W. David Wilson, Jeffrey Lakritz, Consulting Editors W. David Wilson ā¢ Jeanne Lofstedt ā¢ Jeffrey Lakritz Coughing, an important and normal respiratory defense mechanism, is the sudden, forceful, noisy expulsion of air through the glottis to clear mucus, particles, and other material from the tracheobronchial tree and glottis. The mucociliary escalator and the cough reflex are the major protective mechanisms that function together to remove material from the respiratory tract.1,2 Particles trapped in mucus are carried toward the trachea by continuous waves of ciliary motion, starting at the level of the terminal bronchioles. Coughing is associated with development of increased intrathoracic pressure and generation of high-velocity airflow out of the respiratory system. This rapid expulsion of air may increase removal of secretions accumulating within the tracheobronchial tree proximal to the level of the segmental bronchi.3 Coughing is thus a normal physiologic and protective response and a common clinical sign associated with a variety of cardiorespiratory disorders. Coughing is an involuntary reflex; however, this reflex may be voluntarily initiated or suppressed.2,3 The reflex pathways involve sensory receptors present within the airway epithelia, from the larynx to the respiratory bronchioles, with nerve fibers conducting afferent impulses within vagal, glossopharyngeal, trigeminal, and phrenic nerves.1,3,4,5 The receptors, mainly irritant receptors, are particularly numerous within the proximal airways (trachea and bronchi), especially around the hilus of the lung and bifurcation of the bronchi.1,4 Other receptors capable of activating the cough reflex are found within lung parenchyma, the pleura, and elsewhere. Brainstem neuronal pathways for the cough reflex in the horse are not well characterized; however, in other species, afferent signals to the medulla oblongata are received and efferent fibers pass back down to the respiratory system within the vagal, phrenic, intercostal, and lumbar nerves and motor portions of the trigeminal, facial, hypoglossal, and accessory nerves to supply striated and smooth muscles of the larynx, tracheobronchial tree, diaphragm, intercostal muscles, abdominal muscles, and secretory glands of the respiratory tract.1,5–7 Irritant receptors are stimulated by mechanical deformation induced by compressing the trachea, by bronchoconstriction, inert dusts such as carbon, irritant gases such as ammonia, inflammatory conditions, and by biological mediators such as histamine, prostaglandins, or bradykinins (Boxes 5-1 and 5-2).2,4 Sensitivity to mechanical stimulation varies along the airway, and in horses the upper airway cough receptors appear to be less active than observed in many other species.8 For example, a stomach tube inadvertently passed into the trachea frequently does not induce coughing until it reaches the carina. Similarly, endoscopic examinations often reveal large pools of exudate within the trachea of horses with limited history of coughing.6 Foals with Rhodococcus equi pneumonia often develop a tracheal rattle, reflecting accumulation of exudates within the trachea, but may not cough. Repeated stimulation of irritant receptors over several hours does not appear to diminish sensitivity but may lead to changes in threshold.4 Bronchoconstriction is a consistent component of cough,3,7,8 and stimuli resulting in a cough may also induce reflex bronchoconstriction through parasympathetic inervation.4 However, cough and bronchoconstriction are separate airway reflexes.7 Whereas stimulation of irritant receptors in large airways induces both cough and reflex bronchoconstriction independently, stimulation of nerve endings in smaller airways does not directly initiate coughing but does cause bronchoconstriction. Bronchoconstriction may then initiate coughing indirectly.4 The role of bronchoconstriction in the induction of coughing is supported by studies in people demonstrating that administration of bronchodilators is associated with reduction in the frequency or severity of coughing in asthma.9,10 These studies indicate that mediator release (blocked by cromolyn) causes bronchoconstriction (blocked by bronchodilators), which stimulates irritant receptors (blocked by local anesthetics) and causes coughing.4 Much of the airflow obstruction in horses with recurrent airway obstruction (RAO) appears to be mediated through these pathways and is eliminated with atropine, a parasympatholytic bronchodilator.4 The mechanical events that produce coughing occur during four phases: inspiration, compression, expression, and relaxation.2–4 The first three phases are necessary to create the decreased airway cross-sectional area and high airflow rates needed for an effective