How I approach… The old coughing dog

Introduction to the dog with a chronic cough

The chronically coughing elderly dog is a frequent presentation to the first-opinion veterinarian, with common causes being airway collapse (tracheobronchomalacia) and inflammatory airway disease (chronic bronchitis or lymphocytic airway inflammation). Additional disorders associated with chronic cough include repeated airway insult due to aspiration injury and bronchiectasis. While acute onset is often expected in dogs with an infectious disease, organisms such as Bordetella and Mycoplasma can also cause chronic clinical signs similar to those seen with these other causes. Unfortunately, there are few distinguishing features that make these conditions easy to diagnose definitively, and many of these disorders are found concurrently 1,2 or sequentially in the individual dog, thus the clinician may require expanded diagnostic testing – and must maintain vigilance – when managing a coughing dog. 

Tracheal collapse is one of the most common causes of cough and airway obstruction in the otherwise healthy dog. It is characterized by dorsoventral flattening of the cartilaginous rings with elongation of the dorsal tracheal membrane into the airway lumen. The etiology is unknown, but some affected dogs have been shown to have a reduced number of chondrocytes in their tracheal rings, which decreases the rigidity of the cartilage. The condition can affect the cervical and/or intrathoracic trachea, and bronchial collapse (bronchomalacia) can be present alone or in combination with tracheal collapse. The cervical trachea collapses during inspiration, while the intrathoracic portion collapses on expiration. Bronchial collapse can be static or dynamic on expiration. Collapse leads to mechanical irritation of the opposing mucosa, which enhances mucosal edema and inflammation and perpetuates further coughing.

Canine chronic bronchitis is an inflammatory condition defined by the presence of cough for more than 2 months of the year for which no specific etiology can be identified. Inflammatory damage to the airways results in epithelial cell hypertrophy and squamous metaplasia, goblet cell hypertrophy, submucosal gland hyperplasia, and mucosal/submucosal inflammation, edema and fibrosis. These result in an increase in the amount and viscosity of mucus and chronic irritation within the airway. Typically, airway inflammation is neutrophilic in nature, although in some affected dogs lymphocytic inflammation can be seen 3.

Bronchiectasis is characterized by irreversible dilatation of the bronchi, and it is often accompanied by suppurative airway secretions. It can result from poorly controlled inflammatory or infectious lung disease, aspiration injury, or smoke inhalation. Loss of normal airway tapering leads to mucus stasis and recurrent pneumonia. 

Aspiration of gastrointestinal contents or micro-aspiration injury may play a role in development of all the diseases described above, and could also exacerbate causes of cough 4.

Incidence and prevalence

When presented with a coughing dog, patient signalment assists in prioritizing the list of differential diagnoses. Cervical tracheal collapse is common in smaller dogs (Pomeranians, Poodles, Chihuahuas, Yorkshire Terriers) while bronchomalacia occurs in both large and small breeds. Similarly, large or small breed dogs can cough due to bronchitis or bronchiectasis. Bronchiectasis is more common in certain breeds, particularly the Cocker Spaniel, but also the Malamute and Standard Poodle, while micro-aspiration or aspiration injury is more likely in animals affected by laryngeal dysfunction, such as older, large breed dogs 5.

History

Most older dogs with coughing related to airway disease (collapse or inflammation) are healthy except for the presence of unrelenting cough. In fact, dogs with tracheal collapse are often young at the onset of signs and then display waxing and waning clinical signs throughout life. The condition can be exacerbated by endotracheal intubation, weight gain, infection or inflammation. Other dogs with tracheal or airway collapse can present in middle age with either intermittent or severe clinical signs. The cough is typically described as being paroxysmal, dry, and “honking”, particularly after drinking, eating or exercise, with excitement, or in hot or humid conditions. Dogs that have bronchomalacia are more likely to have exercise intolerance and expiratory effort in conjunction with the cough.

Chronic bronchitis can result in a cough that is harsh or moist, depending on the type of secretions in the lower airways and the severity of disease. Exercise intolerance or expiratory effort can develop late in the disease course, and pulmonary hypertension can lead to syncope in severely affected animals. However, there are usually few other clinical complaints or historical findings.

Dogs with bronchiectasis can have a moist, productive cough related to accumulation of suppurative secretions. The disease syndrome most closely resembles pneumonia, is of variable severity, and is partially responsive to antibiotics. 

