History: History and signalment can be important in aiding diagnosis of horses with respiratory diseases. Important points include:

Physical examination: The physical examination should be performed in a quiet environment with the horse relaxed and rested. Examination during and after exercise may be required subsequently.

Systemic and extra-thoracic signs that may occur with lower respiratory tract disease include:

Respiratory rate, pattern and character: The rate, depth, and pattern of breathing should be noted, as should any abnormal sounds associated with breathing. This is best done by distant observation with the horse in a quiet location. The breathing is most easily assessed by observing the movement of the costal arch whilst standing caudolaterally to the horse. Alternatively, the respiratory rate may be measured by feeling airflow with a hand placed close to the external nares.

Nasal discharge:

Cough:

Palpation:

Auscultation: Auscultation is performed on both sides of the chest and over the cervical trachea. Lung sounds will vary depending on the body condition and depth of breathing. In fat horses, airflow sounds may be difficult to appreciate; these sounds can be accentuated by using a re-breathing bag (place a large plastic bag over the nose and let the horse breathe into and out of the bag until the arterial carbon dioxide tension increases, thereby increasing the depth of respiration) (Figure 6.1) or by temporarily occluding the nostrils. The normal margins of the lung fields are level with the tuber coxae at the 18th rib, mid thorax at the 13th rib, shoulder at the 11th rib, and then curving down to the level of the elbow (Figure 6.2).

Adventitious sounds (i.e. abnormal sounds) include crackles (rales), wheezes (rhonchi) and pleural friction sounds.

Percussion: Percussion may be used to detect pleural and superficial pulmonary lesions. It is less sensitive than ultrasonography. A plexor and pleximeter may be used, or, alternatively, the fingers can be employed to sharply tap over the intercostal spaces (Figure 6.3). The area of percussion is similar to the area of auscultation, remembering that there is an area of cardiac dullness (larger on the left than the right) in the ventral thorax. The entire chest on both sides should be percussed working in parallel lines from dorsal to ventral and anterior to posterior.

Endoscopy: Endoscopy allows direct visualization of many parts of the respiratory tract, and the use of flexible fibreoptic or videoendoscopic equipment has become an essential component in the evaluation of respiratory tract diseases. The parts of the tract that are accessible to examination are determined by the equipment available. Although a narrow 1 or 1.2 m endoscope will permit examination of most parts of the upper respiratory tract in adult horses, it is unlikely to have sufficient length to allow examination of the bronchial tree – an instrument of 2 m or longer is needed for bronchoscopy. Fine paediatric endoscopes may be required for foals. Adequate disinfection of the instrument between horses is essential to prevent cross-infection.

The horse is restrained as necessary. A nose twitch is usually sufficient, and this helps stabilize the head. Chemical sedation is sometimes required. It is helpful to have three people to perform the examination: one holds the horse and twitch; one stabilizes the endoscope at the nostril, and one has charge of the controls. For routine examinations, the endoscope is introduced through one nostril and passed along the ventral nasal meatus into the nasopharynx. In most horses, an endoscope can be introduced from the nasopharynx into the larynx without inducing significant coughing. However, in horses affected by lower airway diseases, this manoeuvre may induce a severe cough response. Coughing may be abolished by spraying a dilute lidocaine solution over the larynx (via a catheter passed through the accessary port of the endoscope). Sedation with xylazine, detomidine or romifidine may also diminish the cough response.

The trachea can be examined in its entirety if a sufficiently long endoscope is used (>1.8 m). Strictures of the tracheal lumen (congenital, iatrogenic or traumatic) can be observed, although radiography may yield more useful information. Discharges often accumulate at the thoracic inlet (Figure 6.4), and their nature (mucus, purulent material, blood) can be assessed and samples obtained by aspiration. A haemorrhagic discharge observed after exercise is suggestive of exercise-induced pulmonary haemorrhage. Foreign bodies such as brambles may lodge in the distal trachea/main bronchi and are an uncommon cause of chronic coughing – they can usually be identified and retrieved during endoscopy.