cough. Coughing is a maximum expiratory flow maneuver that begins with deep inhalation to expand lung volume, increase elastic recoil, and dilate the airways through the tethering effect exerted by surrounding lung tissue.11 Closure of the glottis is followed by forced expiratory efforts involving the rib cage, abdomen, and diaphragm, which increase pressure in the abdominal, pleural, and alveolar spaces to over 500 mm Hg.4 The glottis then opens suddenly, allowing the elevated alveolar pressure to rapidly accelerate gas flow from the respiratory tree.2,4,11 This high-velocity gas stream shears exudate from the airway walls and lumen and carries it to the nasopharynx, from which point it exits the respiratory tract as nasal discharge or is swallowed.11 Airflow stops before the animal has exhaled to residual volume because the glottis closes or the driving force provided by the muscles abates.4 The characteristic sound of coughing is produced by vibration of laryngeal and pharyngeal tissues, narrowing and deformation of airways, vibration of surrounding lung tissues, and turbulent gas flow vibration in the airway.4,7 During forced expiration such as occurs with coughing, the increased pleural pressure is transmitted to the intrathoracic airways and alveoli. Intraalveolar pressure exceeds pleural pressure by an amount equal to the elastic recoil pressure of the lung.4,11 A pressure gradient thus exists between the alveoli and atmospheric pressure at the nostrils and mouth. At a point in the airways known as the equal pressure point (EPP), the pressure in the airway lumen equals the pleural pressure because the elastic recoil pressure has dissipated.2–4,11 In the intrathoracic airways rostral to this point, the intraluminal pressure is lower than the pleural pressure; therefore a transmural pressure gradient exists, which causes dynamic compression or collapse of the more proximal airways.2,4,11 The location of the EPP is determined by elastic recoil pressures, and thus indirectly by lung volumes, and the frictional resistance to flow in the airways between the alveoli and the EPP.4 At high lung volumes, elastic recoil pressure is high, resistance in peripheral airways is low, and the EPP is typically located in larger intrathoracic airways that resist collapse because they have cartilaginous support for their walls.4 As lung volume decreases, elastic recoil forces decrease, airway resistance increases, and the EPP moves more peripherally, subjecting lower and less well-supported parts of the tracheobronchial tree to dynamic compression.3,4,11 Dynamic compression reduces the cross-sectional area of intrathoracic airways and thus increases the velocity of airflow through the narrowed segment.4 These spikes of accelerated airflow promote more effective shearing of mucus and debris from the airway wall and lumen during coughing.11 Dynamic compression also increases the resistance to airflow.4 Once dynamic compression of peripheral (smaller) airways occurs, further increases in expiratory effort cause greater narrowing of the airway; thus the flow rate that can be generated at a given lung volume does not increase beyond a certain maximum point (maximum expiratory flow).4 Maximum expiratory flow decreases progressively as the animal exhales because lung volume declines. Thus coughing beginning at high lung volumes achieves the highest airflows.4 However, only the larger intrathoracic airways are compressed and subjected to these spikes of higher airflow velocity.2,4,11 As lung volume falls or if cough is initiated at lower lung volumes, smaller airways are dynamically compressed and cleared of mucus.2,3,11 The effectiveness of coughing can thus be improved by repeating it several times in succession, either in a progression from high to low lung volume during the same breath or by inhaling between breaths.4 Because maximum airflows are lower in smaller airways, coughing is probably less effective in clearing material from the smaller airways than from the larger ones.4 In the alveolar regions of the lung, gas flow is too slow for coughing to be an effective means of clearance.11 Because only intrathoracic airways are dynamically compressed and subjected to these spikes of higher flow velocity during forced expiration, the extrathoracic trachea should be less effectively cleared by coughing.4 However, horses and other animals with long necks do not seem to have great difficulty in clearing their airways, although they do lower their heads and straighten their airways during coughing to assist the clearance process. In diseases such as RAO that cause narrowing of the airway lumen, the increased resistance to airflow, especially in peripheral airways, causes the EPP to move more peripherally toward the alveoli, resulting