April 17, 2009
Feline asthma is the most common cause of coughing in cats. It is also known as bronchial asthma and allergic bronchitis.
It is considered an allergic disease driven by T-helper 2 lymphocytes against an inhaled allergen. Cytokines are produced which perpetuate the disease.
The pathogenesis includes activation of inflammatory cells, induction of hyperreactivity in airways, synthesis of allergen-specific antibodies, and remodeling of airway tissues.
Serotonin is the primary mediator that contributes to airway smooth-muscle contraction; serotonin is found in mast cells. Inhaled antigens within airways cause acute mast-cell degranulation and thus a release of serotonin. This results in a sudden contraction of the airway smooth muscle.
The disease initially manifests as coughing with the cat assuming a crouched down, extended neck position. The cough is generally non-productive. The disease is often progressive, resulting in bronchiectasis and emphysema.
Cats with severe cases exhibit expiratory dyspnea, wheezing, open-mouth breathing and cyanosis. Harsh lung sounds, crackles and prolonged expiratory phase of respiration also occur in some cats.
The antigens that initiate serotonin release are usually undiagnosed, but the common suspects are grass and tree pollens, house dust mites, smoke (cigarette or fireplace), sprays (hair sprays, flea sprays, household deodorizers), dusty cat litter and flea powders. Food allergy is also a consideration.
Cigarette smoke is becoming a greater suspect in smokers’ households because the pollutants gravitate to the floor or carpet; a cat’s respiratory intake is on or near this level.
A preliminary diagnosis is often made clinically based on the characteristic cough, especially in cats living in an area with a high incidence of allergens or during high-allergen seasons. Radiographs are often used to strengthen the diagnosis.
The most common lung pattern is interstitial, but bronchial and alveolar patterns have been reported. Some cats have normal chest radiographs, and the degree of disease is not always consistent with the radiographic changes.
The right middle lung lobe may collapse in some cats. Rarely, right heart enlargement and lung overinflation, aerophagia or emphysema may occur. A peripheral eosinophilia occurs in about 30 percent of affected cats.
The diagnosis may be confirmed with a bronchial wash or a bronchial alveolar lavage. An increase in eosinophils is expected, but eosinophils (up to 25 percent of leukocytes collected) are commonly found in the respiratory tract of normal cats. Cultures should be performed, but a large portion of healthy cats will have positive cultures.
Heartworm antigen and antibody testing should be considered because heartworms often cause a steroid-responsive cough in cats and may produce a peripheral or bronchial eosinophilia.
Coughing may also be caused by lungworms (regionally), heart failure (rare), chylothorax, pulmonary masses, ascarid larval migration or bronchial foreign bodies. Potassium bromide is another differential as it has been shown to cause an irreversible asthma-like condition in cats.
Treatment is based on the severity of the clinical signs.
Cats with open-mouth breathing should receive oxygen administered via facemask, nasal catheter, tent (made from a plastic bag) or oxygen cage. Corticosteroids are the most consistently helpful drugs as they directly address the allergic reaction.
In a respiratory crisis, rapid-acting steroids should be administered intravenously. Dexamethasone or prednisolone sodium succinate is recommended. The bronchodilator terbutaline can be given by injection.
Non-crisis cats are treated with long-acting injectable, oral or inhaled corticosteroids. If the disease is seasonal, the long-acting injectable or inhaled drugs may be the best options.
If it is a year-round disease, oral corticosteroids may be the best approach. Although inhaled corticosteroids have fewer side-effects than oral or injectable forms, they are much more expensive. They are administered via a feline-sized face mask and spacer.
Bronchodilators may also be used on an intermittent or continuous basis. They may be given subcutaneously, orally or by inhaler. The inhaler product is much less expensive than the inhaled corticosteroid preparation.
Some asthmatic cats have bacteria, including Bordetella bronchisepticus, or Mycoplasma organisms in their lungs; however, many of these are due to colonization and not a true infection. If treatment is performed, culture and sensitivity on samples taken by lung aspiration or tracheal wash are preferred.
If culture is not available, the preferred drugs are a combination of a fluoroquinolone (enrofloxacin, marbofloxacin, etc.) and amoxicillin plus clavulanate potassium. Alternatively, doxycycline may be used.
Weight reduction, if indicated, is helpful to some cats.
Restricting the cat’s access to the listed allergens, especially dusty kitty litter, aerosol sprays and cigarette smoke, should be tried, but this is usually not curative. A food trial can also be tried in case the allergen is a food product, but this is also a low-yield procedure.
The prognosis is good in the short term; however, some chronically affected cats develop pulmonary fibrosis and emphysema.
Dr. Norsworthy, Dipl. ABVP (feline), practices at Alamo Feline Health Center in San Antonio. He frequently speaks at veterinary conferences. His newest book, “The Feline Patient,” is available from Blackwell.
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