The lower respiratory tract

Chapter 7


The lower respiratory tract





Chapter contents




INTRODUCTION


The horse is unique among domestic animals in the demands that are placed on its respiratory system. The cantering and galloping horse has locomotor and respiratory cycles locked in a one-to-one phase. A galloping horse will take over 150 breaths per min (bpm), having <0.5s to inhale and exhale 12–15L of air. Minor degrees of respiratory disease in the form of small increases in mucopus in the airways or minor degrees of airway spasm will quickly take their toll on athletic performance, although horses with minor lower respiratory tract problems may not show overt signs of disease at rest.


Diagnosis has been made easier with the use of the fiberoptic endoscope and more recently with the videoendoscope. The covert respiratory problems of the relatively young equine athlete may be precursors of more overt problems later in life in the form of recurrent airway obstruction (RAO), previously known as chronic obstructive pulmonary disease (COPD). Therefore, the diagnosis, treatment and prevention of respiratory problems in young horses have both short- and long-term benefits.


Considerable emphasis has been placed on environmental factors. Air quality of stables can affect the incidence and severity of a range of lower respiratory tract problems and in many instances is a valuable adjunct to pharmacologic intervention. The old adage of prevention being better than cure cannot be overemphasized in terms of the respiratory well-being of the stabled horse.


This chapter will review the diagnostic approaches useful in assessing lower respiratory tract disease, therapeutic approaches to some more commonly seen problems and the air hygiene of stables. Exercise-induced pulmonary hemorrhage has been associated with a wide range of equine athletic events and this condition is dealt with separately for this reason.



DIAGNOSTIC APPROACHES TO LOWER RESPIRATORY TRACT DISEASES




CLINICAL EXAMINATION


With gross pulmonary diseases, such as bacterial pneumonia in foals (q.v.) or advanced recurrent airway obstruction (RAO) (q.v.) in adult horses, clinical signs of pulmonary disease will be obvious, possibly even on cursory examination.


Clinical signs can include coughing, which may be in bouts (paroxysmal) and which may expectorate large volumes of abnormal respiratory secretions (RS) via the oral cavity. Other obvious clinical signs can include bilateral nasal discharge, grossly elevated respiratory rate (i.e. ≥25 per min in an adult horse), increased respiratory effort (hyperpnea) and poor exercise tolerance or even weakness, with severe hypoxemia. Pyrexia and depression may occur in infectious respiratory diseases and percussion of the ventral chest will reveal dullness in cases of pleural effusion. Cases with such near-pathognomonic signs of pulmonary disease will present no diagnostic challenge.


Additionally, the history and epidemiologic factors such as age, colostral or vaccination status, numbers of animals involved, transport history, history of contact with donkeys and knowledge of the type of forage, bedding and housing used, may enable a specific type of pulmonary disease to be diagnosed.



Limitations of clinical pulmonary examinations


The above examples of gross pulmonary disease are exceptional and in the majority of cases of equine pulmonary disease the clinical signs will not be pathognomonic. Many, if not most, horses with pulmonary disease do not have a nasal discharge, as all excessive RS produced in the lungs are swallowed.


Many horses with pulmonary disease will not have sufficient change in pulmonary function to increase significantly their respiratory rate or effort to an extent that will be clinically apparent and clearly differentiable from changes due to individual variation, breed differences or physiologic changes induced by clinical examination. While many horses with pulmonary disease have insufficient pulmonary inflammation or loss of pulmonary function to enable a clinical diagnosis to be made clearly, they may have sufficient loss to cause a reduction in exercise performance, especially in high performance animals such as racehorses.


Auscultation of chest sounds is a very subjective and unreliable procedure in horses with mild or even moderate pulmonary disease, with little agreement between different clinicians in such cases. Tracheal auscultation after nasal occlusion or use of a rebreathing bag renders auscultation more accurate.


The presence of coughing is probably the most reliable clinical indicator of pulmonary disease in the horse. Yet this sign is sometimes inexplicably absent in horses with significant pulmonary disease, even when excessive RS production is present.


Clinical diagnosis of pulmonary disease can thus be unreliable.




ENDOSCOPY


Endoscopy is probably the most useful and easily performed diagnostic technique available for equine pulmonary disease investigation. The horse has an insensitive upper respiratory tract that readily allows the passage of an endoscope into the trachea. This procedure (tracheoscopy) is usually termed bronchoscopy. In the absence of respiratory disease, the equine trachea will contain no endoscopically visible or just tiny flecks of RS.


