Feline asthma

What it is

Feline asthma is an inflammatory disease of the bronchi caused by type I hypersensitivity to inhaled allergens (pollens, house dust mites, moulds and others). The Th2-type immune response recruits eosinophils into the airway wall and causes:

• Bronchoconstriction from contraction of bronchial smooth muscle, reversible with bronchodilators.
• Mucus hypersecretion and mucosal oedema.
• Over time, airway remodelling: smooth muscle hypertrophy, gland hyperplasia and fibrosis, which make part of the obstruction irreversible.

It is distinguished from feline chronic bronchitis, with neutrophilic inflammation and no marked bronchoconstriction, although both share signs and are often grouped as feline lower airway disease. Tobacco smoke, aerosol air fresheners and dust from some litters can worsen signs.

Who it affects

Young and middle-aged cats, usually 2 to 8 years old. The Siamese appears predisposed.

Anatomy involved

• Bronchi and bronchioles: thickened walls infiltrated by eosinophils, with the lumen narrowed by smooth muscle spasm and mucus plugs. Air is trapped in the lungs during expiration.
• Lungs: hyperinflation; in chronic cases, emphysema or collapse of the right middle lobe when a mucus plug obstructs its bronchus.
• Trachea: inflammation of the tracheal and bronchial mucosa triggers coughing.

Clinical signs

Chronic or intermittent cough (the cat crouches with its neck extended, sometimes mistaken by owners for trying to bring up a hairball), wheezing and tachypnoea. In asthmatic attacks: dyspnoea with expiratory effort (marked abdominal push on expiration), open-mouth breathing and, in severe cases, cyanosis. Between attacks many cats are normal.

Diagnosis

• Exclusion of other causes: lungworms (Aelurostrongylus abstrusus, heartworm Dirofilaria immitis in endemic areas), infection, heart disease, neoplasia.
• Thoracic radiographs: bronchial pattern ("doughnuts" and "tram lines"), hyperinflation with a flattened diaphragm and sometimes right middle lobe collapse. They may be normal.
• Bronchoalveolar or tracheal lavage: inflammation with eosinophil predominance; culture, including for Mycoplasma.
• Faecal examination (Baermann technique) for lungworm larvae and heartworm tests depending on the region.
• Computed tomography in unclear cases. Blood tests are usually normal; peripheral eosinophilia is inconsistent.

Treatment

In an acute attack: oxygen, minimal handling, a bronchodilator (injectable terbutaline or inhaled salbutamol) and a fast-acting corticosteroid. Long term, corticosteroids are the mainstay: oral prednisolone at first, tapered to the lowest effective dose, or inhaled fluticasone through a spacer chamber and mask designed for cats, which reduces systemic effects. Inhaled salbutamol is reserved for attacks; it should not be used alone long term, because it does not treat the inflammation. Environmental allergens and irritants are reduced and body weight is controlled. Allergen immunotherapy is under study. Asthma is not cured, but most cats have a good quality of life with treatment; severe attacks can be fatal.

Key studies

Diagnosis

• Grotheer et al. (2020), retrospective study of 73 cats with asthma and 24 with chronic bronchitis, classified by bronchoalveolar lavage cytology. Cough (95% and 96%), abnormal lung sounds and dyspnoea were equally common, and radiographic lung patterns were abnormal in 94% and 91%. Only blood eosinophilia differed (40% versus 27%), so airway cytology is needed to tell the two apart.
• Gadbois et al. (2009), radiographs of 40 cats with bronchial disease and 40 controls. A bronchial pattern was present in 37 cats and hyperinflation in 31, but five examiners reached a sensitivity of 71–89% and a specificity of 43–74%, with poor to good agreement between them. Radiographs support the diagnosis but cannot confirm or rule it out.
• Chalifoux et al. (2021), case-control study of cats presented in respiratory distress, 54 with inflammatory lower airway disease and 121 with other causes. A history of cough (odds ratio 7.9) and an abdominal component to breathing (odds ratio 5.8) predicted airway disease, pleural effusion made it less likely, and venous blood gases did not help. Survival was 94% compared with 61% in the other cats.

Treatment

• Reinero et al. (2005), randomised crossover study in 6 cats with experimentally induced asthma. Oral prednisone and inhaled flunisolide reduced eosinophils in lavage fluid (5% and 2.5%, compared with 34% on the control treatment); no treatment changed airway hyperresponsiveness. An experimental model with very few cats.
• Reinero et al. (2009), randomised crossover study in 6 healthy and 5 experimentally asthmatic cats. Racemic albuterol (salbutamol) inhaled twice daily for 2 weeks increased airway inflammation, including lavage eosinophils in the asthmatic cats, whereas the pure R-enantiomer did not. This supports the advice not to use salbutamol alone as maintenance treatment.