Comorbid Conditions, Diagnostic Conundrums, and Differential Diagnoses
Hairballs and reverse sneezing might be mistaken for coughing by caregivers. Taking a thorough history and requesting video footage of the cough should clarify uncertain cases.
Investigating respiratory disease involves looking at the entire airway, including the upper respiratory tract.Asthmatic airway inflammation is typically eosinophilic (with eosinophils >17% on BAL cytology), however a neutrophilic (neutrophils >7%) and mixed inflammatory reaction (eosinophils>17% and neutrophils>7%) may be observed (Johnson and Vernau 2011; Lin et al. 2015). Eosinophilic inflammation may be found in other respiratory diseases (e.g., parasitic) as well as with bromide therapy, so this finding is not specific for feline asthma. Additionally, airway inflammation may be non-uniform between varying lung areas and collecting more than one BAL sample is advisable (Johnson and Vernau 2011; Ybarra et al. 2012).
Chronic bronchitis differs from feline asthma in that it is primarily a non-degenerative neutrophilic airway inflammatory reaction, which causes airway remodeling and fixed irreversible changes in airway resistance. Differentiating feline asthma from feline chronic bronchitis is challenging (Nafe et al. 2010; Reinero 2011; Allerton et al. 2013; Trzil and Reinero 2014; Lin et al. 2015). Unlike feline asthma, sporadic bronchoconstriction does not occur in chronic bronchitis and bronchodilators are ineffective in resolving respiratory distress (Nafe et al. 2010). This differentiates asthma from chronic bronchitis (Trzil and Reinero 2014; Lin et al. 2015). There is, however, significant overlap between these two conditions. The history, physical examination, and radiographic features may be similar between the two entities. While differentiating between chronic bronchitis and feline asthma is ideal, it is often difficult in the clinical setting and also probably of limited value until targeted therapies become available for clinical use and are proven to affect outcome.
Of clinical significance is the sporadic bronchoconstriction seen in feline asthma, which must be managed as a respiratory emergency. Definitive diagnosis of asthma allows pre-emptive measures to be put in place, such as training caregivers for home emergency treatment. Fortunately, in most cases collecting a good history will provide enough information to discriminate between the two clinical entities. Biomarkers, as a tool of differentiating between various inflammatory lung diseases, remain research tools (Nafe et al. 2010; Reinero, Liu, and Chang 2012).Bacterial pneumonia mimics the respiratory signs seen in asthma. Even the absence of visible intracellular bacteria on BAL cytology does not exclude bacterial pneumonia, necessitating treatment trials in selected cases (Johnson and Vernau 2011). In one study, cats with bacterial pneumonia did not have a BAL eosinophil count >21% (Johnson and Vernau 2011). The role of Mycoplasma in feline asthma is uncertain and diagnosing mycoplasma lower respiratory infections is difficult. A 42-day treatment with doxycycline may be appropriate to eliminate this bacterium as a contributing factor, especially in cases that do not undergo lung sampling (Foster and Martin 2011).
Comorbid conditions and complications associated with feline inflammatory lower airway disease include tracheal and bronchial collapse, bronchiectasis, pneumonia, lung lobe torsion, dynamic upper airway obstruction, pulmonary hypertension, and pneumothorax. Further differentials that cause respiratory distress and a change in breathing pattern include pleural effusions, cardiogenic and non-cardiogenic causes of pulmonary edema, and pulmonary fibrosis.
In cases where a complete and thorough workup cannot be undertaken (e.g., caregivers cannot afford or are unwilling to undergo a complete or even a partial investigation, uncooperative patients) a practical approach would include treating for parasites and attempting a therapeutic trial with corticosteroids after the patient's clinical picture has been critically considered.
If at all possible, a minimum database (blood, urine, and fecal analyses) together with thoracic radiographs should be done. If the clinical picture is consistent with asthma, invasive diagnostics (bronchoscopy with BAL) may not be necessary at initial presentation due to the risks involved (Padrid 2014). A good history and physical examination can go a long way to clinically excluding differentials. The longer the history of coughing, in an otherwise healthy cat with a negative examination finding, the greater the probably it has inflammatory lower airway disease; especially considering that feline asthma is one of most common respiratory disorders. A history that is short (days to weeks) in an ill cat is likely to have another diagnosis (e.g., pneumonia) and caregivers should be convinced to undertake some form of diagnostic investigation. Needless to say, this approach is a compromise and caregivers must clearly understand the advantages and risks.