in a reduction in the maximum expiratory flow rate.2,4 In this situation the effectiveness of coughing in clearing the airways is reduced. Thus administration of a bronchodilator may greatly improve the effectiveness of coughing in patients with RAO.4 Cough may be stimulated by bronchoconstriction; excessive mucous production; deposition of inhaled particles in the airways; release of inflammatory mediators; infectious diseases; exposure to hot or cold air; intramural or extramural pressure or tension on the airways (tumor, granuloma, abscess, or decreased pulmonary compliance caused by restrictive disease such as interstitial fibrosis or pleural effusion); sloughing of airway epithelial cells; and enhanced epithelial permeability (pulmonary edema).12 Epithelial sloughing and enhanced epithelial permeability increase the accessibility of cough receptors to the mechanical or chemical agents that stimulate them. Loss of integrity of the epithelial lining of the respiratory tract is a common feature in many respiratory diseases associated with cough (infectious diseases); however, a cause-and-effect relationship between alterations in pulmonary epithelium and cough has not been established.12 Diseases of the respiratory tract may alter the sensitivity of the cough reflex.12 For example, viral infections may increase the responsiveness of cough receptors to stimuli.7,12 Of the many causes of coughing in large animals, viral infections such as equine influenza and infectious bovine rhinotracheitis (IBR) are particularly important because they cause outbreaks of respiratory disease that have acute onset of coughing as a prominent feature and that are frequently associated with persistence of coughing for prolonged periods after signs of acute disease have abated. These features of viral infections reflect the decreased effectiveness of mucociliary clearance resulting from virus-induced injury to ciliated epithelial cells, together with exposure and sensitization of irritant airway receptors, which lead to persistent bronchial hyperreactivity.4,13 Affected individuals show bronchoconstriction and coughing in response to mildly irritating stimuli such as dust, stable pollutants, cold air, dry air, and exercise that would not normally cause coughing. Coughing subsides only when the airway epithelium has healed, which may take up to 7 weeks.4 Although the role of mucus in coughing is not clear, normal amounts of mucus may have a protective effect by coating the epithelium with a layer that separates the receptors from irritants.4 Conversely, excessive accumulation of mucus in the airways such as occurs in horses with RAO may mechanically stimulate irritant receptors and cause coughing.4 Fluid flushed over the tracheobronchial epithelium stimulates irritant receptors, particularly if the fluid is hypertonic or hypotonic.9,10,14 Coughing is also stimulated by fluid that lacks permeant anions (i.e., anions that have a hydrated size and membrane-penetrating characteristics similar to the chloride anion).15 Coughing is a prominent feature of cardiac disease in many species, although cardiac diseases are not often encountered in large domestic animals. Failure of the left side of the heart as a result of congenital defects, valvular stenosis or incompetence, conduction disturbances, myocardial disorders, or restrictive pericardial disease causes an increase in pressure in the pulmonary venous return from the lung. This results in transudation of fluid from the pulmonary capillaries into the pulmonary parenchyma and airspaces (cardiogenic pulmonary edema) and causes swelling of the mucosal lining of small airways.2 These changes stimulate cough receptors and initiate the cough reflex. Coughing that occurs secondary to cardiac disease is usually chronic, although acute-onset coughing may be observed with ruptured mitral chordae tendineae and bacterial endocarditis.16 The history should include questioning relative to the patient, the cough, the environment, and management. The age of the affected animal is important because many conditions have a marked age incidence. For example, R. equi pneumonia occurs primarily in foals younger than 6 months of age, equine herpesvirus type 4 predominantly affects weanling and yearling horses, and RAO affects primarily mature stabled horses. The use of the horse; its state of fitness; the presence of vices such as crib biting; and any history of contact with other horses at shows, events, sales, racetracks, or breeding farms should be determined. Recent stressors such as transportation, surgery, strenuous activity, or weaning should be determined because these are known risk factors for conditions such as pneumonia and pleuropneumonia. The duration of ownership of the