What disease(s) are we really talking about?
Although feline asthma is among the most commonly diagnosed respiratory conditions in cats, there is still a lot of confusion
about how to define or classify this condition or how to differentiate it from other lower respiratory tract problems.
In recent years a number of different terms have been proposed to help classify disorders of the lower respiratory tract of
cats. In many cases, these are still very general terms that primarily describe localization (e.g. bronchi) or relevant diagnostic
test results (e.g. inflammatory cells in an airway wash). In other cases, there has been an attempt to describe the etiology
(e.g. allergy) or make distinctions based on clinical signs (e.g. bronchitis when cough is predominant, asthma when reduced
airflow or dyspnea). From this larger discussion of terminology it has been suggested, and largely accepted, that feline
non-infectious lower airway (or bronchial) diseases can be divided into two general categories of chronic bronchitis and asthma.
There seems to be general agreement that a diagnosis of asthma should imply certain characteristic features including: 1)
chronicity with variable and recurrent clinical signs, 2) reduction in airflow, 3) bronchial hyperresponsiveness, 4) increased
mucous production, 5) lower airway inflammation, and 6) lack of a specific infectious etiology. Some authors also remain
adamant that asthma in cats has allergic basis and is associated with spontaneous bronchoconstriction, unlike other lower
airway inflammatory diseases. Overall, however, there is currently no widely accepted, standardized specific criterion for
making a specific diagnosis of feline asthma (as opposed to chronic bronchitis).
From a practical standpoint, there is so much overlap in the clinical and diagnostic features of feline asthma and feline
chronic bronchitis that distinguishing the two conditions in a clinical setting can be quite challenging. For an individual
veterinarian, cat, or client it may also not make that much difference given our current level of understanding of the two
conditions and currently available therapeutic options.
Numerous factor have been implicated in the development of asthma in humans, including allergies, environmental irritant substances/pollutants,
exercise, stress, and medications. Almost all of these factors have also been reported to cause lower respiratory tract disease
in cats, but in general, feline asthma has been considered to be an allergic condition, while respiratory problems secondary
to the other factors are often classified as separate conditions.
Cases of true allergic disease, or asthma, likely represent type I hypersensitivity reactions. Initiation of the process
occurs when potential allergens are inhaled, taken up and processed by dendritic cells in the airways, and then presented
in conjunction with MHC II molecules to na´ve CD4+ lymphocytes. In susceptible individuals, when these lymphocytes are activated
in the presence of appropriate co-stimulatory molecules, the immune response is polarized towards a Th2 response. The cytokines
produced by this (especially IL4, IL5, and IL13) orchestrate an inflammatory response in which allergen-specific IgE is produced
and eosinophils, basophils, and mast cells all become involved. When the cat is re-exposed to the allergen, IgE bound to
mast cells becomes cross-linked and results in degranulation, leading to further exacerbation of the inflammatory cascade.
Tissue responses and injury resulting from degranulation of mast cells and eosinophils include smooth muscle contraction,
increased vascular permeability, edema, damage to / sloughing of the protective epithelial lining of the airways, enhanced
local neural responsiveness. Long term changes can include epithelial metaplasia and proliferation, hyperplasia of mucous
glands with excess mucous production, impaired mucociliary clearance, hypertrophy and hyperplasia of smooth muscle, fibrosis,
and emphysematous changes in the pulmonary parenchyma.
In non-allergic bronchitis, similar tissue injury can be seen as a result of the oxidative damage caused by neutrophilic inflammation.