13 Jul 2022
Ann Derham
Job Title
Lower airway inflammatory conditions are a common cause of poor performance and ill-thrift in horses. The underlying cause of lower airway inflammation can be non-infectious or infectious.
A thorough investigation is required to correctly diagnose the underlying issue and treat the patient correctly. New techniques have been developed in recent years, allowing for a greater assessment of the lower respiratory tract in horses, which, in turn, has resulted in better management of lower respiratory tract inflammation in these patients. This article’s aim is to run through the approach to investigation and diagnosis of lower airway inflammation in horses.
Age is an important factor to consider, as some conditions are more typical in certain age groups. For example, the mild-moderate form of equine asthma is seen in young horses (younger than seven years), the severe form is more common in older horses (older than seven years), while in foals, an infectious inflammatory airway condition is far more likely.
Getting a detailed history of the patient is a critical step. A good history can help you to narrow down the differentials list, and offer clues as to the underlying cause/issue(s). Some important information that is needed from the client includes:
A thorough clinical examination is the most important part of examining any horse with lower airway inflammation. While it won’t allow you to make a primary diagnosis, a thorough clinical exam and a detailed history should enable you to make a decision about underlying causes, and what diagnostics are then required.
Prior to beginning the clinical examination, carefully watch the horse at rest in its stable. This will allow you to get a more accurate resting respiratory rate and will also allow you to observe if an abdominal effort to the breathing can be seen, or a heave line is present. Following this, the clinical examination should be performed, with particular attention paid to the following:
Once the clinical examination is finished, a rebreathing exam (Figure 1) should be performed. This involves placing a plastic bag over the horse’s nose (large enough to allow for several breaths). The horse breathes the same air in and out, increasing the horse’s carbon dioxide levels, resulting in the horse taking deeper breaths. The bag is then removed, causing the horse to take very deep breaths initially before returning to normal respiration. During the rebreathing exam, the lung fields are carefully auscultated, as you can hear the movement of air in the periphery of the lungs. The horse’s tolerance for this exam is very important, and the bag is removed immediately if the horse starts to cough or become distressed.
An accurate diagnosis of the underlying disorder is a prerequisite for successful management of lower airway inflammation in horses. In some cases, clinical signs and history alone may be enough for a diagnosis. Depending on the underlying cause(s), different diagnostics may be required. Coughing, nasal discharge, dyspnoea, fever, exercise intolerance or increased mucus on endoscopy are strong indicators for collecting samples from the lower respiratory tract.
The most common techniques performed are now detailed.
In unresolved cases (such as around poor performance), it may be necessary to do both a TA and BAL to assess the overall health of the respiratory tract. The most important consideration when choosing a technique is whether microbiologic culture of the tracheobronchial secretions is indicated.
This should be performed in any horse (adult or foal) that is presented with signs of respiratory distress, respiratory issues coupled with a fever or with a recent history of long-haul transport. Ultrasound is the preferred method for diagnosing pleuropneumonia in horses and allows for localisation and determination of the extent of the disease, it also allows the clinician to characterise the pleural fluid (if any), which is usually found in the most ventral part of the thorax. Thoracic ultrasound can also be used to guide thoracocentesis if needed.
Cytology should be performed on all BAL and TA samples, as without cytology inappropriate treatment protocols may be initiated. Key points to remember when looking at cytology include:
Microbial cultures should be interpreted with caution. The presence of bacteria alone from a tracheal aspirate is not enough to confirm bacterial infection as they may also be due to sample contamination or a transient lower airway population.
Aspirates from lower respiratory tract infections (bacterial) will also have increased mucus, total cell and neutrophil counts, along with degenerative neutrophils. Typically, tracheal aspirate samples are plated on blood agar, anaerobic, fungal and neomycin plates. Quantitative cultures determine the number of colony-forming units of each species, which provides further information regarding the significance of a species being identified3.
Lower airway inflammation is a common cause of poor performance and ill-thrift in horses.
A thorough history and clinical examination will help the clinician decide which diagnostics are most appropriate (BAL versus TA, or both). Cytological examination should be performed in all cases, and microbial culture is indicated when a concern exists regarding an infectious process. Thoracic ultrasound is required for any cases with suspected pneumonia/pleuropneumonia.
