CANINE BRONCHITIS


INTRODUCTION

The purpose of the respiratory system is to provide an area where the body can absorb oxygen during inspiration and eliminate carbon dioxide during expiration. The respiratory system consists of two portions: the conducting portion and the respiratory portion. The conducting portion transports air to and from the lungs. This consists of the nose, pharynx (throat), larynx (voice box), trachea (windpipe) and bronchii (small air passages). The respiratory portion, where the gas exchange occurs, consists of the bronchioles (microscopic air tubes) and lungs.

Before entering the lungs, the trachea (windpipe) divides into a right primary bronchus which goes to the right lung, and a left primary bronchus which goes to the left lung. After entering the lungs, each bronchus divides many times, establishing a network of small air tubes. This network of air tubes (bronchioles) resembles the branches of a tree. The small, microscopic bronchioles terminate in the alveolus, the area of the lungs where air exchange takes place.

Bronchitis is an inflammation of the inside lining of the bronchial tubes. Chronic bronchitis refers to bronchitis of long duration.

Chronic noninfectious bronchitis is generally seen in older dogs. It is common in poodles, terriers, and beagles. Chronic infectious tracheobronchitis (also called "kennelcough") is more common in young dogs.

Coughing is the major clinical symptom associated with chronic bronchitis. Bouts of coughing occur more frequently and become more severe as the condition progresses. Dogs with chronic bronchitis do not tolerate periods of exercise. Shortness of breath, wheezing, gagging, and retching are common symptoms that are observed after exertion. Severely affected dogs may faint after coughing.

Dogs with noninfectious bronchitis usually have a good appetite. Weight loss is generally not a problem. Even though they cough, these dogs are still fairly active. Dogs with infectious bronchitis are often tired, refuse to eat, and have a fever.

Chronic bronchitis (CB) is an inflammatory airway disease, which, in association with tracheobronchial collapse, is probably the most common chronic canine airway disorder. Inflammation within airways causes chronic cough and excessive mucus production. Because, dogs can’t expectorate (spit), it is not always easy to know if the dog is making increased airway mucus. Therefore, the diagnosis of CB is usually based on chronic cough alone.

Because we diagnose CB on the basis of a daily cough, we need to be sure that other causes of chronic cough such as heart failure, heartworm infestation, pneumonia, lung tumor, etc., have been ruled out. This can be complicated, in part, because CB is a disease of older dogs, and these animals may have any of these other, co-existing disorders, which can by themselves, cause cough. Additionally, certain drugs used to treat CB in dogs may be inappropriate and even contraindicated for disorders other than CB. Importantly then, the diagnosis of CB must be made with some degree of certainty to avoid potential complications related to therapy.

CLINICAL FINDINGS IN DOGS WITH CHRONIC BRONCHITIS (CB)

Dogs diagnosed with CB are generally > 6 years of age. There does not seem to be a clear sex or breed predilection although lots of small and toy breeds such as Poodles and Pomeranians have been clinically diagnosed with CB.

History

By definition, dogs with CB have a chronic cough. This cough is generally deeper and “throatier” than the high pitched “honking” cough caused by extrathoracic tracheal collapse, and yet harsher than the “soft moist” cough caused by pneumonia. To figure out if increased mucus production is associated with the cough, ask the client if the cough terminates in gagging, swallowing or choking. If so, the dog is coughing up and then swallowing the mucus.

Some dogs with CB may be otherwise absolutely normal while others will be severely exercise-limited by their disease. The difference is probably due to the amount of cartilage weakening that is present, and the resulting airway collapse that occurs when the easily fatigued dog begins to exercise. These animals are otherwise bright, alert, and in all other respects, systemically well. Chronic bronchitis in dogs does not cause depression, lethargy, anorexia (appetite loss), etc. If these signs are present, you should consider other disorders that cause cough.

Diagnostic Tests

Because the diagnosis of CB is based on a history of chronic cough, it is only necessary to perform those diagnostic tests that help to determine the presence of other disorders that cause cough. A good physical examination is important.

Thoracic Radiographs (x-rays): Thoracic radiographs of dogs with CB may appear normal. This finding does not rule out CB!!!! More commonly however, thoracic radiographs reveal the presence of “doughnuts” and/or “tram lines” which are prominent and thickened bronchial walls seen on end or in parallel, respectively.

Bronchopulmonary Cytology: The types and kinds of cells collected provide information as to the cause of the cough. We can see allergy cells, infection cells, cancer cells, bacteria, fungi, etc. in dogs having a chronic cough

Techniques such as bronchoalveolar lavage (BAL) allow the wash fluid to come into contact with the lung surface and result in retrieval of a higher percentage of alveolar macrophages compared to trans-tracheal washing (TTW).BAL - under anesthesia, an endoscope is sterily inserted through the mouth, into the trachea and bronchi. This not only allows collection of cells, but also visualization of any mechanical abnormalities. TTW is performed with an awake (or, lightly sedated) patient. A catheter is inserted, through the trachea, down into the bronchi; fluid is flushed in and removed for analysis.

Tracheobronchial Culture. A presumptive diagnosis of “bacterial” bronchitis is most commonly made when cultured airway samples grow a mixed population of bacteria. Remember though, airways of all species studied, including dogs, cats and people, retain small numbers of bacteria throughout the day. That is one of the reasons we cough and clear our throat.

