F. J. Derksen

The pathogenesis of chronic obstructivepulmonary disease of horses

Summary Present evidence suggests that chronic obstructive pulmonary disease (COPD) of horses is a delayed hypersensitivity response to inhaled antigens, particularly the thermophilic moulds and actinomycetes that grow in damp hay. Within several hours of exposing COPD-susceptible horses to such hay, neutrophils invade the lung and accumulate in the lumens of airways, particularly bronchioles. The inflammatory response is accompanied by increased levels of histamine in bronchoalveolar lavage fluid, increased plasma levels of the inflammatory mediators thromboxane and 15-hydroxyeicosatetraenoic acid (15-HETE), and a decrease in the production of prostaglandin (PG) E2 by the airway mucosa. During acute exacerbations of COPD, airways exhibit nonspecific hyperresponsiveness and become obstructed as a result of bronchospasm and the accumulation of mucus and exudates. Bronchospasm is due largely to activation of smooth muscle muscarinic receptors by acetylcholine (ACh). Because the in vitro response of smooth muscle to ACh is unaltered, the increase in airway smooth muscle tone is probably a result of activation of airway reflexes by inflammatory mediators and decreases in inhibitory mechanisms such as the intrapulmonary nonadrenergic noncholinergic nervous system and the production of PGE2 in affected horses. The diffuse airway obstruction leads to uneven distribution of ventilation, ventilation/ perfusion mismatching, and hypoxaemia. As a result of the increased respiratory drive caused by hypoxaemia and the presence of airway obstruction, horses adopt a characteristic breathing strategy in which very high peak flows at the start of exhalation rapidly diminish as exhalation proceeds.

Equine Multinodular Pulmonary Fibrosis: A Newly Recognized Herpesvirus-Associated Fibrotic Lung Disease:

Pulmonary fibrosis and interstitial lung disease are poorly understood in horses; the causes of such conditions are rarely identified. Equine herpesvirus 5 (EHV-5) is a γ-herpesvirus of horses that has not been associated with disease in horses. Pathologic and virologic findings from 24 horses with progressive nodular fibrotic lung disease associated with EHV-5 infection are described and compared with 23 age-matched control animals. Gross lesions consisted of multiple nodules of fibrosis throughout the lungs. Histologically, there was marked interstitial fibrosis, often with preservation of an “alveolar-like” architecture, lined by cuboidal epithelial cells. The airways contained primarily neutrophils and macrophages. Rare macrophages contained large eosinophilic intranuclear viral inclusion bodies; similar inclusion bodies were also found cytologically. The inclusions were identified as herpesviral-like particles by transmission electron microscopy in a single horse. In situ hybridization was used to detect EHV-5 nucleic acids within occasional macrophage nuclei. With polymerase chain reaction (PCR), the herpesviral DNA polymerase gene was detected in 19/24 (79.2%) of affected horses and 2/23 (8.7%) of the control horses. Virus genera–specific PCR was used to detect EHV-5 in all of the affected horses and none of the control horses. EHV-2 was detected in 8/24 (33.3%) of affected horses and 1/9 (11.1%) of the control horses. This disease has not been reported before, and the authors propose that based upon the characteristic gross and histologic findings, the disease be known as equine multinodular pulmonary fibrosis. Further, we propose that this newly described disease develops in association with infection by the equine γ-herpesvirus, EHV-5.

Relationship between clinical signs and lung function in horses with recurrent airway obstruction (heaves) during a bronchodilator trial.

During a trial to determine the dose response to the beta2-adrenergic agonist pirbuterol, we judged the severity of airway obstruction by use of a clinical scoring system and compared this to objective data obtained by quantitative measures of lung function. Six horses affected by recurrent airway obstruction were used in this trial. Four hundred and sixty-eight measurements of lung function and clinical scores were obtained from 13 measurement periods when horses received each of 6 doses of pirbuterol. Scores of 1-4 were assigned to degree of nasal flaring and abdominal effort and summed for a total score. The veterinarian scoring the signs did not know the dose of pirbuterol received by the horse and was unaware of the lung function data. Nasal, abdominal and total scores were significantly related to changes in lung function and changes in breathing pattern. There were significant differences between total scores greater than 5 in indices that reflected changes in breathing strategy (peak inspiratory and expiratory flow), peripheral airway obstruction (dynamic elastance), and effort of breathing (maximal change in pleural pressure). Below a total score of 5, there were fewer significant differences in lung function even though measurements of pulmonary resistance and dynamic elastance indicated considerable airway obstruction. Failure of clinical score to reflect this low-grade airway obstruction suggests that airway disease is underdiagnosed and its detection would be helped by the availability of a convenient lung function test.

In vitro responses of airway smooth muscle from horses with recurrent airway obstruction

The in vitro contractile and relaxant responses of tracheal smooth muscle strips (TSM) and third-generation bronchi (3B) of control horses and horses with recurrent obstructive disease (heaves) were compared. Acetylcholine (ACH) sensitivity of the diseased tissues was less than that of tissues from control horses, especially at the level of the third generation (EC50 controls 15 +/- 11 microM vs 81 +/- 17 microM for heaveys). Despite tracheal and bronchial hyporesponsiveness to ACH, these tissues from heavey horses were hyperresponsive to EFS. The inhibitory effect of isoproterenol and electrical field stimulation (EFS) was examined in histamine-precontracted airways pretreated with atropine (1 microM), indomethacin (3 microM), and phentolamine (1 microM). Precontracted tissues of both groups at both levels of airway demonstrated similar concentration-dependent relaxation in response to cumulative doses of isoproterenol (0.1 nM to 0.1 mM). EFS of precontracted tissues caused 80-90% relaxation in the TSM in both groups and 21% relaxation in 3B of the control group. Electrical field stimulation relaxation of precontracted tissues was not seen in the diseased 3B, suggesting a lack of inhibitory innervation. EFS of similarly precontracted tissues plus propranolol (1 microM) resulted in only 46% relaxation in the TSM of both groups but did not reduce the EFS relaxation of control 3B, suggesting the presence of both sympathetic and nonadrenergic noncholinergic nerves (NANC) in the trachea but only NANC in the 3B.

