in dogs and cats than in humans, facilitating access to all bronchial divisions using rigid bronchoscopes.
Videofluoroscopic examination of the trachea is practically indispensable for the functional evaluation of the collapsing trachea or a partially stenosed trachea. The animal should be conscious and in lateral recumbency while the entire trachea is visualized on the monitor. Playback of the observations in slow motion can reveal functional features missed during the initial examination. When recording equipment is not available, comparing an end inspiratory and end expiratory radiograph will give some indication of the functional consequences of the anomaly.
Infectious Tracheal Bronchial Disease
Viral tracheobronchitis in dogs. The most important viruses causing tracheobronchitis in dogs are canine adenovirus type 2 and canine parainfluenza virus. These viruses may damage the respiratory epithelium to such an extent that bacteria and mycoplasmas are able to cause secondary disease. Other viral infections are caused by canine adenovirus types 1, 2, and 3, and canine herpesvirus. Canine adenovirus type 2 and canine parainfluenza virus are transmitted by aerosol droplets. Canine adenovirus type 2 is moderately resistant and can survive for months in the environment, while canine parainfluenza virus is relatively labile, but quaternary ammonium disinfectants are effective against both.
The history and clinical signs of viral tracheitis without bronchitis differ from those of tracheobronchitis. Viral tracheitis is characterized by a history of contact with coughing dogs or a stay in a kennel or visit to another place where dogs are brought together and where viruses may survive. The harsh, dry cough develops 3 to 5 days after initial exposure. In most cases the infection is self-limiting in about 3 weeks. Palpation of the larynx and trachea elicits a cough or paroxysmal coughing. In mild infections there is no fever and the dog is otherwise healthy. The coughing may continue during the night and may also be stimulated by barking and by drinking. It may even cause the dog to avoid barking and drinking.
In viral tracheobronchitis a productive cough is to be expected, for the production of mucus is more abundant in the bronchial tree than in the trachea. The history is usually suggestive of viral tracheitis at the onset of the clinical signs, but it is the change from a dry cough to a productive cough that is the alarming sign. The dog can still appear to be otherwise healthy, but the presence of a productive cough should direct attention to the possible development of bronchopneumonia, for Bordetella bronchiseptica infection is commonly associated with tracheobronchitis.
This condition is also self-limiting, but its natural course may require 3 months. The diagnosis can rest on the history and clinical signs alone, unless they include evidence of complications, such as malaise and fever. Then further clinical examination, laboratory tests, and radiographs of the thorax are needed to estimate the seriousness of the disease. If no complications develop but the disease is not resolved after 3 months of care as described below, radiographic examination and bronchoscopy with bronchial lavage should be performed. An underlying disease such as tracheal hypoplasia or allergic tracheobronchitis may be revealed as the reason for the delay in spontaneous resolution of the tracheobronchitis.
Treatment of viral tracheitis in its usual mild form of laryngotracheitis without fever is symptomatic. House or kennel rest with only short walks and avoidance of excitement is important so long as the dog is coughing. Daily monitoring of body temperature is helpful in following the progress of the disease. The coughing will diminish if the dog is kept calm. Leading the dog on a leash will provoke coughing and should be avoided. When drinking provokes coughing, dogs tend to avoid the water pan. However, a drink of water activates the glands that moisten the laryngeal mucosa, which diminishes the irritation, and thus water should be given orally (20 cc for a dog of 15 kg) several times daily according the frequency of the cough. Excessive coughing may be treated by sedatives, especially during the night. Phenobarbital is satisfactory in a dose of 2 mg/kg once or twice daily, depending on the effect. Most dogs recover from kennel cough without complications, but when it becomes a serious problem in a kennel, intranasal vaccines are recommended for puppies as young as 2 to 4 weeks of age. Intranasal vaccines may contain attenuated canine parainfluenza virus together with attenuated Bordetella bronchiseptica. Attenuated canine adenovirus 2 vaccines are also available).
Treatment of tracheobronchitis without fever can be similar to that of uncomplicated viral tracheitis. Antibiotic treatment with broad-spectrum antibiotics is indicated when fever, malaise, or systemic disease complicate the infection. Further therapy should be guided by the results of special diagnostic procedures. Antitussives should not be used if the cough is predominately productive, since coughing removes obstructive mucus from the bronchial tree. Water should be given orally (20 cc for a dog of 15 kg) several times daily, to prevent desiccation of bronchial mucus or mucopurulent material.
1. Venker-van Haagen AJ. The Trachea and Bronchi. In: Ear Nose, Throat, and Tracheobronchial Diseases in Dogs and Cats. Hannover: Schlütersche Verlagsgesellschaft, 2005: 167-205.