The bronchial tree can be examined as far as the length and diameter of the endoscope permits. The carina (Figure 6.5) normally presents as a sharp angle at the junction of the right and left principal bronchi. Thickening of the angle (owing to mucosal oedema and inflammation) and hyperaemia may be seen in chronic lower airway diseases. A unilateral purulent discharge draining from only one principal bronchus indicates a focal lung lesion on that side (e.g. focal pneumonia, pulmonary abscess or foreign body). Passage of the endoscope down the bronchial tree may induce significant coughing which renders the examination difficult. This reaction can be reduced by repeated infusions of small volumes of dilute lidocaine solution as the endoscope is advanced. The bronchial walls can be assessed for thickening, inflammation and collapse. Lungworms (Dictyocaulus arnfieldi), foreign bodies and endobronchial tumours may be identified.

Tracheal aspiration: Samples of lower airway secretions may be obtained for cytology or culture. Tracheal aspirates can be collected by endoscopy or by a transtracheal technique. In the former, the flexible endoscope is passed in the usual way via the nares into the trachea. When the tip has reached the distal trachea, a catheter is passed down the accessory port until it protrudes from the end. Sterile saline (10–15 mL) is injected through the catheter, which forms a pool at the thoracic inlet. The catheter tip is positioned into this pool and the sample is aspirated. Only a small proportion of the delivered fluid is likely to be retrieved. Aspirates from normal horses contain mainly ciliated epithelial cells and macrophages. Approximately 10% of the cells are neutrophils, although some stabled horses may have up to 40% neutrophils.

Samples collected in this way are suitable for bacterial culture, but culture results must be interpreted with caution since the endoscope is unavoidably contaminated by respiratory pathogens residing in the upper respiratory tract. Guarded aspiration catheters are available that will reduce the risk of contamination of samples obtained in this way. The endoscope must be thoroughly disinfected if samples are being obtained for microbiological culture (and to prevent cross-contamination between horses). Contamination of flexible endoscopes by Pseudomonas spp. is common. If the horse coughs frequently during endoscopic tracheal aspiration, there will be an increased risk of contamination by oropharyngeal organisms.

Samples obtained by transtracheal aspiration are collected aseptically and are suitable for both cytology and culture. An area over the lower third of the cervical trachea is clipped and prepared aseptically, and an over-the-needle cannula is inserted between two tracheal rings into the tracheal lumen. A catheter is then threaded through the cannula and passed to the level of the thoracic inlet where lavage with 20–30 mL of sterile saline is performed (Figure 6.6). In cases of lower airway or pulmonary infection, a Gram-stained smear of a sample obtained in this way may give an early indication of the types of bacteria involved before culture results are available. Both aerobic and anaerobic cultures should be performed in cases of pneumonia/lung abscess.

Several complications may arise following the transtracheal aspiration technique:

Bronchoalveolar lavage (BAL): BAL is used to obtain samples of lower airway secretions, primarily for cytology, from the small airways and alveoli. The technique may be performed through an endoscope (which is lodged in a bronchus prior to lavage) or blindly using a catheter passed per nasum into the bronchial tree.

Cytology of BAL fluid provides a more accurate reflection of the state of the small airways and alveoli than that of tracheal aspirates. BAL is ideally suited to the assessment of diffuse lower airway and alveolar diseases. When performed blindly, the segment of lung being lavaged is unknown, and the technique should, therefore, not be used if focal lung disease is suspected (including exercise-induced pulmonary haemorrhage (EIPH), pneumonia, lung abscess). Endoscopy permits more accurate placement of the lavage tube, but the precise location can still be difficult to assess.

