Lower respiratory tract disease produces typical clinical signs in cats, including chronic cough and wheeze as well as dyspnea
that may have a sudden onset. Owners may report an increase in respiratory rate (>30-40 breaths per minute), increased expiratory
effort and lethargy. Clinical signs may be mild to severe and may be chronic or intermittent.
Diagnosis involves a thorough history and physical examination as well as a minimum database (complete blood count, serum
chemistries, urinalysis, and retrovirus serology). Other diagnostic tests for feline lower airway disease include feline heartworm
serology, thoracic radiographs, bronchoscopy and/or bronchoalveolar lavage with cytology and culture, and fecal analysis.
Differential diagnoses include heartworm disease, viral, bacterial or fungal infection, inhaled foreign body, cardiac disease,
thoracic disease, neoplasia, and pulmonary parasites (ascarids, lungworms, lung flukes). Essentially, a diagnosis of feline
asthma is a diagnosis of exclusion.
Feline asthma is one of a spectrum of conditions under the umbrella of chronic lower airway disease or bronchopulmonary disease
that also includes chronic bronchitis. Feline asthma may also be called allergic airway disease or allergic bronchitis.
Since feline asthma is characterized by airway inflammation and bronchoconstriction, therapy is aimed at reversing these changes.
There are many treatments for feline asthma, including some experimental modalities borrowed from human medicine. Many of
these treatments have not been well evaluated in the cat.
Heartworm Associated Respiratory Disease (HARD)
The cat is considered a resistant, yet susceptible host for Dirofilaria immitis. Worm burdens are much lower in cats than in dogs (average 15 worms in dogs and 1-3 in cats in endemic areas) and about 1/3
of feline infections involve worms of the same sex. Feline heartworm (HW) was first described in the 1920s; awareness has
increased greatly since the introduction of Heartgard for cats in 1997 and the associated marketing campaign. Feline HW remains
a difficult to diagnose, yet fully preventable disease.
Cats are infected with HW in the same way as dogs, but far fewer larvae mature to adulthood. It is difficult to estimate prevalence
of feline HW for several reasons – there is no ideal test, inapparent infections go unnoticed, and some cats die acutely without
a diagnosis. The prevalence of immature infections is higher than the adult infection rate. Based on necropsy surveys of shelter
cats, feline HW is thought to be present at about 5-15% of the canine rate in endemic areas. Certainly wherever canine heartworm
is found, feline HW is present as well.
Many cats will have no clinical signs of HW disease and they will spontaneously eliminate the infection without incident.
Other cats may have clinical signs associated with infection at two possible time points:
1) Upon arrival of immature worms (L5s) in the pulmonary arteries and arterioles in the 3 to 6 month post-infection period.
The high mortality of immature worms stimulates a severe vascular and parenchymal inflammatory response. Pulmonary lesions
may be long-lasting. The clinical response in the cat is termed HARD because respiratory signs predominate (dyspnea, tachypnea,
and cough). The clinical signs may be transient or intermittent. Clinical signs subside as the worms mature. Many cats with
HARD are misdiagnosed as having asthma or bronchitis.
2) Upon death of adult worms, with release of antigens and toxins leading to pulmonary inflammation and thromboembolism. Clinical
signs include rapid onset of respiratory compromise or sudden death (occurs in 10% or more of HW infected cats). Even the
death of 1 adult worm can be lethal by causing circulatory collapse and respiratory failure. Adult worms are able to suppress
pulmonary intravascular macrophage activity and so actually induce little inflammation until they die in 1 to 2 years.
Nonspecific clinical signs associated with feline HW include chronic vomiting (present in 25-33% of cases), lethargy, anorexia,
and weight loss. Less common signs due to aberrant migration include ascites, pneumothorax, chylothorax, neurological signs
(ataxia, seizures, syncope, collapse, blindness, vestibular signs), and hemoptysis. Signs of cardiac disease or failure are
very uncommon in cats with HW.