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Asthma
in humans is a lower airway disease that causes people to cough and
wheeze, and limits their ability to exercise.
These clinical signs occur because the airways in asthmatic humans
are hyper-reactive, and may undergo spontaneous broncho-constriction
(narrowing) when exposed to certain stimuli.
A remarkably similar condition exists in cats.
Cats afflicted with asthma have increased mucus in their airways,
airway inflammation, labored breathing, and wheezing.
Although the condition was initially described in cats over 90
years ago, it is only in the last 5 years that the disorder has been
diligently studied, resulting in new information that has lead to better
therapeutic options for affected cats.
Clinical
signs of asthma in cats may be acute or chronic.
In acute cases, there is a history of sudden onset of labored
breathing.
In most cases, however, asthmatic cats are presented to
veterinarians with a complaint of chronic coughing.
There
are no simple laboratory tests that can diagnose feline asthma with 100%
certainty.
Radiographs (x-rays) provide the most information for veterinarians
presented with a coughing cat.
Radiographs help eliminate other causes of respiratory distress,
coughing and wheezing, such as heart failure, pneumonia, lung cancer, and
inhaled foreign bodies.
Identification of thickened air passages (seen as “doughnuts”
and “train tracks” on an x-ray) is the most important radiographic
finding in a diagnosis of feline asthma.
If the cat is stable and the veterinarian elects to obtain a sample
of the lung secretions, the presence of large numbers of eosinophils, a
type of white blood cell, is consistent with (but not specific for) a
diagnosis of asthma.
Because
the clinical signs of asthma (coughing, wheezing) are mainly due to
constriction of the airways, it is tempting to limit the treatment of
asthmatic cats to the use of bronchodilators (drugs that open up the
airway passages).
This strategy may work for cats with intermittent signs, but is
less likely to work for cats with daily signs.
Examination of the airways in humans with asthma demonstrates the
presence of chronic, continuous airway inflammation, and it is suspected
that the airways of asthmatic cats are similarly affected.
For cats with daily symptoms, opening up the air passages with
bronchodilators often is not enough.
These cats require therapy designed to decrease the inflammation of
the airways.
The
most effective long-term treatment of feline asthma is high-dose
corticosteroids.
These drugs are anti-inflammatory, and they help in removing the
inflammatory cells that surround the airways in affected cats.
Humans that receive high-dose oral steroids often experience
serious side effects that preclude their routine use.
Fortunately, cats are fairly resistant to the health-threatening
side effects of corticosteroids.
Prednisone, given at high doses for two weeks, and then slowly
tapered down to every-other-day therapy, remains the most consistent,
reliable, and effective treatment for asthma in most cats.
Some cat owners, however, find it nearly impossible to orally
medicate their cat.
In this situation, an injectable, long-acting steroid may be given,
however, this is a less desirable approach to treatment, as these
injections can last anywhere from 3 weeks to 3 months, and may be
associated with side effects such as transient diabetes.
When the effects of the injection wear off, the clinical signs
usually return.
Bronchodilators
may be given in addition to the corticosteroids.
Aminophylline, theophylline, albuterol, and terbutaline are some of
the more common bronchodilators that can be given orally to cats.
Drugs that stimulate the beta receptors (beta agonists) in the
lungs (albuterol, terbutaline) are, by far, the most effective
bronchodilators available, and many veterinarians use the injectable form
of these drugs in emergencies, when cats are suddenly presented in severe
respiratory distress.
There is no reason why owners of cats who experience occasional,
severe asthma attacks cannot be taught to administer terbutaline
injections, under the skin, much the same way that insulin is administered
to diabetic cats at home.
Cats undergoing an attack often respond in 15 to 30 minutes after
injection; a second dose may be required in severe attacks.
Depending on the response, the owner may then decide if the cat
needs to be seen by a veterinarian immediately or not.
For
cats that cannot tolerate oral terbutaline, or for those who are on
maximum doses of corticosteroids and terbutaline, another drug,
cyproheptadine, may be added to the armament.
Several years ago, it was discovered that serotonin, a substance
found in the inflammatory cells surrounding the airways of asthmatic cats,
causes constriction of the airways (in cats, but not humans).
Experimentally, anti-serotonin drugs block the serotonin receptors
in the lungs, preventing constriction of the airways.
As for the effectiveness of this drug in a clinical setting, the
jury is still out.
This drug is an antihistamine that often causes an increased
appetite in many cats.
Indeed, appetite stimulation is the most common reason for
prescribing this drug in cats.
I have used this drug in asthmatic cats with some success, and feel
that it is worth trying, since the only side effect (increased appetite)
is easily managed and is not a health threat.
Cyclosporine
is a powerful anti-inflammatory drug commonly prescribed in humans who
have received an organ transplant.
The drug helps prevent rejection of the transplanted organ.
Cyclosporine modulates the immune system is a variety of ways, and
may be beneficial in controlling the inflammatory response that occurs in
the airways of asthmatic cats.
This drug is very expensive, and the oral absorption of this drug
is very unpredictable, necessitating close monitoring of the blood levels
of this drug until a stable therapeutic level is achieved.
Cyclosporine usage should be limited to those cats who are
steroid-resistant, or whose signs are so severe and uncontrolled that
euthanasia is being considered by the owner.
As
a veterinarian specializing in cats, the lack of treatment options other
than oral medication had been frustrating.
Fortunately, a new therapeutic option has become available:
administration of medication via inhaler.
Metered dose inhalers (MDIs) are commonly used in asthmatic humans.
They allow high concentrations of drugs to be delivered directly to
the lungs, avoiding or minimizing systemic side effects.
Inhaled drugs relieve symptoms of asthma much faster than oral or
injectable drugs.
The inhalers are designed for people to coordinate the activation
of the device during a slow, deep inhalation, for optimal lung delivery.
Obviously, this cannot be controlled in children or in animals.
Addition of a spacer allows the MDI to be used in children and
pets. The
spacer is a plastic chamber about the size of an inner roll of toilet
paper. The
inhaler fits on one end of the chamber, and a specially designed facemask
attaches onto the other
end. 
To
use the MDI in an asthmatic cat, the owner attaches the inhaler to the
chamber and presses the inhaler twice to deliver the medication into the
chamber.
Doing it this way avoids disturbing the cat with the noise made by
the activation of the inhaler.
The owner places the mask end of the spacer over the cat’s face
and lets the cat breathe in and out about a dozen times.
Several
drugs are available in inhaler formulation.
Short acting beta agonists such as albuterol (brand name:
Proventil; Schering-Plough; Ventolin; Glaxo Wellcome) may be used
to immediately relieve bronchoconstriction in asthmatic cats.
Salmeterol (Serevent; Glaxo Wellcome) is a long-acting
beta-receptor agonist that takes a little longer to take effect (15 to 30
minutes), but lasts more than 12 hours.
It is not recommended for an acute asthma attack, but may be used
in conjunction with daily corticosteroid therapy.
Inhaled
corticosteroids are the most potent anti-inflammatory drugs available.
Their use in people has been shown to improve asthma control and
normalize lung function.
Systemic side effects are minimized because very little drug is
absorbed into the bloodstream; most of the drug remains in the lungs,
where it exerts its anti-inflammatory effects.
There are many inhaled steroid preparations.
Controlled clinical studies in cats are lacking, but anecdotal
evidence suggests that these drugs hold promise for treatment of asthma in
cats, and protocols for therapy with inhaled medications are just now
being published.
This new development allows owners of asthmatic cats, and
veterinarians to “breathe a little easier”.
Updated 2/9/06 |
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