Endoscopic BAL technique: A 120 cm (or longer) endoscope is passed in the usual way to the carina and then into a mainstem bronchus. It is advanced down the bronchial tree until it wedges in an airway (usually about a fourth to sixth generation bronchus if an endoscope with an external diameter of >8–10 mm is being used). Lavage is performed via the accessory channel of the endoscope with or without a catheter. Coughing can be severe as the endoscope passes down the bronchial tree. In most horses this abates after 10–20 seconds, but in horses with airway inflammation coughing can be persistent, in which case this can be reduced by infusing a small volume of dilute lidocaine solution (0.4%) at each bronchial division.

The volume and type of fluid used for BAL is a matter of personal preference. Most clinicians use sterile pre-warmed saline, infused in 50 mL aliquots to a total volume of about 300 mL. The retrieval rate for this fluid varies, but is usually in the region of 50–80%. Normal BAL samples will appear foamy due to the presence of surfactant, and this indicates a good sample. The samples should be pooled together prior to evaluation.

Blind BAL technique: Commercially available nasobronchial BAL tubes should be used for the purpose when the technique is performed blindly. The tubes usually have an inflatable cuff at the distal end. With the horse’s head extended, the tube is passed in the same way as the endoscope through the nasopharynx, into the trachea until a cough reflex is elicited. Infusion of dilute lidocaine solution (as for the endoscopic technique) will alleviate the coughing, and permit the tube to be advanced down the bronchial tree (the tube most commonly enters the right dorsocaudal lung). When the tube becomes lodged and will not advance any further, the cuff is gently inflated with 5–10 mL of air, lavage fluid is instilled and the sample is recovered by manual or automated aspiration (as for the endoscopic technique) (Figure 6.7a,b). Excessive negative pressure for aspiration (manual or automated) will collapse small airways, limiting the recovery volume, and may result in blood contamination of the sample.

Complications of BAL are minimal. A mild neutrophilic inflammatory response in the lavaged lung segment is expected. Occasionally, mild pyrexia can be detected for about 24 hours after the procedure.

Thoracic radiography: In adult horses, only lateral views are possible. Four overlapping fields are often required to cover the entire thorax in adult horses (Figure 6.8). An air gap is used between the patient and the film to avoid the use of a grid. Lesions that can be detected include pulmonary consolidation/pneumonia, lung abscesses, advanced EIPH lesions, lung infiltrates such as neoplasia, granulomas and fibrosis, and pleural effusions.

Diagnostic ultrasound: Ultrasonography is well suited to the investigation of pleural diseases, especially pleural effusions, where the technique can provide some clues as to the nature of the fluid (i.e. transudate or exudate), as well as the presence of adhesions and loculations. Consolidated lung, pulmonary abscesses and pulmonary masses may also be imaged if these lesions are adjacent to the pleural surface. Ultrasound waves are not transmitted through aerated lung, so the technique is of little value in airway disease or focal lung diseases where the diseased area lies deep to the lung surface. The ribs can also be imaged by ultrasound.

Either linear array, curvilinear or sector scanners can be used. Superficial structures may be examined using a high-frequency probe (7.5–10 MHz), but for deeper structures (>15 cm) a lower frequency probe should be used. The intrathoracic structures are imaged by placing the probe in the intercostal spaces and scanning both sides of the chest in a systematic fashion. The normal inflated lung has a highly echogenic surface. The reverberation artefact caused by the echogenic interface of pleura and aerated lung obscures the visualization of deeper structures. The pleural surfaces glide smoothly against each other as the lungs inflate and deflate. The two pleural surfaces normally appear as one echogenic line. Ribs are echogenic at their surface, smoothly marginated, convexly curved and cast a shadow. In the caudal thorax, the diaphragm may be visualized as a thin, smooth, echogenic line between the aerated lung and the cranial border of the liver.

