Managing equine acute lower airway disease

Causes of acute LRT dysfunction

• Severe asthma (heaves, summer pasture asthma)
• Pulmonary oedema
• Bronchopneumonia
• Eosinophilic pneumonia
• Interstitial lung disease
• Near drowning
• Smoke inhalation
• Neoplasia
• Anaphylaxis
• Pulmonary thromboembolism

Assessment of the breathing pattern can yield valuable diagnostic information. Most dyspnoeic horses have an increased rate and depth of breathing. Slow deep breathing is a concern because it suggests severe URT or LRT obstruction, when airflow is so severely restricted that the breathing rate cannot be elevated. Fast shallow breathing occurs with conditions which make breathing painful (eg pleuropneumonia), restrictive or interstitial lung diseases (eg pulmonary oedema) and pleural effusion/ pneumothorax. Horses which have dynamic collapse of intrathoracic airways (eg asthma) have dyspnoea predominantly during expiration, resulting in a prolonged and laboured expiratory phase, and exaggerated expiratory contraction or‘heave’of the abdominal muscles. In contrast, inspiratory dyspnoea results in a prolonged and laboured inspiratory phase and exaggerated activity of the intercostal muscles during inspiration. In both types of dyspnoea, the nostrils are usually dilated throughout the breath cycle.

Investigation may also require endoscopy, cytology and bacteriology of airway secretions, thoracic radiography and ultrasonography, arterial blood gas analysis and/or pulmonary biopsy. Management of acute LRT disease includes removal of inciting causes (eg organic dusts, allergens), bronchodilators, anti-inflammatory drugs, intranasal oxygen, frusemide, antimicrobials and salbutamol.

Asthma

The most common cause of acute LRT dysfunction is readily recognised when severe. However, as a consequence of increased client awareness, cases are increasingly presented early in the disease course, when they have only occasional coughing or mild exercise intolerance, and before they develop overt expiratory dyspnoea. Consequently fewer cases are now diagnosed confidently from history and clinical findings alone, and there is increased reliance on the use of ancillary diagnostic techniques. Controlled exposure to hay/straw may be done to exacerbate clinical signs and thereby aid diagnosis of mild cases. Asthma is typically effectively managed by removal of inciting organic dusts and allergens, administration of bronchodilators and glucocorticoids, and if severe, intranasal oxygen supplementation. Failure of asthmatic horses to respond adequately may indicate 1) incorrect diagnosis – consider other disorders such as equine multinodular pulmonary fibrosis, 2) inadequate dust/allergen control – this is common - visit and assess environment and management, 3) failure of bronchodilator therapy – B2 -agonists provide a clinically evident benefit effect only in some horses - consider assessing the response to atropine – and note that airway obstruction may be due to mechanisms other than bronchoconstriction, such as mucus plugging and airway oedema, or 4) irreversible airway remodelling including bronchiectasis, but significant pulmonary fibrosis and emphysema are not thought to occur.

[...]