Q. Please provide a brief review of diagnosis and treatment of feline asthma.
A Dr. Philip Padrid at the 21st American College of Veterinary Internal Medicine Forum in Charlotte, N.C. gave an excellent lecture on diagnosis and treatment of feline asthma. Some relevant points made in this lecture are provided in this review.
Asthma is a chronic inflammatory disease within the bronchi and bronchioles that causes cough, wheeze and exercise intolerance. These signs are the result of a decrease in airflow through airways that are narrowed from excessive mucus secretion, airway wall edema and bronchoconstriction. There are no signs or laboratory tests available in veterinary practice that proves asthma in cats. The other possible causes of acute dyspnea, wheeze and cough in cats are chronic bronchitis, heart failure, pneumonia (bacterial, rarely fungal), pulmonary malignancy, respiratory parasitism (heartworm, Aelurostrongylus) and inhaled foreign body.
Pulmonary function abnormalitiesIn the last 10 years, research scientists at the University of Illinois, Tufts University, University of California, Davis and the University of Chicago of have shown pulmonary function abnormalities in cats with signs of chronic lower airway inflammation. Some of these cats have increased lung resistance that resolves after treatment with terbutaline (a beta-2 agonist) indicating the presence of reversible bronchoconstriction in these cats.
Additionally, some of these cats experience dramatic bronchoconstriction after exposure to low levels of methacholine, a drug that has minimal effects on pulmonary function when used in equivalent doses in non-asthmatic cats.
This is an important finding as the first demonstration of spontaneous, naturally-occurring airway hyperreactivity in a non-human species. Additionally, research scientists at Michigan State University have begun to unfold the chromosomal location of genes that may play a role in the development of asthma in the feline species.
The primary signs of asthma include cough and wheeze. These signs are frequently caused by airway smooth muscle contraction. It is tempting to use bronchodilators to reverse the airway smooth muscle spasm. In fact, this is the logical treatment for cats with intermittent signs. It is worth recalling that human (feline?) asthmatic airways show evidence of chronic ongoing inflammation whether or not the patient is symptomatic. Therefore, cats with daily signs should be managed toward decreasing the underlying inflammatory component of the disease.
Long-term treatmentCorticosteroids are the most effective long-term treatment for feline asthma. Corticosteroids have many beneficial effects in this setting, including inhibiting the synthesis of genes for cytokines that are important in generating airway inflammation. However, the side effects of chronic, high-dose oral steroid treatment in cats are well known and include pancreatitis, diabetes mellitus and significant behavioral changes in addition to polyuria, cystitis and inappropriate urination.
Fortunately, inhaled steroids are now available that do not cause systemic side effects, and this treatment has greatly enhanced one's ability to successfully treat cats with asthma.
Corticosteroids (fluticasone propionate "Flovent" and bronchodilators (albuterol "Proventil" or "Ventolin") can now be given effectively by inhalation to cats with asthma. Both classes of drugs are available as metered-dose-inhalers (MDIs) for humans with asthma. The MDI is used with a "spacer" specifically designed for use with cats and small dogs (Aerokat, see www.aerokat.com). The spacer is a plastic chamber the size of a cardboard inner role of toilet paper. The MDI fits into one end of the spacer, the other end of the spacer has an attachment for the facemask. The owner first attaches the MDI and the facemask to the spacer, and then actuates (presses) the MDI to fill the spacer with medication. The owner then places the facemask gently over the cats' mouth and nose. The cat is allowed to breathe in and out 10-15 times with the mask in place, and the treatment is completed.
Fluticasone propionateThis inhaled steroid is fluticasone. It comes in three doses (44, 110 and 220 mcg per actuation). The 220 mcg dose is usually used. This drug has virtually no side effects. It has the clinical effect of oral prednisone 1 mg/kg q12h. There are a few potential problems/limitations.
First, the drug takes about 7 to 10 days to reach full effect.
Second, the facemask needs to be properly fit for the animal to ensure that the drug is being inhaled; it should snugly fit around the muzzle including the corners of the mouth. The highest dose (220) cost is about $120 per month. Finally, the owner may not be aware when the canister is empty.
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AlbuterolAlbuterol is a bronchodilator. It only comes in one dose. Albuterol is more rapid acting than the oral, subcutaneous or intramuscular forms of terbutaline.
It is also more effective than the theophylline compounds (Theo-Dur). It may be used daily or as needed for the asthmatic cat already on daily steroids if there is increased cough, wheeze or increased respiratory rate and effort at rest. This drug is often prescribed as needed for cats with intermittent signs of asthma (not daily signs).
Potential side effects include musculoskeletal twitchiness, excitability, insomnia and anorexia. These side effects in cats are very uncommon. In fact, this drug is very safe. It has been reported that profound overdose (cat chews canister) can cause problems with potassium regulation.
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Additional treatmentsIn general, antibiotics are rarely indicated for cats with asthma and are appropriate only when there is good evidence of superimposed airway infection. A true infection may be assumed if the positive culture was obtained after growing the organisms in culture without using an enrichment broth such as thioglycolate.
This is because the concentration of aerobic bacteria recovered from the airways of healthy cats rarely exceeds 5 x 103 organisms/ml. In contrast, growth of a single organism recovered without the use of enrichment broth implies >105 organisms/ml, and this is consistent with an infected airway.
Prophylactic or long-term antibiotic therapy is not indicated unless there is documentation of a chronic airway infection. However, Mycoplasma species have been isolated from the airway of as many as 25 percent of cats with signs of lower airway disease. In contrast, Mycoplasma is not cultured from the airway of healthy cats. Therefore, Mycoplasma has the potential to cause significant structural damage to airway epithelium, and it may be reasonable to treat any cat for Mycoplasma if they do not have an immediate positive response to corticosteroids (within three to five days).
Cyproheptadine is an antihistamine and antiserotonin compound that has been used in feline medicine to increase appetite. Although there have been many anecdotal reports that cyproheptadine is sometimes helpful in these asthmatic cases, cyproheptadine is generally not recommended for these cases.
Anti-leukotriene (or receptor) inhibitors block production of leukotrienes (LTC4, D4) or ligation of these molecules to their receptor(s). In general, these drugs have limited, but significant, effectiveness in treating some human asthmatics with moderate to severe disease. Leukotrienes increase mucus production, induce cellular edema and promote airway smooth muscle constriction in humans and other species.