Commonly identified abnormalities include:

Thoracocentesis: Thoracocentesis allows the collection of samples of pleural fluid for analysis (including cytology and bacteriology); it can also be used in the treatment of pleuritis by allowing the drainage of large amounts of fluid. The precise site at which the chest is entered will vary depending on the amount of effusion; ultrasound guidance can be helpful. The usual site is either the right side at the 7th intercostal space, or the left side at the 7th or 8th spaces in the ventral third of the chest. Care must be taken to avoid the lateral thoracic vein, the liver, and the heart. The area is clipped and local anaesthetic infiltrated under the skin and into the intercostal muscles down to the pleura. A stab incision in the skin allows the introduction of a cannula (such as IV cannula, teat cannula or metal bitch urinary catheter) which is pushed into the pleural cavity along the cranial border of the rib (Figure 6.11). Fluid will normally drain by gravity, but care must be taken to avoid introduction of air into the cavity. Normal horses have a small quantity of clear, watery and pale straw-coloured fluid. The nucleated cell count of the fluid is low (<4 × 10⁹/L) with approximately 70% neutrophils, 20% large mononuclear cells, and small numbers of lymphocytes and eosinophils. The total protein content is normally less than 30 g/L. Effusions are classified as transudates, modified transudates, exudates or chylous.

Pleuroscopy (thoracoscopy): Direct endoscopic examination of the pleural cavity is occasionally helpful in the evaluation of selected cases of pleuropneumonia, thoracic abscesses, pericarditis and thoracic neoplasia. It may also be helpful in aiding biopsy of pulmonary and pleural masses. Either rigid endoscopes (such as arthroscopes or laparoscopes) or flexible endoscopes may be used.

Haematology: Haematological alterations in lower respiratory tract diseases are often minimal or non-specific. Hyperfibrinogenaemia, hyperglobulinaemia, hypoalbuminaemia and mild anaemia may be observed in infectious, inflammatory and neoplastic diseases.

Other diagnostic tests: Other diagnostic techniques that may be usefully employed in some situations include lung biopsy, pulmonary function tests (such as intrapleural pressure measurement) and arterial blood gas analysis.

Aetiology: RAO is strongly associated with stabling and feeding hay. In most cases, a pulmonary hypersensitivity associated with inhalation of organic dusts, primarily hay and straw dusts, occurs; this results in inflammation of the small airways. Hypersensitivity to fungal and thermophilic actinomycete spores (major component of hay and straw dust, especially from ‘heated’ bales) is involved in most cases. Spores of Faenia rectivirgula (formerly known as Micropolyspora faeni), Aspergillus fumigatus, etc., have been implicated. Recently it has also been suggested that RAO may result from non-specific inflammatory responses to inhaled pro-inflammatory agents, including moulds, endotoxin, particulates and noxious gases which are present in the breathing zone of stabled horses.

Pathogenesis: Inhaled fungal and actinomycete spores (and other possible allergens) deposit in the bronchioles where specific hypersensitivity reactions are initiated.

Horses with RAO exhibit a modified type 2 cytokine response characterized by the production of interleukin-4 (IL-4) and IL-13 mRNA, but not IL-5 mRNA in BAL cells. Interferon-gamma mRNA is also elevated, suggesting a mixed response. Non-antigen-specific inflammatory responses are also probably involved. Inhalation of endotoxins (that may be present in hay and straw dusts) may result in lower airway inflammation, although the concentrations of endotoxin in the air of most stables is relatively low. Moulds can also cause a non-specific inflammatory response (i.e. not due to an allergic mechanism). Noxious gases (such as ammonia, hydrogen sulphide and methane) may also cause airway inflammation, especially in horses with RAO that also have non-specific airway hyperresponsiveness.

The net results of these reactions are:

These reactions result in small airway obstruction; airflow is impeded, especially during expiration. Most of these reactions are reversible if exposure to the offending allergens/pollutants is eliminated. In longstanding cases, some structural changes to the bronchioles may occur, e.g. metaplasia of the epithelium, hyperplasia of mucus-secreting goblet cells. In severe cases, chronic airflow obstruction results in alveolar overinflation and eventually areas of emphysema. In rare cases, secondary infection by opportunist bacteria may occur.

Epidemiology:

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