SCIENCE SUPPLEMENTS – Equine Respiratory Disease: Part 3. Diagnosis of common equine respiratory diseases

Dr Kirstie Pickles

Last week we looked at some of the more common causes of respiratory disease in horses, Catch up HERE .

This week we will discuss how these respiratory conditions can be diagnosed.

Clinical Examination

A veterinary investigation of respiratory disease will always start with a clinical examination. The vet will be looking for:

  • Abnormalities in breathing pattern or rate: horses with respiratory disease often have increased respiratory rates and effort. Increased effort while breathing in indicates an upper respiratory tract obstruction (nose to larynx) whereas increased effort breathing out indicates a lower respiratory tract problem (windpipe and lungs).
  • Presence of nasal discharge: the type of discharge (watery, purulent etc) and whether it comes from one or both nostrils is important. Discharge from both nostrils generally indicates lung disease. Purulent (yellow, thick) nasal discharge usually indicates infection.
  • Rectal temperature: a fever usually indicates an infection.
  • Lung auscultation: listening to the lungs with a stethoscope will detect any abnormal sounds such as crackles or wheezes.
  • Rebreathing exam: when a doctor listens to your chest, they ask you to breathe deeply so it is easier to hear any abnormal lung sounds. As horses are less cooperative, we often make them breathe into a large plastic bag so that they rebreathe their exhaled carbon dioxide which makes them breathe deeper (Fig. 1). Surprisingly to many owners, most horses tolerate this procedure very well.
Fig. 1:

Fig. 1: A black plastic bag is being used to perform a rebreathing exam to make the horse breathe deeper to facilitate auscultation of abnormal lung sounds.

Endoscopy

The next step in determining the cause of any abnormal respiratory signs is usually to perform endoscopy. An endoscope is basically a long flexible tube with a video camera at the end which allows the user to look into the airway (Fig. 2). The horse is may be lightly sedated (unless laryngeal function is being examined) and the endoscope passed up the nose, through the larynx and down the windpipe. The origin of any nasal discharge can be determined (Fig. 3) and any respiratory tract abnormalities identified.

Fig 2

Fig. 2: Tip of an endoscope showing the video camera, light source and flushing channel.

Fig. 3: Endoscopic picture of a horse’s trachea (windpipe) showing a wide stream of mucopurulent material in a horse with severe equine asthma.

Fig 3

Collection and Analysis of Respiratory Secretions.

Respiratory secretions can be collected by performing tracheal wash and bronchoalveolar lavage (lung wash). These two techniques are complementary and both should ideally be performed rather than just one. Lung washes are the gold standard for initial diagnosis of disease but tracheal washes are ideal for monitoring disease in competition horses. A tracheal wash collects secretions from the trachea (windpipe) using a narrow tube passed down the endoscope. If there is very little mucus in the trachea, a small amount (20ml) of saline is washed into the trachea first to help collect a sample (Fig. 4).

Fig 4

Fig. 4: Endoscopic catheter (black arrow) collecting saline and mucus (white arrow) from the trachea in a tracheal wash.
A bronchoalveolar lavage (BAL, ‘lung wash’) is performed following a tracheal wash. A small amount of local anaesthetic is flushed into the trachea (to limit coughing during BAL) after the tracheal wash and then the endoscope is removed. A long flexible tube (BAL catheter) is then passed up the nose and down into the lungs until it is wedged in a main airway. Saline is then inserted into the lung down the tube and sucked back out again ‘washing’ the lung surface and collecting cells as it does so. This is a very safe procedure and, despite flushing about 300 ml of saline into the lungs, there is absolutely no risk of the horse drowning! Most of the saline is retrieved and the small amount remaining is quickly absorbed by the lungs.
The samples collected by tracheal wash and BAL can be processed and analysed by a laboratory to inform the vet about the type of disease process occurring which guides treatment. For example, increased numbers of neutrophils (a type of white blood cell) in the absence of any bacteria indicates equine asthma (Fig. 5). These samples can also be cultured if an infection is suspected, which then helps guide antimicrobial therapy.

Fig. 5: Cells from a BAL sample from a horse with severe asthma showing many neutrophils.

Fig 5

Imaging

Sometimes imaging techniques such as ultrasound or x-rays are also used to give additional information. Ultrasound can be used in any age of horse but some practice x-ray machines will only be able to x-ray foal chests as adult chests as so thick they require very high-powered machines. Ultrasound is particularly useful for identifying fluid surrounding the lungs (Fig. 6), abscesses, masses and pneumonia. Additionally, ultrasound-guided sampling and draining of fluid in the chest cavity makes this procedure much safer.

Fig. 6: Ultrasound image from a horse with pleuropneumonia (infection of the lung and

Fig 6

chest cavity) showing an abnormal collection of fluid surrounding the lung (white arrow) and a collapsed lung lobe (red arrow).

Other

Blood samples are not particularly useful when evaluating respiratory disease, apart from perhaps giving an idea of severity and chronicity of infection. In general, it is more useful to collect samples from the respiratory tract for analysis.
Commercially available allergy testing currently has no value in diagnosis of equine respiratory disease.

NEXT WEEK: FINAL PART – Treatment and Prevention of Equine Respiratory Disease

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