Introduction

Asthma in human beings is a chronic inflammatory disease within the lower airways (bronchi and bronchioles) that causes cough, wheeze and exercise intolerance. These clinical signs are the result of a decrease in airflow through airways that are narrowed from excessive mucus secretion, airway wall edema and bronchoconstriction. "Feline asthma" is a remarkably similar condition that has been recognized in the veterinary literature since at least 1911, when Dr Hill described cats with increased airway mucus, airway inflammation and the clinical signs of labored breathing and wheezing.

Diagnosis

There are no clinical signs or laboratory tests available in routine veterinary clinical practice that are pathognomonic for asthma in cats. The tests we can perform are most valuable to exclude other common causes of acute dyspnea, wheeze and cough including heart failure, pneumonia, pulmonary malignancy, respiratory parasitism and inhaled foreign body. Fortunately, none of these diseases routinely causes clinical signs of asthma in an otherwise healthy cat. Therefore, we can usually make the correct diagnosis of feline asthma if we examine only a few clinical signs and radiographic findings, including:

1.  A history of a sudden onset of labored breathing that is quickly relieved (usually) with some combination of oxygen, bronchodilators and steroids.

2.  In some cases however, the only clinical problem is chronic cough, or unrelieved dyspnea.

3.  The most important radiographic finding is bronchial wall thickening and air trapping. These changes are usually described as "doughnuts" and "tramlines."

4.  When airway cytology is available, cats with asthma usually have evidence of airway inflammation including large numbers of eosinophils recovered from tracheobronchial secretions. This is not specific for asthma however, as many healthy cats also have large numbers of eosinophils within their respiratory tract.

Pathophysiology

Although there are many potential causes of asthma, the airways respond to inhaled irritants or immunologic stimuli in a limited number of ways:

1.  Airway epithelium may thicken (hypertrophy), evolve to a different structure (metaplastic change), or simply become damaged (erode or ulcerate).

2.  The structures responsible for producing mucus (goblet cells and submucosal glands) may enlarge and produce excessive amounts of a particularly thick form of mucus.

3.  Bronchial smooth muscle will often spasm and may become hypertrophied.

These changes are associated with cellular infiltration of the bronchial mucosa and submucosa, and this tissue may also become edematous. The resulting clinical signs of cough, wheeze, difficulty breathing and decreased exercise capacity are due to airway narrowing (and airflow reduction) from excessive mucus secretions, airway edema, airway narrowing from cellular infiltrates, and airway smooth muscle constriction. Cough may also result from stimulation of cough-mechanoreceptors located in airway epithelium that is inflamed and contracted. This is an important concept, because the effects of even a small degree of airway narrowing to produce clinical signs can be dramatic. For example, a 50% reduction in the diameter of an airway results in a 16-fold reduction in the amount of air that flows through that airway. It is easy to imagine why any movement that requires an increase in the depth of breathing, such as chasing a mouse, will be tremendously altered if there is a 16-fold decrease in the amount of air that comes into the lungs during normal respiration. The important take-home message is that small changes in airway size result in dramatic changes in airflow through that airway. The clinical implications of this finding are twofold. First, relatively small amounts of mucus, bronchoconstriction etc can partially occlude airways and cause a dramatic fall in airflow. On the other hand, therapy that results in relatively small increases in airway size may cause a dramatic improvement in clinical signs.

How Should We Treat Cats With Asthma?

Treatment of feline asthma if symptoms are intermittent (do not occur daily): In these cases I prescribe an albuterol inhaler with instructions to use "as needed". The assumption in these cases is that cats with symptoms that do not occur daily do not have significant chronic inflammation that requires daily anti-inflammatory therapy. When acute signs do occur (cough, wheeze, difficulty breathing) they can usually be effectively and quickly treated with the inhaled bronchodilator. If signs become more frequent the cat should be re-evaluated to determine if more aggressive treatment should be initiated (see next step).

Treatment of Feline Asthma if Symptoms Occur Daily

High Dose, Long Term Corticosteroids

The most consistent, most reliable, and most effective treatment for feline asthma is high dose, long term oral corticosteroids. I begin treatment of asthmatic cats with prednisone, 1-2 mg/kg PO BID for 10-14 days. At this point the majority of newly diagnosed cats with asthma will feel and act much better. Once a beneficial response to oral prednisone has been documented (usually within 3-5 days) I begin inhaled steroids as I wean the patient from oral prednisone.