animal, its previous health, and the geographic location of origin, if it was recently purchased, may help identify regional diseases not normally seen in the area (e.g., systemic mycosis, lungworms, or silicosis in horses) or indicate the degree of stress likely to have been recently experienced. For recently purchased feedlot cattle and sheep, it should be determined if the animals were preconditioned before sale, their place of origin, the number of sale yards through which they have passed, and the duration of transportation. When evaluating the potential for contagious diseases or those related to common environmental conditions, the vaccination status of the affected animal and herdmates and the presence of similar clinical signs in other in-contact animals sharing common facilities or common airspaces should be determined. In nursing animals the vaccination status of the dam is important. A history of other signs such as anorexia, nasal discharge, weight loss, exercise intolerance, stridor, lymphadenopathy, facial swelling, diarrhea, colic, and edema may provide important clues to the cause of the current problem. Weight loss occurs in many acute and chronic diseases, both infectious and noninfectious. Anorexia may indicate that eating is too painful for an animal, for example, because of a painful nasopharyngeal lesion or because the animal is devoting so much effort to breathing that it will not eat. Environmental considerations such as the introduction of new animals into the environment or the return of animals from shows, sales, training centers, or breeding farms should increase the suspicion of infectious viral or bacterial diseases such as IBR in cattle or influenza in horses. For diseases such as R. equi pneumonia in foals, it is useful to know if similar cases have been seen on the premises in the past. The type of housing or pasturing facilities should be evaluated, particularly with regard to airspace, ventilation, sanitation, stocking density, dust, shade, and shelter. Equine stabling facilities that have enclosed barns, particularly when the stalls face a central arena or are located under a hay storage loft, promote the spread of contagious agents and almost invariably increase the concentration of dust, mold spores, and noxious gases such as ammonia and tractor exhaust fumes in the environment. The type of feed and bedding, storage facilities, and feeding arrangements should be evaluated, especially in chronically coughing horses suspected of having allergic lung disease. Quantitative measures of ventilation and environmental quality can be made and can prove helpful in case management and in monitoring the effectiveness of measures to reduce environmental dust and other pollutants.17 Similarly, the sampling of air in barns or paddocks using special devices that allow quantitation of bacterial pathogens such as R. equi may prove to be useful for assessing risk of infection. The quality of the feed should be evaluated visually and by smell, particularly with regard to the presence of mold spores. General management, pasture management, and parasite control measures should also be evaluated, especially if lungworm infection is suspected. Horses with lungworm infection almost invariably have a history of current or previous co-grazing with donkeys. The season and seasonal incidence of recurrence of the coughing can provide useful clues (e.g., marked seasonal worsening of signs in the late autumn when horses are stabled suggests chronic allergic respiratory disease). Similar to the conditions mentioned for horses, cattle pastured during the warmer months of the year and housed during the colder months may demonstrate recurrent episodes of respiratory dysfunction and cough associated with allergies to dusts present in stored hay or bedding. The history should also include the nature of previous treatments and the response. Important historical features of the cough include the time and speed of onset; frequency; duration (chronic if longer than 1 month); relation to feeding, housing, or weather; relation to exercise and timing during exercise; improvement, deterioration, or other change since onset; and the presence of and similarity to previous episodes of coughing or respiratory disease. Many coughs are exacerbated by exercise because exercise places greater stress on the respiratory tract. The resulting rapid airflow improves mobilization of secretions and also irritates the airway directly. Coughs that occur during eating suggest specific or nonspecific sensitivity to molds, pollen, or other dusts in feed; inflammatory conditions of the larynx or pharyngeal region; laryngeal or pharyngeal problems, including complications of surgery, that interfere with swallowing or guarding of the