Page 100 - Problem-Based Feline Medicine

92 PART 2 CAT WITH LOWER RESPIRATORY TRACT OR CARDIAC SIGNS

Chronic coughing in cats is often caused by feline
INTRODUCTION
asthma/bronchitis complex.
Unlike dogs, coughing is rarely associated with con-
MECHANISM?
gestive heart failure (CHF).
Coughing is a reflex rapid expulsion of air in response
to mechanical or chemical irritation of the pharynx, lar-
DISEASES CAUSING COUGHING
ynx, trachea, bronchi and small airways, to free the air-
way of foreign material.
FELINE ASTHMA/BRONCHITIS
Afferent impulses from epithelial receptors reach the
COMPLEX***
cough center in the medulla via the vagus nerve. A com-
plex series of events transpires from this point, namely:
Classical signs
● Air is inspired to 2–3 times the tidal volume.
● There is closure of the epiglottis and vocal folds. ● Cough is a prominent sign.
● Contraction of the diaphragm, intercostal and exter- ● Expiratory dyspnea with wheezing.
nal abdominal muscles occurs against the closed ● Status asthmaticus –severe acute
glottis, which increases intrapleural pressure respiratory distress.
(Valsalva maneuver) and decreases the luminal ● Cyanosis, open-mouth breathing,
diameter of the small airways. wheezing.
● Sudden opening of the glottis occurs with rapid
expulsion of air from the small airway at speeds of
Pathogenesis
75–100 miles per hour.
● Explosive release carries the offending particulate Exposure of the respiratory epithelium to an
matter clear of the lower airways. inhaled antigen results in multiple inflammatory
responses that create airway obstruction, namely:
In acute conditions (aspiration, inhalation of noxious
● Edema.
fumes, etc.) this cough reflex is a protective mecha-
● Cellular infiltration into airway walls.
nism. In the chronic state, however, it becomes a
● Epithelial hypertrophy or metaplasia with erosion
debilitating problem. Chronic airway irritation
or ulceration.
decreases the threshold of the cough receptor, which
● Excessive mucus production.
brings about more coughing and more mucosal damage
● Decreased mucus clearance.
and irritation.
● Inflammatory exudation within the lumen.
● Hyperreactivity of the airway smooth muscle caus-
WHERE? ing bronchoconstriction fibrosis.
● Emphysema.
Cough receptors are found along the entire airway, but
are especially dense at the larynx and the carina. All Coughing results from enhanced airway sensitivity.
receptors are innervated by the parasympathetic system.
Wheezing is generated by air forced through narrowed
In cats, receptors have been found as far distally as the airways, caused by mucosal thickening (epithelial
alveoli. hypertrophy, infiltration and edema) and bronchocon-
striction.
WHAT? Crackles result from excessive airway mucus.

The most common cause of acute coughing in the cat is The systemic inflammatory reaction, increased work of
foreign body aspiration, often associated with hairballs breathing and airflow limitation all create systemic
trapped in the oropharynx. lethargy.
Aspiration of food or fluid can be more serious, creat- There are multiple inflammatory mediators involved
ing aspiration pneumonia. in the airways of cats: