Feline asthma (Proceedings)

Asthma is a common, although poorly defined, condition in cats. Some authors attempt to classify cats with lower airway diseases into asthma or chronic bronchitis. Clinical signs include cough (occasionally productive), wheezing and respiratory distress. Some cats with asthma cough their entire lives but seem otherwise unaffected while some cats have episodes of severe respiratory distress separated by months to years. At this point, perhaps asthma can be best described as a lower airway disease with some components of asthma (reversible bronchoconstriction) and some component of chronic bronchitis.

The pathophysiology of asthma in cats in incompletely understood. The airways of asthmatic cats may respond in a limited way. The lining (epithelium) may hypertrophy or undergo erosive or metaplastic changes. Mucous production is increased and the mucosa and submucosa may develop edema and may be infiltrated with inflammatory cells. The bronchial smooth muscle frequently hypertrophies, resulting in narrowing of the airways. The clinical signs of cough and respiratory distress are easily appreciated as excessive mucus and smooth muscle hypertrophy limits the flow of air. Airflow rates are highly dependent on the diameter of the airway. The hallmark of human asthma is bronchial reactivity with reversible bronchoconstriction. Recent investigations have suggested that the relationship between T lymphocytes and eosinophils locally in airways may be particularly significant.

These figures represent the changes in airway caliber that occur with lower airway disease. Significant increases in airway resistance are present.

Feline airways

The history of affected cats is generally either chronic coughing or an acute onset of respiratory distress. In some cats, seasonal variations are detected and city cats seem to have a higher incidence than cats from more rural environments. Anecdotally, cats from smoking households appear to be at increased risk.

Physical examination may be normal in cats with chronic cough or may reveal moderate to severe respiratory distress. Pronounced crackles or wheezes may be present on auscultation. The expiratory phase in particular may appear pronounced or lengthened. Some cats have a history of severe respiratory infection during kittenhood.

Radiographs will often demonstrate an increased bronchial pattern ("donuts"), with hyperinflation. Occasionally the right middle lung lobe is collapsed. Radiographs are also very useful for excluding other causes of respiratory distress such as congestive heart failure. Radiographs may also appear normal.

Routine blood testing is usually normal although peripheral eosinophilia (> 1500 μL) may be present in asthmatic cats. A transoral tracheal wash may be performed to evaluate cytology of the airways. Asthmatic cats often have an increased eosinophil count although other inflammatory cells may be present in wash samples as well. Occasionally, a bacterial infection may be present and may complicate diagnosis and therapy. It is important to realize that at tracheal wash may be stressful in a cat that has respiratory compromise. In our practice, tracheal washes are performed much more frequently in cats with chronic cough than those with acute distress. Our protocol involves non-stressful placement of an intravenous catheter, pre-oxygenation for 5 minutes, then induction with propofol (2-10 mg/kg slow iv to effect), placement of sterile endotracheal tube and then washing with 2-3 ml aliquots of sterile saline. A sterile specimen cup may be placed at the end of the tracheal tube to collect any secretions that are expectorated. The sample should be submitted for cytology and aerobic culture. Some cats appear to have a secondary bacterial infection, particularly those with right middle lung collapse. Mycoplasma infection may also play a role in triggering an asthmatic response in some cats.

Pulmonary function tests such as tidal breathing flow-volume loops, measurement of dynamic compliance or lung resistance, and barometeric whole-body plethysmography have also been used to document feline asthma in referral hospitals.

In most cases, it is appropriate to exclude other causes of airway inflammation or cough. Aelurostrongylus abstrusus (lungworm) infections may also develop in cats. Clinical signs may appear initially similar to feline asthma. However, affected cats are usually younger and live primarily outdoors as a snail/slug intermediate host is required. Larvae may be identified on a tracheal wash or via a Baermann fecal. Fenbendazole is used for treatment. Occasionally other internal parasites may migrate through the lungs and cause an allergic response (ascarids) in young cats. Heartworm infection is another possible trigger for allergic lung disease in cats. In recent years, heartworm infection has been described more frequently in cats, even northern or indoor cats. Diagnosis may be challenging as affected cats have a small worm burden, but may be made through serology or occasionally echocardiography.

Treatment of the asthmatic cat includes glucocorticoids (prednisone or long-acting reposital preparations such as Depo-Medrol®) and bronchodilators. (See table 1) Commonly used bronchodilators include theophylline and beta- 2 agonist like terbutaline or inhaled albuterol. Newer proposed treatments include leukotriene antagonists, cyclosporine A, anti-interleukin 5 antibodies or cyproheptadine. Individual cats may vary in their response to various therapies. In an emergency setting, the severely asthmatic cat ("status asthmaticus") should be treated with oxygen, minimal handling, injectable rapid-acting glucocorticoids, and possibly beta-2 agonists. If a response is not seen within 6-12 hours, it may be wise to re-consider the diagnosis. Long-term therapy tends to reflect both the owner and the clinician's preferences. My usual choice is prednisone or Depo-Medrol® for those cats that have had moderate to severe signs. Some cats do very well with either theophylline or terbutaline. I have also treated some cats with Accolate® successfully, although these were cats that did not tolerate glucocorticoids well due to other conditions (eg diabetes/ congestive heart failure).

Table 1. Medications used in the treatment of feline asthma

Aerosol medications represent an exciting opportunity for some asthmatic cats. Flovent (glucocorticoid) is commonly used at dose ranges from 44 to 220 Q 12-24 hours. All inhaled steroids are very expensive, particularly in contrast to oral prednisone. The AEROCAT system has been the most widely used, and is available at www.aerocat.com/ Aerosol medications will be absorbed and may have systemic effect.

The prognosis is usually good, although some cats have recurrent bouts and require frequent medications.

Reference available upon request