lower airway; or esophageal obstruction (choke). The effort required to cough and associated pain may also give clues to the cause of the cough. The character of the cough should be evaluated because certain features of a cough tend to point toward its origin and possible causes but are by no means pathognomonic for either. Coughs originating in the upper airway are usually of acute onset, loud, harsh, coarse, dry, hacking, and nonproductive in character. Painful upper airway conditions such as acute pharyngitis, strangles, or necrotizing laryngitis can make the cough more muted. Lower airway coughs are usually soft, deep, and productive (mucus, pus) and tend to be more persistent than coughs originating in the upper airway. However, chest pain frequently attenuates coughing in horses with pleuropneumonia. In horses the fixed position of the larynx within the nasopharynx usually precludes expectoration of sputum into the mouth; thus the productive nature of the cough is difficult to assess. Excess mucus or exudate that is coughed up into the pharynx is usually swallowed; thus a cough can be productive without evidence of a nasal discharge. Swallowing efforts that follow a cough generally indicate that the cough is productive. The physical examination should include both distant and close evaluation of the patient and the environment. In addition to a detailed examination of the respiratory and cardiovascular systems, a general physical examination should also be completed so that diseases in other systems can be detected and systemic manifestations of cardiopulmonary disease can be evaluated. The attitude of the animal, the respiratory rate and character, the presence of excessive intercostal or abdominal respiratory effort or of a āheave lineā or ābarrel chest,ā the presence and nature of respiratory distress or stridor at rest, and the presence and character of any nasal and ocular discharge should be noted before the animal is restrained. The following should then be determined: (1) rectal temperature, pulse rate, pulse rhythm and character, mucous membrane color, and capillary refill time; (2) symmetry of airflow from each nostril; (3) odor from the nostrils and mouth; (4) facial symmetry and swelling; (5) resonance or painful response on percussion of the maxillary and frontal sinuses; (6) enlargement of submandibular, parotid, retropharyngeal, prescapular and other regional lymph nodes; (7) enlargement of the parotid salivary glands or thyroid gland; (8) swelling, pain, or palpable abnormalities in the retropharyngeal region; (9) palpable swelling or flattening of the cervical trachea; and (10) masses at the thoracic inlet and palpable turbulence such as a tracheal rattle in the extrathoracic airway. The oral cavity should also be examined. If spontaneous coughing is not heard during the physical examination, a cough should be induced after auscultation of the airways by pinching the larynx or trachea, and it should be ascertained whether the induced cough sounds like the animal’s spontaneous cough. Pinching of the trachea generally causes normal animals to cough once or twice, whereas it often induces paroxysmal coughing in animals with lower airway disease. The laryngeal or tracheal cough reflexes show increased sensitivity in most infective and inflammatory airway diseases.16 The larynx, trachea, lungs, and heart should be carefully auscultated, as should both sides of the chest, in a quiet environment with the animal at rest and after the rate and depth of respiration have been increased by application of a rebreathing bag, by temporary occlusion of the nostrils, or by light exercise (see Cyanosis, p. 63). Auscultation of the trachea and larynx should be performed at an early point in the examination, before a rebreathing bag is applied, in order to assess the potential for upper airway obstruction during the examination of the thorax. Auscultation permits detection of turbulent airflow, increased or decreased bronchovesicular sounds, wheezes, crackles, pleural friction rubs, or pleural fluid splashes, all of which indicate disease of the airways, pulmonary parenchyma, or pleura. Wheezes and crackles reflect airway narrowing and dynamic airway collapse, respectively, and are both evidence of small airway disease.16 A small (12 L) or medium-size (52 L) plastic trash bag makes an adequate rebreathing bag; a plastic rectal sleeve is satisfactory for foals or calves if the opening is stretched so that it does not occlude the nostrils. The response to application of a rebreathing bag should also be noted. Most horses tolerate this procedure well and breathe more deeply, whereas animals with chest pain often do not. Normal horses do not cough when the bag is applied unless they have been eating