Chronic bronchitis and asthma are both chronic inflammatory diseases that occur in the lower airways (bronchi and bronchioles). In practice, it is often difficult to distinguish between the two disorders. Chronic bronchitis is defined in part by the presence of a daily cough; asthma is defined in part by acute bronchoconstriction that may resolve spontaneously or with treatment. Bronchoconstriction may or may not cause cough in an individual cat (known as cough-variant asthma). Thus, cats with a daily cough may have either disorder, while cats without cough may have asthma but do not have chronic bronchitis. Both disorders may result in wheeze and exercise intolerance. Finally, human asthma is a syndrome in which the causes may vary greatly, including allergy, exercise, and cold air, in individual patients. For purposes of this discussion, asthma in cats refers to allergic asthma.
Several "asthma genes" have been implicated in the pathophysiology of human asthma, and this is also an active area of research in veterinary medicine. The Siamese breed seems to be overrepresented, suggesting an inherited trait.
There are no precise actuarial data. Chronic bronchial disease (both bronchitis and asthma) probably affects 0.75% to 1% of the feline population in general and may affect 5% of the Siamese breed.
Siamese and Siamese-related breeds appear to be at risk. Cats of any age may develop chronic bronchial disease, although young and middle-aged cats (1 to 8 years) most commonly have the disorder. Male and female cats appear to be equally at risk.
Unknown. The causes of asthma and bronchitis probably differ. The most common causes of asthma in humans include house dust mites, feline "dander," and cockroaches; chronic bronchitis is the result of cigarette smoke in humans, although this association has not been shown in cats.
Unknown. In humans with asthma, there is a 50% chance of developing asthma if both parents are asthmatic. Living indoors or outdoors is not causally associated with development of chronic bronchitis or asthma in cats. Cigarette smoke increases symptoms in human patients with established disease; passive cigarette smoke has been linked to bronchial disease in children. No data confirm this phenomenon in cats, although common sense suggests that exposure to cigarette smoke is probably harmful.
Unknown. In humans, cigarette smoke causes cough, airway wall thickening, and hypertrophy of the mucus-secreting apparatus. Asthma is more complex and is better thought of as a "syndrome" with many causes. The most common form, "allergic asthma," results from a complex interplay of activated T lymphocytes, antigen, antigen-processing cells, various cytokine proteins and chemical messengers, and activated eosinophils and other inflammatory cells in the bronchial mucosa. The inflammatory results of these processes include excessive mucus secretion, airway wall edema, and, in asthmatic patients, bronchoconstriction. Airway muscle hypertrophy and hyperplasia are more common in asthmatic than in bronchitic airways. Basement membrane thickening is a feature of asthma in humans but has not consistently been found in cats.
Cough, mucus production, wheeze, open-mouth breathing, and exercise intolerance directly resulting from the pathophysiologic derangements described above.
Cough and increased respiratory rate and effort are common to asthma and chronic bronchitis. There is usually a gag/choke or swallow at the end of the cough, signifying excess mucus production. Asthmatic patients may have symptom-free periods, whereas a daily cough is the defining feature of bronchitis.
Physical examination may be unremarkable. Most cats with chronic bronchitis or asthma have increased bronchovesicular lung sounds heard diffusely. Wheeze may be heard, especially during expiration.
Respiratory rate may be increased, and there may be nasal flaring. Cyanosis occurs in severe cases.
There are no clinical signs or laboratory tests available in general practice that are pathognomonic for asthma or bronchitis. We pursue diagnostic testing in cats with cough, tachypnea, and/or wheeze to determine the presence of other common causes of these signs-for example, heart failure, pneumonia, pulmonary cancer, respiratory parasitism, and inhaled foreign bodies.
Asthmatic cats often present with a history of sudden onset of labored breathing that is quickly relieved with some combination of oxygen, bronchodilators, and steroids. In some cases, the only sign is chronic or intermittent cough. Patients with chronic bronchitis have a history of chronic daily cough with gag/choke or swallow at the completion of the cough, representing excessive mucus secretion.
The most common radiographic change common to both feline asthma and bronchitis is peribronchial cuffing, often described as doughnuts or tram lines (Figure 1). Other common causes of cough and increased respiratory effort include pneumonia, heart failure, cancer, and heartworm disease. These disorders usually create radiographic abnormalities that differ from the classic findings of hyperinflation and bronchial markings seen in chronic bronchitis or asthma.