Small airway disease: Bronchitis in dogs and cats (Proceedings)

Chronic bronchial disease

Bronchial obstruction can develop due to inflammatory infiltrates (eosinophils, neutrophils, or macrophages) or hypertrophy of bronchial tissues. The result is obstructive airway disease. Increased airway resistance with resultant expiratory dyspnea characterizes obstructive airway disease. Chronic bronchitis in dogs may be caused by congenital abnormalities in the structure and function of airway cilia, parasitic infestation, severe viral or bacterial infection (chronic, untreated periodontal disease), inhalation of noxious irritants, and immune-mediated phenomena. Airway epithelium may hypertrophy, undergo metaplastic change, erode, or ulcerate. Goblet cells and submucosal glands may hypertrophy and produce excessive secretions. Bronchial smooth muscle may hypertrophy. With diffuse airway disease, the airways narrow as a result of endobronchial mucus, edema and hypertrophy of the submucosa and bronchial smooth muscle contraction. The consequences of increased airway resistance in the small bronchi are; increased expiratory pressures result in further airway collapse and expiratory dyspnea and increased work of breathing.

Chronic bronchitis in dogs usually occurs in small and toy breeds 8 years of age or older. The dogs usually have a chronic (greater than 2 months duration) cough that may become productive (terminal retch). The dogs with chronic bronchitis usually have no other systemic signs of disease but may be exercise intolerant. When possible the primary source of infection should be treated. Heartworm disease, periodontal disease and chronic pyoderma may serve as sources for continued infection and should be treated.

Feline bronchial disease

There is no clear terminology for the bronchial obstructive diseases in the cat. Bronchitis is inflammation of the airways. Asthma generally implies a reversible bronchoconstriction related to hypertrophy of smooth muscle in airways, hypertrophy of mucous glands, and infiltrates of eosinophils. Asthma in cats is primarily due to Type I hypersensitivity reactions; the etiology is generally undetermined. Cats with bronchitis not due to asthma generally have infiltrates of neutrophils or macrophages as well as hypertrophy of mucous glands, hyperplasia of goblet cells, excessive mucous, and ultimately fibrosis secondary to chronic inflammation. Etiologies include bacterial infection, mycoplasmosis, viral infection and parasitic infections.

Cats with bronchitis can be of any age; chronic bronchitis usually develops in middle-aged to older cats. There is no obvious breed or gender predilection. Primary presenting complaints include cough, dyspnea, and wheezing. Some cats will have a terminal retch following cough. Physical examination abnormalities include cough, dyspnea, and crackles, and wheezes in the pulmonary tissues. Increased bronchovesicular sounds may be the only abnormality noted on auscultation. If dyspnea occurs, it commonly has a pronounced expiratory component. Open mouth breathing or panting commonly occur during periods of stress.

CBC is generally normal with the exception of eosinophilia in some cats with asthma. Thoracic radiographs reveal primarily a bronchial pattern. Overinflation and air trapping is seen in some dyspneic cats with chronic disease. Air bronchograms are commonly seen in some dyspneic cats with bronchitis due to bacterial infection. Cytology of transtracheal wash samples reveals increased mucus with variable numbers of eosinophils, neutrophils, and macrophages. Bacteria may or may not be visualized. Aerobic and Mycoplasma culture as well as antibiotic susceptibility testing should be performed regardless of the type of inflammatory cell and whether or not bacteria are seen.

Cats with eosinophilic TTW cytology should be assessed for dirofilariasis using adult antigen detection tests or newly developed antibody tests. Fecal flotation (Toxocara and Toxascaris in kittens), Baermann examination of feces (Aelurostrongylus), and fecal sedimentation (Paragonimus) should be performed in cats with eosinophilic TTW cytology particularly if indoor-outdoor and from parasite endemic areas.

Respiratory parasitism

The most common respiratory parasites are seen mainly seen in cats and include Aelurostrongylus abstrusus, Capillaria aerophilia, and Paragonimus kellicotti are responsible. Migratory parasitism due to Toxocara and Toxascaris occurs almost exclusively in kittens although it has been seen in puppies. Toxoplasma gondii induces pneumonia in some neonatal kittens but rarely causes respiratory tract disease in adult cats.

Cats are infected by Capillaria by ingesting larvated eggs or infected transport hosts like earthworms. Paragonimus eggs are passed in cat feces and release a miracidium that infects snails. Following a developmental period in the snail, cercaria are released that infect crayfish. The cat ingests infected crayfish. The organism penetrates the intestinal tract, migrates through the peritoneal cavity, diaphragm and pleura to complete its development in the alveoli and bronchi. Adults live in pairs in the lungs leading to a fibroblastic and mononuclear cell infiltrate that forms a cyst around the adults. Adult Aelurostrongylus live in the terminal bronchioles. Eggs are passed into the alveoli; larva are released in alveoli migrate up the trachea to the mouth, and are swallowed. Larvae passed in feces infect snails and slugs that act as intermediate hosts. Transport hosts including rodents, lizards, and birds ingest infected intermediate hosts; ingesting the transport host infects cats. Larvae are released, penetrate the esophagus, stomach, and intestine and migrate to the lungs in the bloodstream. Inflammation is induced by migrating stages and adult stages.

Aelurostrongylus abstrusus, Capillaria aerophilia, and Paragonimus kellicotti infections of cats are generally subclinical. A dry cough is the most common clinical sign. Infection by Aelurostrongylus and Paragonimus occasionally lead to severe respiratory distress. Bronchial patterns predominate with each parasite; Paragonimus induces solid of cavitary interstitial nodules. Pneumothorax develops in some cats infected with Paragonimus. Peripheral eosinophilia and eosinophils on TTW cytology are common. Ova or larva may be demonstrated on TTW cytology. Capillaria ova are demonstrated on fecal flotation. Paragonimus ova are best demonstrated on fecal sedimentation. Aelurostrongylus larvae are best demonstrated by Baermann technique.

Bronchiectasis

Bronchiectasis is the permanent dilation of the bronchial walls as the result of chronic inflammation. In small animals, the condition is typically central and cylindrical (vs. peripheral and saccular in cattle). Routine radiographs may be sufficient to make the diagnosis in severe cases by visualizing dilated bronchi. Bronchoscopy or bronchography are required in less severe or focal cases. There are also congenital forms of bronchiectasis usually associated with immotile cilia syndrome (Kartengener's syndrome). Bronchiectatic airway segments do not by themselves affect pulmonary function. However, bronchiectasis is commonly caused by bacterial infection and these dilated airway wall segments may serve as a reservoir acting as a nidus for chronic bacterial infections.