recently, whereas horses with airway irritation caused by pneumonia or other conditions often cough paroxysmally in response to application of a rebreathing bag. The time taken to regain the normal respiratory rate and character after removal of the bag provides a reasonable, qualitative indicator of ventilatory reserve. Normal animals recover quickly, within a few breaths, whereas respiration may be altered for several minutes in animals with significant lung disease. The chest wall should be palpated to detect pleural friction rubs or lesions such as rib fractures, and the symmetry of chest expansion should be determined. Young calves with chest deformation (unilateral or bilateral) and severe inspiratory and expiratory respiratory distress should be evaluated for fractured ribs that are causing tracheal compression/stenosis (see Chapter 31; p. 566 for horses and p. 581 for more on ruminants). Temporary tracheostomy does not relieve this respiratory embarrassment. Unilateral chest pain (pleurodynia) often reduces chest excursion on the affected side. Both sides of the chest should be percussed systematically to detect changes in resonance and chest pain. The caudoventral percussion border in the normal horse traces from the level of the tuber coxae at the seventeenth intercostal space (the horse normally has 18 pairs of ribs) to the level of the tuber ischii at the fifteenth intercostal space to the midthorax at the thirteenth intercostal space to the level of the point of the shoulder at the eleventh intercostal space; it continues as a curving line to a point 1 to 3 inches above the olecranon. The normal caudoventral percussion boundary in cattle and small ruminants traces from the eleventh intercostal space at the level of the lateral edge of the epaxial musculature to the ninth rib at a level halfway between the costochondral junction and the lateral edge of the epaxial musculature to the fifth rib at the olecranon (cattle, goats, and sheep normally have 13 pairs of ribs). Percussion is an important diagnostic tool in all large animals, but it is most useful in foals, calves, and goats. The precise percussion boundaries, degree of resonance, and auscultation findings are influenced by age, size, body condition, fitness, and hair coat, as well as by disease processes; a gas-filled abdominal viscus can also confuse interpretation. Hyporesonance (dullness) may indicate pulmonary consolidation, large mass lesions, cardiomegaly, pleural effusion, or other pleural disease. Free pleural fluid usually causes an abrupt change from normal resonance above a horizontal fluid line to hyporesonance below this line. Hyperinflation or pneumothorax may cause hyperresonance with or without expansion of the normal percussion boundaries. A painful response to percussion may indicate pleuritis or some other inflammatory process involving the parietal pleura. Painful responses may be dramatic and potentially injurious to the examiner. In cattle the presence of thoracic or cranial abdominal pain should be ascertained by application of upward pressure to the xiphoid area with the knee or by application of downward pressure with both hands just caudal to the withers. Animals with cranial abdominal or thoracic pain resist these maneuvers and may make an audible or auscultable grunting noise. Signs such as jugular distention or pulsation (or both) and peripheral edema, which may indicate heart failure, should be noted; and the heart should be auscultated to detect murmurs, dysrhythmias, muffling of heart sounds, or other abnormalities that would indicate the need for further cardiovascular diagnostic procedures. A complete blood count including fibrinogen concentration usually reveals nonspecific findings but may be useful in the evaluation of cases in which primary or secondary inflammatory conditions are suspected and in conditions such as pulmonary thromboembolism secondary to caudal vena cava thrombosis (CVCT) in cattle and guttural pouch mycosis in horses in which blood-loss anemia is likely to be a complicating problem. Acute viral infections often induce a transient anemia and leukopenia, predominantly lymphopenia, followed by a monocytosis during recovery. Neutrophilia and hyperfibrinogenemia are features of many inflammatory conditions but are usually most marked when bacterial infection is involved. Some parasitic diseases (e.g., lungworm infection in cattle) may induce eosinophilia, as may allergic conditions, but eosinophilia is not a common feature of either lungworm infection or allergic RAO in the horse. Nasal or nasopharyngeal swabbing with direct cytologic examination and culture or molecular diagnostic testing are indicated for confirmation of viral or bacterial infections that involve the upper