Most equine pulmonary disorders are associated with increased production of RS, which are usually mucopurulent in nature. Purulent RS occur with bacterial pneumonias and secondary bacterial infections. The overproduction of RS is often accompanied by a reduction in the mucociliary clearance mechanisms, and both factors result in an accumulation of RS in the horizontal (thoracic) trachea.


Excessive tracheal RS can be graded from 1 to 5 based on volume. The endoscopic presence of a pool of RS in the trachea is a most sensitive indicator of the presence of pulmonary disease. In younger horses, excessive RS usually result from viral infections or their sequelae but in older animals are usually due to RAO (q.v.). Some animals can perform to a high athletic level despite the presence of grade 1 or 2 tracheal RS. This suggests that, in these animals, insufficient small airways are involved to significantly affect airflow, even during exercise.


Tracheal mucosal inflammation is usually absent in chronic pulmonary diseases unless coughing is present, but bronchial mucosal inflammation will be commonly seen. Mucosal inflammation can be recognized as a blunting of the carina and bronchial divisions, along with redness and possibly the presence of prominent mucosal vasculature.


After spraying a few milliliters of local anesthetic (e.g. 0.2% lidocaine) on the carina, it is possible to advance the endoscope to examine the main bronchi. With pulmonary abscessation (q.v.) or exercise-induced pulmonary hemorrhage (EIPH) (q.v.), it may even be possible to localize the source of pus or blood to a particular lung segment. Rarely a tracheobronchial foreign body may be present and can be removed. With anterior thoracic masses such as mediastinal lymphosarcoma (q.v.) or abscessation, a collapsed mainstem bronchus may be observed. In dyspneic horses, a dynamic collapse of the intrathoracic tracheal and bronchial walls may be seen during expiration. With obstructed pulmonary venous return, e.g. with cardiac disease or anterior thoracic masses, prominent veins will be seen on the lateral tracheal wall.


Pleuroscopy, i.e. the insertion of an endoscope through a surgical incision in the chest wall, after drainage of pleural fluid, may be of diagnostic value in horses with pleural effusion. Ultrasonography is, however, a less invasive and generally more useful technique for such investigations.



BACTERIOLOGIC EXAMINATIONS



Nasal or nasopharyngeal swabs


If a profuse bilateral nasal discharge is present, particularly in the presence of a cough and bilateral submandibular lymphadenitis, this is suggestive of infectious pulmonary disease. Such nasal discharges will often contain RS from the lower respiratory tract, but also contain variable numbers of upper respiratory tract (URT) bacteria.


Culture of nasal discharge is unreliable in the diagnosis of pulmonary bacterial infections, but is of value in the diagnosis of strangles (q.v.), a primary URT infection, although culture or polymerase chain reaction (PCR) analysis of guttural pouch lavage fluid is a much more sensitive method of strangles diagnosis. With chronic unilateral nasal discharges, which indicate a unilateral URT infection, nasal bacteriology is frequently worthless, as the recovered bacteria are usually secondary to sinusitis or to mycoses of the nasal cavity or guttural pouches. However, the recovery of pure and heavy fungal growths can suggest a URT mycosis (q.v.).



Transendoscopic tracheal aspirates


Cultures of RS, aspirated directly from the trachea using a bronchoscope, are more useful than nasal or nasopharyngeal swabs. However, they usually still have bacterial contamination from the URT and/or from the endoscope, even if the greatest care is taken during the nasal, nasopharyngeal and laryngeal passage of the endoscope.


URT contamination can be reduced by sealing the distal end of a sterile transendoscopic catheter with a plug of sterile agar, prior to inserting the sterilized endoscope into the trachea. Before RS collection, this plug is flushed out into the trachea, where it is harmless. More complex techniques such as telescopic sheathed catheters and endoscope sheaths have been developed to overcome URT contamination of tracheal aspirates. Contamination of RS by saliva can be reduced by starving the animal for 30 min prior to bronchoscopy and by rapid sample collection.


Direct transendoscopic aspiration of RS from the trachea is more desirable than a tracheal wash as it allows quantitative bacteriology to be performed. Enumeration of the numbers of bacteria present is also very important in assessing the significance of recoveries. Isolates ≥106 colony forming units (CFU)/mL RS are usually significant, particularly if a single type of a potentially pathogenic bacterium is present. Isolates ≤104CFU/mL are usually insignificant, especially when a mixed growth is obtained. However, it is essential that anaerobic as well as aerobic cultures be performed.