Bronchoscopy

Tracheal collapse is common in dogs with CB. This finding does not rule out CB, but instead reflects concurrent tracheal disease in association with CB.Bronchoscopy can detect collapse that may not be visible on an xray.Dogs with intrathoracic airway collapse respond only marginally to therapy, and in general, have a less fortunate prognosis.

THERAPEUTIC OPTIONS

Chronic bronchial inflammation, regardless of cause, causes mucosal and airway wall thickening, mucus hypersecretion, and some degree of airway smooth muscle constriction. The resulting signs are the defining features of canine CB and include cough and exercise intolerance. The primary treatment of CB is based entirely on controlling airway inflammation. The guiding principle of any therapy must always be “if in doubt, do no harm.”

Corticosteroids

Steroids have been used to treat humans with bronchial disease for over 50 years. They are clearly the most effective treatment for this disorder, although potentially debilitating side effects limit their use in this clinical setting. Even though steroids are not primary anti-cough medicines, by decreasing inflammation they may decrease stimulation of airway sensory nerves that are responsible for initiating cough in canine CB. Additionally, steroids decrease the volume of mucus produced by bronchitic airways. Steroids are the most effective drugs available to treat dogs with CB, and should be considered the mainstay of chronic therapy. Prednisolone given orally is usually the first line of treatment. The first week or two of treatment will cause the most dramatic decrease in clinical signs and this is usually as good as the dog will ever get on prednisone. Tapering continues to the lowest effective dose that controls > 75% of the cough. If the cough returns using a dose of prednisone that causes significant side effects inhaled steroids (flovent) twice daily can be used. Because inhaled steroids have less systemic effects, inhalers are an appropriate delivery system.

Bronchodilators

Bronchodilator therapy by inhalation is safe and easy to administer (albuterol). Because it is not clear which dogs with CB will benefit from bronchodilator therapy, it can be attempted in any dog with CB that does not have a great response to steroids.

Antibiotics

Bacterial infection probably doesn’t play a significant role in most cases of canine CB. A positive culture result obtained from a tracheobronchial wash does not necessarily imply the presence of a clinically significant airway infection and should not lead to antibiotic therapy, unless there was a pure bacterial culture on a primary culture plate. This is because bacteria can be recovered from the airways of healthy cats, dogs, and humans. Of course, if a primary culture is returned, antibiotic treatment should begin based on culture and sensitivity data.

Cough Suppressants

Chronic airway inflammation causes production of lots of thick mucus, probably as a protective mechanism to trap the offending irritant from reaching the lung. Coughing is very important to clear this mucus and should be thought of as a protective physiologic reflex. However, there are many cases in which the cough is dry and non-productive. In these situations, the cough is not protective and serves to further irritate the airway, leading to a vicious cycle of cough-irritation-cough. In addition, some dogs with chronic cough are unable to sleep and may awaken their owners at night. Occasionally, some dogs with chronic cough may become syncopal. In each of these clinical settings, cough suppression may be indicated. Hycodan (hydrocodone bitartrate) is a prescription anti-cough medication.

In practice, the most common side effects of high doses of hydrocodone in dogs are drowsiness and constipation. One teaspoon of metamucil can be used for the constipation and a reduction in the dose of hydrocodone is suggested.

Other Drugs

Drugs which break-up, or "loosen" the mucus have been suggested as a form of therapy for dogs with airway disease associated with excessive secretion of mucus. Drugs such as guiafinesin and acetylcysteine are capable of breaking the disulphide bonds that are partially responsible for the particularly viscid nature of airway mucus. Saline, inhaled from a nebulizer is very helpful and has no side effects.

  • Aerokat (inhaler for dog = AeroDawg) Aerokat (inhaler for dog = AeroDawg)
  • Flovent inhaler (steroid) Flovent inhaler (steroid)
  • Albuterol inhaler (bonchodilator)Albuterol inhaler (bonchodilator)
(These are Metered Dose Inhalers - MDI's)

Each MDI holds a specific number of doses, and it is important to keep track of the doses used, so that you know when it is time to replace it. The MDI will not be completely empty, but it will not deliver an accurate metered dose of the drug, after you have used the number of doses indicated. The 120 dose Flovent 220 mcg MDI (orange, cost $107) will need replacement after 30 days at 2 puffs each morning and night, so a reminder to get it refilled on your calendar. The 200 dose Albuterol 17g MDI (gray, cost $16) will last 50 days if 2 puffs, twice a day schedule. Some dogs are able to decrease to once a day or only use Albuterol when having an attack. Sometimes lower dose MDI's are prescribed. The benefit of the inhalers is that the medication is LESS likely to affect the entire body and gets directly into the lungs.

The masks that come with the AeroDawg spacer pictured above, does not fit normal dogs well. We suggest ordering an anesthetic face mask from your veterinarian, in the appropriate size for your dog.

Pictured are the masks available. The 3 smaller ones work well for cats, and for small dogs with flatter faces. The other 2 can be obtained in a variety of sizes for dogs.

Emergency treatment may employ bronchodilators, oxygen, rapid-acting glucocorticoids, and epinephrine. If your cat has heart disease, the attending veterinarian should be advised since epinephrine is best avoided.

Extreme respiratory distress constitutes an emergency and the dog should receive immediate attention.

PROGNOSIS AND CONCLUSIONS

Canine CB is a common, progressive, and chronic airway disorder. Signs can be greatly improved but the disease is not curable. Establishment of excellent client communications is critical so that client expectations are realistic and so that the therapeutic regime established by the clinician is adhered to.