airway. Because a large number of bacteria normally inhabit the nasopharynx, only the presence of Streptococcus equi or some other pathogen not considered part of the resident flora can be considered significant in horses. When clinical signs suggest a viral respiratory infection, especially when an outbreak of coughing occurs in several animals, molecular detection by polymerase chain reaction (PCR), antigen detection by antigen-capture enzyme-linked immunosorbent assay (ELISA), or virus isolation from nasal or nasopharyngeal swabs, tracheal wash, and/or buffy coat (ethylenediaminetetraacetic acid [EDTA] blood) samples collected during the acute phase of the disease are indicated, as may viral serologic tests on acute and convalescent serum samples.18 Under these circumstances, sampling as many of the most severely affected, often younger, animals as early as possible in the disease course maximizes the chances of establishing an etiologic diagnosis. Nasal swabs are slightly easier to collect than nasopharyngeal swabs and yield similar results when sensitive PCR tests are used19; however, nasopharyngeal swabs are preferred for virus isolation by many diagnostic laboratories. Nasal swabs are best collected using 6-inch Dacron or Rayon (not cotton) swabs, whereas nasopharyngeal swabs are best collected using longer, larger swabs (e.g., Fox 16-inch Procto Swabs, Allegiance Health Care, Hayward, Calif.). Swabs should be placed in capped tubes for submission for PCR testing or into viral transport medium for immediate submission to the laboratory if virus isolation is to be requested. It is best to use the transport medium supplied by the respective laboratory to ensure compatibility with viral culture systems. Antigen-capture ELISA (e.g., influenza Directigen Flu A test, Becton-Dickinson, Franklin Lakes, N.J.) and PCR tests are available for testing of nasal or nasopharyngeal swabs and other samples to confirm a diagnosis of viral infection.18 These rapid-screening tests are sensitive, less cumbersome to perform than virus isolation, do not require such stringent conditions for handling and transporting samples, and provide results sufficiently quickly that specific control measures can be implemented. However, they do not yield an isolate that can be used to monitor genetic and antigenic evolution of the viral agent. Serologic testing often provides only retrospective information, but it can prove helpful in the formulation of future control measures including vaccination. The larger the number of animals tested, the more informative are the results of serologic testing in a herd or flock. Serologic testing is also indicated when pneumonia caused by Coccidioides immitis or other fungal agents is suspected. Endoscopic examination of the nasal passages, conchae (turbinates), pharynx, larynx, and trachea allows the presence, nature, and source of exudates and the presence of anatomic or functional abnormalities or mass lesions to be noted. Endoscopic examination of the upper airway of the horse to evaluate for partial airway obstruction is best performed without the use of sedatives or tranquilizers if possible because these alter the tone and function of the muscles supporting laryngeal and pharyngeal anatomy and function and may confuse interpretation of endoscopic findings. In the horse the interior of the guttural pouches may be examined by advancing the endoscope through the pharyngeal openings of each guttural pouch using a guidewire such as a closed biopsy instrument passed through the biopsy channel of the endoscope and into the pouch. Most coughing horses have lower airway disease, which may be reflected in accumulation of endoscopically visible exudate in the horizontal trachea, particularly after exercise. Lungworm larvae may be grossly visible in the trachea of horses with lungworm infection. Deeper bronchoscopic examination is necessary to determine whether exudate is a reflection of diffuse airway disease or whether it arises from a specific area of the lung, such as would occur with a pulmonary abscess or a foreign body lodged in a bronchus. Sedation may or may not be necessary to permit endoscopic examination of the extrathoracic airways in the standing horse; however, sedation with xylazine (1.1 mg/kg IV), detomidine (0.04 mg/kg IV), or romifidine (0.04 to 0.12 mg/kg IV) supplemented with butorphanol (0.1 to 0.2 mg/kg IV) is recommended to facilitate bronchoscopy and to dampen the cough reflex, as is spraying of the carina and bronchial branches with 2% lidocaine via the biopsy channel of the endoscope to reduce coughing during the procedure. Tracheobronchial aspiration and bronchoalveolar lavage (BAL) can be performed by introduction of appropriate catheters via the biopsy channel of the