Transtracheal aspirates


Transtracheal aspirates constitute the most accurate technique for obtaining pulmonary RS samples for bacteriologic culture. After local anesthesia has been performed on a clipped and prepared distal midline cervical site, a 12–14G needle or cannula is inserted into the trachea. A sterile catheter is then aseptically inserted through the needle and directed caudally along the floor of the trachea to the thoracic inlet, where RS usually accumulates in the diseased animal.


If RS cannot be directly aspirated after repeated attempts, 20–50 mL sterile saline can be flushed down the catheter and the resultant “tracheal wash” aspirated. This latter step will, however, preclude quantification of isolates. Occasionally a subcutaneous abscess or emphysema (q.v.) will develop after tracheal puncture.





VIROLOGIC EXAMINATIONS


Virus isolation is useful only during the very early febrile stages of respiratory infections. Nasal or nasopharyngeal swabs should immediately be placed in virus transport medium, cooled and transported within 24 h to a specialized laboratory. If an equine herpesvirus infection (q.v.) is suspected, a blood culture is also worthwhile. Because of the short duration of virus shedding and the fragility of such isolates, even with good collection, transport and laboratory techniques, many viral cultures will be negative.


Many immunologic techniques are being developed worldwide that can give rapid diagnosis of viral infections, including an ELISA test for equine influenza (q.v.) that can give a diagnosis within 24 h, PCR tests for tissue samples for equine herpesvirus (q.v.), and PCR tests for tissue and semen samples for equine viral arteritis (q.v.) that can give results within a couple of days.


Retrospective diagnosis of viral respiratory infections can be obtained by virus serology. An initial sample should be taken as early as possible during the course of the disease and a second 2 wk later. The information from such virologic studies may not be available rapidly enough to help the clinical management of individual cases but it can give invaluable information for the development of effective vaccine strategies.



RESPIRATORY TRACT SECRETION CYTOLOGY


RS cytology is a most useful ancillary diagnostic technique, particularly with chronic pulmonary diseases. RS cytology can be performed on tracheal RS or preferably on BALF samples. With pleural effusions, cytology may be used to diagnose thoracic neoplasia, which is commonly due to mediastinal lymphosarcoma (q.v.).





BALF cytology


BALF cytology is easier to examine, enumerate and interpret than tracheal RS cytology. BALF cytology also correlates very well with pulmonary histopathology. The areas lavaged during bronchoalveolar lavage are primarily the distal smaller airways and alveoli. With localized pulmonary disease such as pulmonary abscessation (q.v.), it is possible to lavage a normal area and so obtain misleading results. BALF cytology is currently considered to be the most sensitive diagnostic technique for the diagnosis of RAO ( Table 7.1).



For transendoscopic BALF collection, a minimum endoscope working length of 180 cm is required in an adult Thoroughbred, but a 300 cm endoscope is required for BAL of the diaphragmatic lobe. Having entered a main bronchus, the endoscope is then advanced into a secondary or tertiary bronchus until it is wedged. The accessory lobe bronchus, off the right mainstem bronchus, is a convenient site in Thoroughbreds, especially when using a 180 cm endoscope.


BALF samples may also be obtained with a proprietary BAL catheter or even with a foal stomach tube. BAL catheters tend to lavage the caudal diaphragmatic areas and so are useful in examinations for EIPH. Unlike transendoscopic BAL, BAL catheters do not permit a specific lung area to be lavaged.


Following wedging, 250 mL lukewarm saline is quickly infused into the lung and immediately aspirated, approximately 50% usually being recovered. Cytology is best performed on a cytocentrifuge preparation.



BALF cytology in pulmonary disease


Neutrophilia, i.e. the presence of ≥5% neutrophils in BALF, can occur: (1) permanently in symptomatic RAO, inflammatory airway disease (IAD) (q.v.) or summer pasture obstructive pulmonary disease affected horses; (2) for some weeks with viral respiratory infections; and (3) along with high numbers of intracellular bacteria and possibly with toxic neutrophils in bacterial pulmonary infections.


Eosinophilia, i.e. the presence of ≥3% eosinophils in BALF, can occur: (1) with eosinophilic interstitial pulmonary diseases (q.v.); (2) transiently at pasture (due possibly to Parascaris migration [q.v.]); and (3) in lungworm (q.v.) infection.


If the majority of BALF macrophages in horses performing hard work contain hemosiderin, this may indicate that clinically significant EIPH is present. Nearly all horses in hard work have some hemosiderophages in their RS.