Appetite is not usually affected in dogs with airway disease, and many dogs are overweight, which creates added stress on the respiratory system. Dogs with aspiration-related disease can have a history of vomiting, regurgitation or difficulty swallowing, along with lower respiratory tract signs, but those with micro-aspiration or aspiration pneumonitis can have more subtle signs, such as lip-smacking, or a cough after drinking or lying down. They might also display a retch, gag or cough due to laryngeal accumulation of secretions. Other dogs might have laryngeal signs and display voice change, although this is not always the case either. In one study, almost 20% of dogs with chronic cough demonstrated laryngeal paresis or paralysis in the absence of any history or clinical signs of laryngeal disease 5.

Physical examination

The physical exam should focus initially on respiratory rate and effort. Distinguishing inspiratory from expiratory effort is helpful, because inspiratory effort reflects disease outside the thorax while expiratory effort indicates intrathoracic disease. Dogs with severe cervical tracheal collapse can display inspiratory effort and stridorous respiration. This must be distinguished from laryngeal dysfunction, which occasionally can be found concurrently in individuals with tracheal collapse. More importantly, laryngeal paralysis can result as a consequence of tracheal ring surgery, so it is important to establish normal laryngeal function if this surgery is planned. 

Expiratory effort or an expiratory push is classic for intrathoracic airway collapse, bronchomalacia or bronchitis. An expiratory honk is consistent with intrathoracic collapse of a large airway. Some dogs with dramatic expiratory effort will herniate the cranial lung lobes through the thoracic inlet during breathing or coughing, and this can be visualized or palpated at the base of the neck 6. Crackles (inspiratory and/or expiratory) on thoracic auscultation can indicate opening and closing of airways with airway collapse, or can be heard with mobilization of secretions in dogs with bronchitis or pneumonia associated with bronchiectasis. Expiratory wheezes are considered typical for bronchitis, but are heard in a minority of patients. Induction of a cough during the physical examination can be helpful to confirm the nature of the cough observed at home, but increased tracheal sensitivity is only a reflection of airway inflammation and does not indicate a specific disease process.

Accurate assessment of body condition score (BCS) is very important to determine the contribution of obesity to respiratory signs and in devising a therapeutic plan. Most dogs with airway disease are overweight or obese, which worsens respiratory effort and augments airway collapse. On a scale from 1-9, 5/9 is usually considered ideal, although most respiratory patients would benefit from achieving a BCS of 4/9. 

Careful cardiac auscultation is indicated in all respiratory patients due to the commonality of concurrent cardiac murmur in many small breed dogs. Additionally, respiratory disorders can lead to pulmonary hypertension, and detection of a new heart murmur could signal the development of a secondary cardiac condition that would complicate management of the existing respiratory condition. 

Diagnostics

Blood tests are indicated during the work-up of a coughing dog, both to help prioritize differential diagnoses and to assess the safety of anesthesia. In addition, although the diagnosis of tracheal collapse can be strongly presumed based on the signalment, history, and physical examination findings, a diagnostic work-up should be performed to define concurrent disorders and to provide appropriate therapy. Routine blood tests are typically normal, although a stress leukogram (neutrophilia, lymphopenia, and monocytosis) is not uncommon. Bronchiectasis with pneumonia or aspiration pneumonia would be anticipated to result in neutrophilia, perhaps with a left shift. Peripheral eosinophilia should raise concern for possible eosinophilic lung disease, more common with the severe forms of pulmonary eosinophilia. Liver enzyme elevation and even mildly increased bile acids are common in dogs with airway collapse for reasons that remain obscure 7, although theories include hypoxemia and fatty infiltration of the liver. 

Cervical and thoracic radiographs are not reliable in diagnosing airway collapse 8 but can help elucidate concurrent pulmonary diseases – such as pneumonia or bronchiectasis – and cardiac disorders. When both inspiratory and expiratory lateral views are obtained, it can improve visualization of variations in luminal dimensions: on full inspiration, it is anticipated that the cervical trachea will collapse, while on expiration radiographs can reveal attenuation of luminal dimension in the intrathoracic region or at the large bronchi (Figure 1). Overall, radiographs suffer from a high number of false positives, yet they underestimate the degree of collapse, do not always identify the appropriate site of collapse, and are unreliable in documenting intrathoracic airway or lobar bronchial collapse. Fluoroscopy is more helpful in evaluating dynamic airway obstruction, and also allows correlation of airway collapse with cardiac and respiratory cycles. Cranial herniation of the lung through the thoracic inlet due to disruption of the fascial connections in the area has been reported in 70% of fluoroscopic studies in dogs with cough 6. Inspiratory and expiratory computer tomography (CT) can also document airway collapse 9, although it can be challenging to obtain a cross-sectional image for all bronchi. 

