Asthma is quite common in cats and can be as severe as it is in humans. Feline chronic bronchitis and feline asthma are two of the most common respiratory diseases in cats. Although clinical signs such as coughing, wheezing and exercise intolerance are similar, it remains controversial whether chronic bronchitis and asthma are two separate diseases in cats. The main pathological differences between the two diseases is that asthma is primarily an eosinophilic inflammatory condition whereas bronchitis is primarily a neutrophilic inflammatory process. Serotonin appears to be the primary mediator in feline mast cells that contribute to airway smooth muscle contraction
Contents
|
Symptoms
Symptoms of asthma in cats is a bit different to humans. They do not show the ‘wheezing’ that people get. We do not know if they get the feeling of tightness in the chest, but their breathing is certainly more obvious as the disease worsens. Probably the only sign some cats with asthma show is a dry cough (similar to a smoker’s cough) three or four times a day. Usually their chest has a rattle to it when you hold them while they are coughing. Quite often, the cat will hunch over, extend its neck forward and cough in a ‘crane stance.’ Asthma can occur at any age, but it commonly occurs in younger cats over 6 months of age and under three years.
Cause
Cigarette smoke, pollen, dust, carpet mites, stress, and a genetic cause in some breeds such as Siamese and Orientals. The clinical syndrome of feline bronchial disease results from exuberant and persistent inflammation within airways. Regardless of the trigger(s) for allergic bronchial disease, symptoms are instigated by activation of CD4+ lymphocytes and initiation of Th2 immune response. Activation of lymphocytes toward Th2 immunity results in induction of specific cytokines that are protective against parasitic infection or Type 1 hypersensitivity. Th2 immunity is characterised by production of IL-4, IL-5 and IL-13. The interleukins, especially IL-5, are important in differentiation and maturation of eosinophils. Degranulation of feline eosinophils results in elaboration of major basic protein, myeloperoxidase and ribonuclease, responsible for damage and destruction of epithelial lining of airways1.
Smoking – pros and cons
Although smoking is enjoyable to those who smoke, passive smoking is a real problem for cats. Cats living in homes where people smoke cigarettes are more than twice as likely as other cats to acquire a deadly form of cancer known as feline lymphoma, according to a first-of-its kind study in cats conducted by scientists at Tufts University School of Veterinary Medicine and the University of Massachusetts. The study, entitled “Environmental Tobacco Smoke and Risk of Malignant Lymphoma in Pet Cats,” is published in the August 1 issue of the American Journal of Epidemiology. The authors conclude that these findings offer a compelling reason for further study of the relationship between passive smoke and non-Hodgkins lymphoma in humans, which is similar to lymphoma in cats. In sharing their living environments with humans, cats are exposed to many of the same environmental contaminants as their owners, including tobacco smoke. Exposure levels in cats continuously kept indoors may actually be higher than those of human household members, who often spend extended periods of time outside their homes. Cats may become exposed by inhaling the smoke or by ingesting it when they groom themselves and lick particulate matter off of their fur.
Diagnosis
Diagnosis of asthma is not easy, but often X-rays show ‘gun shot’ white areas, which are caused by calcification or hardening of airways. Often the veterinarian will try a short course of cortisone medication to confirm his/her suspicions. Inflammatory bronchial disease should be considered a diagnosis of exclusion. It is considered highly likely when chronic cough or acute onset of respiratory distress is found in young to middle-aged cats. Cells obtained from a tracheal wash (passing small quantity of fluid into trachea under a general anaesthetic) are often rich in eosinophils (see pic below). The cell type of allergy known as the “eosinophil” is copious in the secretions of an asthmatic patient. But even this finding is made complicated since eosinophils do occur in normal feline respiratory secretions. Further, parasitic infections such as lungworm and heartworm also lead to eosinophil-rich respiratory secretions but hopefully other tests have been used to rule these infections out.
Although genetic influences have been noted in human patients, this has not been investigated in cats. Leukotrienes have been implicated in human asthma, however leukotriene metabolites are not increased in bronchial fluid or urine of cats with allergic airway conditions.
Treatment
Antiinflammatory agents
Inflammation is believe responsible for the pathogenesis of bronchial disease and cats normally are given a cortisone tablet every second evening to coincide with the cat’s normal cortisone levels. Corticosteroids reduce inflammation by inhibition of phospholipase A, the enzyme responsible for the initial metabolism of arachidonic acid. Corticosteroids also decrease migration of inflammatory cells into the airways, thus decreasing the concentration of granulocyte products.
An alternative antiinflammatory agent, cyproheptadine, a serotonin-receptor blocker can have good effects. Cyclosporine, an inhibitor of T-lymphocyte activation, attenuates bronchoconstriction. Cyclosporine levels in blood are unpredictable and must be measured weekly, thus making this a less than desirable alternative to corticosteroid therapy.
Bronchodilators
The ß-agonist, Bricanyl (terbutaline) inhaler is the recommended drug of choice for use in cats. It results in bronchodilation from direct relaxation of airway smooth muscle. Intravenous terbutaline has also been trialled, with recommended doses of 0.01 mg/kg parentally every 12 hrs or 0.625mg orally every 12 hrs. Theoretically, down-regulation of ß-receptor density could occur with chronic use, resulting in decreased efficacy of the drug, however this is rarely seen clinically.
Another ß-agonist, Ventolin (salbutamol) inhaler, can be used in cats, but has many side effects, especially on the heart.
Theophylline may provide some relief from clinical signs by preventing acute attacks of bronchoconstriction in predisposed cats. Anecdotally, doses of 10mg/kg orally each evening has had beneficial effects.
Antibiotics should be prescribed based on culture and sensitivity from tracheal washes. If infection with mycoplasma is suspected, a clinical trial of doxycycline (3-5mg/kg orally every 12hrs) can be prescribed. If parasitic infection with lungworm (Aleurostrongylus spp) is suspected, fenbendazole can be administered at 50mg/kg orally every day for 10 days, or ivermectin (300µg/kg orally once a week for 3 weeks).
Atropine, although a potent bronchodilator should not be used because it thickens bronchial secretion and encourages mucous plugging in airways. Also, beta-blockers such as propranolol.
References
1. August, J.R. (2006). Consultations in feline internal medicine. Elsevier Saunders, Missouri