PULMONARY FUNCTION EXAMINATIONS


If sufficient pulmonary tissue is diseased, there may be some measurable decrease in lung function. Pulmonary function measurements are primarily of value in the diagnosis and monitoring of severe pulmonary disease. Because of the absence of patient cooperation in veterinary medicine, in contrast to human medicine where measurements such as forced expiratory volume are standard tests, lung function tests must be performed at rest, unless treadmill facilities are available. Unless trained, many horses object to face masks and/or pneumotachographs and may react to their presence by breath holding or hyperventilating, both of which will invalidate results. The use of sedation to facilitate pulmonary function measurement can also influence results.


There are large reserves in lung function, especially in the smaller airways, where it is estimated that most of the bronchioles must be obstructed before any detectable change in lung function occurs. Consequently, lung function examinations, especially at rest, are very insensitive with small airway disease, and will frequently show no abnormalities, even in the presence of moderate bronchiolar disease.


Significant but unexplained short- and long-term variations in pulmonary function values have been found even in normal trained horses examined under ideal conditions. Pneumotachography, which is required to derive dynamic compliance or pulmonary resistance values, requires expensive equipment and expertise for accurate calibration, recording and interpretation of results.


These factors combine to restrict the usefulness of many equine pulmonary function examinations to group studies, using trained experimental animals. Arterial blood gas analysis (normal PaO2 85–100 mmHg) and maximal intrathoracic pressure measurements (normal dmaxPpl 1–4 mmHg), if obtained in the relaxed, unsedated horse, are useful examinations that can objectively quantify lung dysfunction. However, they rarely enable pulmonary disease that is not clinically apparent to be diagnosed. Additionally, even when pulmonary function tests indicate dysfunction, they give no indication of etiology.



THORACIC RADIOGRAPHY


Thoracic radiography is of great value for the diagnosis and monitoring of pulmonary disease in foals such as neonatal or Rhodococcus pneumonias (q.v.). In adult horses, due to their large chest mass, thoracic radiography is of limited value.


The majority of adult equine pulmonary diseases, including viral infections, RAO, interstitial diseases and low grade EIPH, result in diffuse parenchymal lesions that will not be consistently detected radiographically unless very severe disease is present. Additionally, even minor variations in thoracic radiographic technique in adult horses can cause large perceived differences in chest films. However, major lesions, especially if focal, including severe EIPH, large abscesses or pneumonic lesions, pleural effusions, pneumothorax and diaphragmatic hernias, can be usefully assessed radiographically.




PHARMACOLOGIC APPROACHES TO TREATMENT OF LOWER RESPIRATORY TRACT DISEASES



INTRODUCTION


Viral, hypersensitive and bacterial lower respiratory tract (LRT) diseases generally induce common pathophysiologic changes in the lower respiratory tract. Inflammation is central to these changes and leads to failure in mucociliary clearance, impaired airway and pulmonary defenses and increased susceptibility to airborne environmental irritants (including dusts and noxious gases), allergens, opportunist agents, viral and bacterial pathogens and endotoxin (Gram-negative bacterial lipopolysaccharide and lipo-oligosaccharide).


The pathogenesis of allergic LRT disease is further complicated because many horses progress from specific reactivity to allergens to non-specific hyperreactivity to a variety of airborne irritants. Maintenance or exacerbation of clinical signs then occurs without exposure to the initiating allergen(s).


The central role of stable air hygiene in LRT disease pathogenesis, especially allergic LRT disease, means that pharmacologic approaches represent only one aspect of management. The most important components of successful therapeutic regimens are twofold: (1) environmental changes to improve stable air hygiene, and (2) rest if severe disease is present. Irrespective of the initial cause of respiratory disease, inhaled environmental challenge must be minimized. The importance of stable air hygiene and its influence on airway and pulmonary function is discussed below. Exercising horses with LRT disease leads to delayed recovery or worsening of the condition, possibly with permanent impairment of pulmonary function, and will occur despite any pharmacologic precautions that are taken to attempt to keep the horse in exercise.


Exercise exacerbates airway and pulmonary inflammation by a variety of mechanisms, principally involving mechanical damage by large tidal flows and high respiratory rates. The stress of exercise also significantly impairs pulmonary defense mechanisms by further compromising mucociliary clearance and decreasing the phagocytic and cytotoxic activity of pulmonary macrophages and lymphocytes.


Common pitfalls in therapy and management regimens include inability or unwillingness of the owner to improve the air hygiene of adjacent and communicating airspaces; placing too much reliance on the beneficial effect of soaking hay rather than considering silage/haylage as alternatives; turning out to pasture but using hay and straw in the field shelter; using paper or shavings in stables but as deep litter that allows heavy fungal and mold build-up; failure to use appropriate combinations and doses of drugs, and not choosing the most effective delivery route.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on The lower respiratory tract
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