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How I approach… The old coughing dog
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The old dog that presents with a chronic cough can offer a number of diagnostic possibilities, as Lynelle Johnson describes here.
Lynelle R. Johnson
DVM, MS, PhD, Dip. ACVIM (Small Animal Internal Medicine), Dept. of Medicine and Epidemiology, University of California Davis (UCD), USA
Dr. Johnson graduated from The Ohio State University College of Veterinary Medicine in 1987 and pursued a Residency in Small Animal Medicine and Master of Science at the University of Illinois before doing a post-doctoral Fellowship in Radiation Oncology at Colorado State University. She is currently Professor in the department of medicine and epidemiology at UCD, with a special interest in canine and feline respiratory conditions, having authored two textbooks on the subject.
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Key points
- Coughing in the older dog is often related to airway inflammation, airway collapse or bronchiectasis.
- Definitive diagnosis can require blood tests, radiography and airway sampling under anesthesia.
- Chronic diseases typically need long-term management and can be variably controlled, but rarely cured.
- Cough suppressants are necessary in some cases, but can occasionally result in trapping of secretions and worsened disease.
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).
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