Thoracic radiography in dogs with bronchitis can show a bronchial pattern (Figure 2) or increased number and thickness of airway walls, but in some cases radiographs can also be relatively unremarkable. Bronchiectasis is characterized by dilatation of airway walls and lack of normal tapering towards the periphery (Figure 3), however radiographs are relatively insensitive for confirming this condition. Use of CT to assess airway diameter is more sensitive and documents the extent of disease. 

Bronchoscopy can be used to confirm tracheal and airway collapse and to grade the severity of disease (Figure 4). It is likely the best method available for diagnosing bronchomalacia involving multiple lobar segments (Figure 5) and can also confirm the dynamic nature of disease in smaller airway segments. Bronchoscopy can also identify bronchiectasis (Figure 6) or other irreversible changes such as bronchitis nodules or inflammatory proliferations into the airways (Figure 7). Finally, it allows collection of an airway sample for documentation of infectious or inflammatory airway disease (Figure 8). However, bronchoscopy in dogs with airway collapse can be risky, especially in obese or anxious individuals with severe tracheal sensitivity or marked expiratory effort. Anesthesia can result in loss of active respiratory maneuvers that keep airways open, resulting in failure to recover properly from anesthesia. Additionally, excitement during recovery can result in excessive abdominal effort that potentiates lower airway collapse. 

If bronchoscopy is not available and an airway sample is desired, a tracheal wash sample can be collected for cytology and culture; this might be performed if the animal is anesthetized for an elective procedure such as dental prophylaxis or mass removal. Slow recovery from anesthesia is advisable, with oxygen supplementation and adequate sedation as well as cough suppression.

Treatment

Initial therapy

In the emergency situation where a dog with chronic cough has developed acute respiratory distress due to development of infection, stress, or aspiration-related disease, calming measures are indicated. Supplementation with oxygen and a cool environment are essential. Judicious use of acepromazine (0.01-0.04 mg/kg SC, IM or IV) can be used in combination with butorphanol (0.1-0.4 mg/kg SC, IM or IV), with one or both drugs repeated as needed. Thoracic radiographs can be helpful in determining whether or not administration of antibiotics, anti-inflammatory agents, or mucolytic therapy is required, although repeat airway sampling is needed in some instances.

Chronic management

Owners should be aware that dogs with a chronic cough due to inflammatory or degenerative airway disease virtually always continue to cough. The goal of therapy is to control clinical signs by at least 50% with an acceptable level of intervention, although hopefully even better control can be achieved, and this relatively conservative goal will help manage owner expectations. 

When a complete diagnostic work-up has been performed, treatment is tailored to the results obtained. For infection with Mycoplasma, doxycycline is recommended, while Bordetella infection can require nebulization with gentamicin 10. Owners can be instructed to purchase an ultrasonic or compressed air nebulizer which creates particles 2-5 µm in size that will reach the lower airways. The antibiotic can be placed in the nebulizer cup and administered for 10-20 minutes daily for up to 6 weeks. Aspiration injury does not always require management with antibiotics 11, and in some cases use of acid suppression, in conjunction with physical changes to feeding (e.g., an elevated feeding bowl) to manage laryngeal dysfunction or gastroesophageal reflux disease, can be helpful in alleviating the cough. For suspected bilious vomiting syndrome, consider feeding a small meal before bedtime so that the stomach is not empty for a long period of time. Where a gastrointestinal problem is suspected, a full work-up should also be recommended, including assessment of vitamin B₁₂/folate and abdominal ultrasound. 

In dogs with chronic bronchitis, anti-inflammatory therapy with corticosteroids can be used to break the cycle of mucosal damage and to reduce excessive production of secretions. Oral prednisone or prednisolone can be used at relatively high doses initially (0.5-1.0 mg/kg PO q12h for 5-7 days) and then tapered as rapidly as possible while maintaining control of the cough. Some dogs require alternate day therapy for prolonged periods of time. Exacerbations of disease are treated with an increase in prednisone dosage to the point that effectively controls clinical signs. Animals that cannot be controlled on oral glucocorticoids, or those that suffer excessively from side effects associated with corticosteroid use, can be treated with inhaled steroids by using a facemask and spacing chamber for delivery. In general, inhaled corticosteroids are preferred to oral treatment in order to facilitate weight loss and to lessen other side effects of glucocorticoids, such as panting that can worsen upper respiratory inflammation. In one study, dogs with ACVIM class B2 or C heart disease diagnosed with bronchomalacia based on clinical examination and imaging findings were treated with steroids (fluticasone propionate via a metered dose inhaler with spacing chamber, at 110 mcg/puff, 1 puff BID) without having bronchoscopy performed. All demonstrated at least 50% reduction in cough, as well as improved quality of life, and the owners expressed satisfaction with the ease of treatment 12. This study might suggest that inhaled steroids could be considered appropriate care for some dogs suspected of bronchomalacia even in the absence of a definitive diagnosis. 