endoscope. Tracheal aspiration with cytologic studies and quantitative or semiquantitative aerobic and anaerobic culture, or PCR testing, of collected samples is indicated in the evaluation of patients suspected of having disease of the lungs or pleura, particularly if an infectious cause is likely. Tracheal wash samples collected using the percutaneous transtracheal technique are preferred for bacterial culture because they are not contaminated by oropharyngeal organisms.20,21 Collection of samples by introduction of a sterile catheter through the biopsy channel of a presterilized endoscope inserted within a sterile sheath, or by introduction of a double-sheathed catheter via the biopsy channel, is acceptable and less subject to complications than is the transtracheal technique, but samples obtained transendoscopically may be contaminated with Pseudomonas species and anaerobic bacteria despite use of a guarded technique.21 If tracheal aspiration is to be performed percutaneously, the procedure should precede endoscopic examination of the lower airways to avoid contamination. Culture may fail to reveal the primary bacterial pathogen, especially in animals that have been treated with antibiotics. The diagnostic value of tracheal wash samples can be improved by discontinuing antibiotic therapy for at least 24 hours before collecting samples, rapidly processing samples, using antibiotic removal devices and selective culture media and, if necessary, repeating sampling. Culture results should be evaluated in relation to the clinical signs, clinical experience (especially on the farm of origin of the patient), results of cytologic evaluation, and response to treatment. The trachea is not a sterile environment; therefore culture of small numbers of bacteria without cytologic evidence of infection is of questionable significance. Samples collected by BAL are less useful than those collected by tracheal aspiration for evaluating infectious lower airway diseases and focal pulmonary conditions because the technique samples secretions from only a limited area of the caudodorsal lung on one side and is subject to contamination during introduction of the BAL tube through the nasopharynx. However, BAL yields better samples for cytologic assessment and is therefore preferred for evaluation of patients with generalized, noninfectious lower airway diseases such as RAO or silicosis.18 BAL samples can be collected under endoscopic guidance by means of catheters introduced through the biopsy channel of a suitable endoscope (<9 mm in diameter and longer than 180 cm) or by using commercially available BAL tubes. The advantage of collection via endoscopy is that specific areas of the lung can be sampled, although gaining access to the cranial lung lobes is difficult with any method. Differential cell counts in BAL fluid of normal horses vary depending on age, environment, activity, and other factors. In young horses (<6 years of age), the distribution of macrophages, lymphocytes, neutrophils, mast cells, and eosinophils is on average 65%, 30%, 3%, 0.5%, and 0%, respectively.22 In horses older than 6 years of age, the neutrophil population may reach 15% in normal horses, with a corresponding decrease in the proportion of lymphocytes and macrophages.22 The percentage of neutrophils in normal calves is generally within the range of 5% to 20% depending on calf age, type, health status, and sampling method.23,24 Neutrophils are usually the predominant cell type in BAL fluid or tracheal washes of horses with RAO and in animals with bacterial infections. The neutrophils usually show degenerative changes when significant bacterial infection is present. Large numbers of eosinophils are a feature of parasitic infections such as lungworms, some allergic conditions, and infiltrative eosinophilic interstitial pneumonia. The presence of hemosiderin-laden macrophages is an indication of recent pulmonary hemorrhage, and finding refractile intracytoplasmic granules in macrophages or giant cells should raise the suspicion of silicosis, especially in animals showing radiographic evidence of marked pulmonary interstitial disease. Noting the presence, number, location (intracellular or extracellular), and Gram stain characteristics of bacteria permits rational selection of antibiotics for treatment while awaiting the results of culture and susceptibility tests.
Alterations in Respiratory Function
Cough
Definition
Pathophysiology
Neural Pathways Involved in the Cough Reflex.
Mechanics of Coughing
Cough Stimuli.
Approach to Diagnosis of Coughing
History.
Physical Examination.
Complete Blood Count.
Nasal or Nasopharyngeal Swabbing.
Endoscopic Examination.
Tracheal Aspiration.
Bronchoalveolar Lavage.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