Inhaled Medications

Corticosteroids and bronchodilators can now be given effectively by inhalation to cats with asthma. Both classes of drugs are available as metered dose inhalers (MDI's) for humans with asthma. Proper use of an MDI requires the patient to coordinate inhaling with the actuation of the device, and this has proven to be surprising difficult for most patients. This is also not realistic for infants and young children. An alternative was developed to allow these individuals to use the MDI's without the need to coordinate their breathing. Thus, the MDI is used in conjunction with a "spacer" designed for used with infants and small children, and a facemask specifically made for cats. 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 end of the spacer that connects with the facemask has an inner rubber gasket that acts as a one-way valve so that the medication within the spacer can only leave the spacer during an inhalation.

The client first attaches the MDI and the facemask to the spacer, and then actuates (presses) the MDI twice to fill the spacer with medication. The client then places the facemask gently over the cat's mouth and nose. The cat is allowed to breath in and out 7-10 times with the mask in place, and the treatment is completed.

The "Spacer"

Company: Trudell Medical (http://www.aerokat.com/)

Why Use a Spacer? It acts as a temporary storage area for the medication to sit in until the animal breathes in. The specific use and prescription for inhaled steroid and bronchodilator medications will be discussed during the course of the lecture.

Summary of Use of Inhaled Medications

Inhaled steroids and bronchodilators are the standard of care to treat humans with asthma. Over the past 4 years we have treated more than 100 steroid-dependent asthmatic cats with twice-daily Flovent, and Proventil on an as needed basis. Approximately 80% of these patients no longer use oral prednisone. The methods described above are effective, practical and very safe, and avoid the complications associated with chronic oral steroid use.

Antibiotics

There is no objective evidence that bacterial infection plays a significant role in the cause or continuation of feline asthma. Similarly, there is no objective evidence that antibiotic therapy has any effect on the duration or intensity of signs displayed by the cat with asthma. It is important to remember that the clinical signs of asthma frequently wax and wane in severity as well as in frequency of occurrence. Anecdotal reports describing the therapeutic effect of antibiotics in controlling asthmatic symptoms are consistent with the "waxing and waning" nature of the symptoms in non-treated cases.

A positive culture result obtained from a tracheobronchial wash does not necessarily imply the presence of a clinically significant airway infection, and should not automatically prompt the clinician to initiate antibiotic therapy. In 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 inferred from the growth of a pure bacterial culture on a primary culture plate, from material obtained from tracheobronchial secretions. 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 (in humans). Antibiotic therapy is then based upon sensitivity data. Prophylactic or long term therapy should also be avoided unless there is documentation of a chronic airway infection. Documented chronic airway infection is extraordinarily uncommon in feline asthma.

There is a possible exception to the statements made above. Mycoplasma species have been isolated from the airway of as many as 25% of cats with signs of lower airway disease. In contrast, Mycoplasma is not cultured from the airway of healthy cats. For this reason, and because Mycoplasma has the potential to cause significant structural damage to airway epithelium, it may be prudent to treat any cat with a Mycoplasma positive airway culture with an appropriate antibiotic.

Anti-leukotriene (or Receptor) Inhibitors

There has been a great deal of recent interest in drugs that 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 moderate to severe asthma in humans. These drugs have not been studied in cats. We have found that LTE₄, the metabolic product of LTC₄ and LTD₄ metabolism, is found in increased amounts in urine of cats with asthma, but not normal cats or cats with non-asthmatic disorders such as kidney failure or pancreatitis. This suggests that leukotrienes are produced in increased amount in cats with asthma. However, direct instillation of LTC₄ into the airways of normal cats has no adverse effect on feline airway structure or function. It is likely that increased leukotriene production, like prostaglandin production, is a clinically non-significant by-product of the general inflammation in asthma. There are no current data that suggest that anti-leukotriene drugs will have an important role in the treatment of cats with asthma.

Summary and Conclusions

Human asthma is not a curable disease, although spontaneous resolution is common in adult asthmatics who developed asthma in childhood. This may or may not be true of cats with asthma. Some cats may be only mildly symptomatic and others may suffer life threatening illness. An important new development in our understanding of this disease is the occurrence of airway inflammation even when patients are symptom free. It is therefore imperative that we direct our therapeutic attention towards the underlying chronic inflammation in addition to the acute clinical signs of cough, wheeze and increased respiratory effort. Client education is also critical so that our clients develop realistic expectations of the effectiveness of these treatments for their pets.