Dogs that fail to respond to anti-inflammatory therapy may benefit from the addition of extended-release theophylline (10 mg/kg PO BID) to reduce the effort of breathing and potentiate the effect of corticosteroids. This drug is classified as a bronchodilator, but because dogs with bronchitis and bronchomalacia do not actively bronchoconstrict, this mechanism of action is not pertinent here. Instead, the drug might work through effects on intracellular calcium or adenosine antagonism. Side effects of theophylline include anxiety, anorexia, and diarrhea, but with gradual introduction it is generally well tolerated. Alternately, some dogs with bronchitis or bronchiectasis suffer from excessive accumulation of secretions, and these animals can benefit from saline nebulization to aid in evacuation of mucus. As above, owners can be instructed to purchase a suitable nebulizer and use sterile saline vials once or twice daily to help hydrate secretions. 

In dogs with tracheal collapse and bronchomalacia, a narcotic cough suppressant can be required if coughing persists after inflammation has been controlled. Butorphanol (0.55 to 1.1 mg/kg PO BID-QID) and hydrocodone (0.22 mg/kg PO BID-QID) are most useful; they generally have to be given frequently in the initial stages of disease, then tapered downwards after 24-48 hours. The animal has to be kept very sedate initially to break the cough cycle, but if the dose of narcotic remains high the drug will lose its effectiveness. Similarly, if the drug is started at a low dose and titrated upwards, the animal will become addicted, and the medication will not be effective in controlling the cough. Other drugs, such as tramadol (2-5 mg/kg PO BID-TID) or gabapentin (5-10 mg/kg PO BID-TID), can be considered for use, but they are not as effective.

Finally, obesity is a common problem in the canine population in general and it seems to be over-represented in animals with chronic respiratory disease. Obesity results in poor lung expansion, reduced thoracic volume, and increased work of breathing, and this can lead to worsened cough and respiratory effort. A weight loss program should be recommended, because this alone can result in improvements in gas exchange and reduction in cough. 

The first step in such a program is an accurate assessment of BCS. In a dog with an ideal score (5/9), the ribs and hip bones can be palpated readily and may be visible in smooth-coated breeds; the waist is obvious from above and from the side. For each point above the ideal, a dog would be considered 10% overweight. The second step is to calculate the calories currently being consumed by the dog, with the goal of achieving weight loss through a reduction in calories using its usual diet. Animals can be started on 80% of their current caloric intake. Alternately, the resting energy requirement (RER), (as calculated by 70 x (body weight in kg)^0.75) can be used to determine the daily calories necessary. Use of a low fat, restricted calorie content diet can improve participation in a weight loss program by enhancing satiety and thus reducing food-seeking behavior 13. The high-fiber content reduces the tendency to overeat and improves stool character. However, with a weight loss goal of 1-2% per week, it can easily take months for an appropriate and sustainable weight to be achieved; a 20% weight loss from a 6/9 to a 4/9 will take 20 weeks, or 5 months. Therefore, it is important to give owners specific guidelines, and frequent follow-up with clients improves compliance 14. Providing owners with the opportunity to offer low calorie treats to their pets can also be useful, enhancing the overall success rate. When possible, the animal should be encouraged to participate in gradually increasing amounts of exercise, but collars should be avoided and exposure to excessive heat and humidity minimized. 

Finally, it is worth noting that dogs with cervical tracheal collapse that fail aggressive medical and dietary management can require placement of external prosthetic rings for stabilization of the trachea, or insertion of an internal stent for intrathoracic tracheal collapse. Research is currently being conducted to establish possible methods for stabilization of individual bronchi.

Conclusion

The clinical presentation of the dog with tracheal or airway collapse is generally relatively characteristic, although ruling out concurrent infectious or inflammatory airway disease and confirming the site and degree of airway collapse requires fluoroscopy and airway sampling. Control of cough is best achieved when a definitive diagnosis has been established, but there are multiple reasons why this is not always achieved. Owners may not understand the value of performing specific diagnostic tests or may be unable to afford them. They may also fear placing their dog under anesthesia for advanced imaging or airway sampling, and in some cases the veterinarian shares these fears; here it is necessary to choose the most rational therapy that can benefit the patient without causing harm. Obesity must be aggressively managed with dietary and behavioral modification.