Effect of tracheal mucus and tracheal cytology on racing performance in Thoroughbred racehorses

REASON FOR PERFORMING STUDY Accumulations of mucus within the trachea are often found during endoscopic examinations of the airways of poorly performing racehorses, but the clinical importance of this finding is unknown. OBJECTIVES To determine the effect of tracheal mucus, pharyngeal lymphoid hyperplasia (PLH) and cytological indices of tracheal aspirate on racing performance in Thoroughbred horses assessed by race place and whether the horse was raced. METHODS Endoscopic examination of the nasopharynx, larynx and trachea was performed, and a tracheal aspirate obtained monthly at Thistledown racetrack from April to December, 2002 and 2003. Horses received a score of 0-4 for the degree of PLH and 0-4 for the amount of mucus visible in the trachea. The tracheal aspirate was assessed for turbidity, and total and differential cell counts. Generalised estimating equations models were used as repeated measures models for each risk factor and the level of association assessed through the risk factors P value in the model. RESULTS Moderate to severe tracheal mucus (2-4) was a risk factor for poor racing performance. There was no association between degree of PLH, cell counts or turbidity of tracheal wash fluid and racing performance. However, horses that raced had higher total neutrophil counts in tracheal wash aspirates than horses that did not race. CONCLUSIONS Grades 2-4 tracheal mucus should be considered a potential cause of poor racing performance in Thoroughbred horses. CLINICAL RELEVANCE Because moderate to severe tracheal mucus accumulation, and not increased tracheal neutrophils, was a risk factor for poor racing performance, functionally significant airway inflammation may best be confirmed by the presence of mucus rather than increased number of neutrophils in the trachea.

Efficacy of three corticosteroids for the treatment of heaves

This study used a cross-over design to compare the efficacy of 3 corticosteroids for the relief of airway obstruction and inflammation in 9 heaves-affected horses. The severity of airway obstruction and inflammation was quantified by measurement of lung function and by bronchoalveolar lavage fluid (BALF) cytology, respectively. Airway obstruction was induced by stabling the horses and they remained stabled during the 10 day treatment period. Lung function was measured before treatment (baseline), at Days 3, 7, and 10 of treatment, and after 30 days at pasture. BALF cytology was investigated at baseline, Day 10, and at pasture. All 9 horses received the following 4 treatments in random order: no treatment, daily oral prednisone tablets (1 mg/kg), daily i.v. dexamethasone solution (0.1 mg/kg), and i.m. dexamethasone-21-isonicotinate (0.04 mg/kg) every 3 days. When horses received no treatment, lung function did not change significantly during stabling but improved at pasture. In all horses, daily i.v. administration of dexamethasone solution improved lung function within 3 days to levels as good as or better than those measured at pasture. Dexamethasone-21-isonicotinate was rapidly effective in 8 of 9 horses. The other horse did not respond to this drug. Prednisone tablets were without effect on Days 3 and 7 of treatment, but by Day 10, 5 of 9 horses showed some improvement in lung function. Dexamethasone i.v. solution decreased the percent neutrophils in BALF at Day 10. Other treatments had no effect on BALF cytology. These results demonstrate that dexamethasone rapidly relieved airway obstruction in heaves-affected horses. Oral prednisone had inconsistent effects but may be beneficial in some horses after more than a week of treatment.

Effect of laryngoplasty on respiratory noise reduction in horses with laryngeal hemiplegia

REASONS FOR PERFORMING STUDY Laryngoplasty is the technique of choice for treatment of laryngeal hemiplegia, with the aim of improving airway function and/or eliminating respiratory noise. However, there are no quantitative data in the literature describing the effect of laryngoplasty on upper airway noise or its relationship to upper airway mechanics in horses with laryngeal hemiplegia. OBJECTIVES To determine whether laryngoplasty reduces respiratory noise in exercising horses with laryngeal hemiplegia; and to establish whether the degree of upper airway obstruction can be predicted by upper airway noise, or the degree of arytenoid abduction correlated with airway obstruction and noise production. METHODS Six Standardbred horses with normal upper airways during maximal exercise were used. Respiratory sounds and inspiratory transupper airway pressure (Pui) were measured in all horses before and after induction of laryngeal hemiplegia and 30, 60 and 90 days after laryngoplasty. Inspiratory sound level (SL) and the sound intensity of the 3 inspiratory formants (F1, F2 and F3, respectively) were measured using a computer-based sound analysis programme. The degree of abduction was graded by endoscopic visualisation 1, 30, 60 and 90 days post operatively. Linear regression analysis was used to determine correlations between Pui, sound indices and grades of arytenoid abduction. RESULTS In laryngeal hemiplegia-affected horses, Pui, inspiratory SL and the sound intensity of F1, F2 and F3 were significantly increased. At 30 days following laryngoplasty, the sound intensity of F1 and Pui returned to baseline values. The sound intensities of F2, F3 and SL were significantly improved from laryngeal hemiplegia values at 30 days post operatively, but did not return to baseline at any measurement period. Sound level, F2 and F3 were significantly correlated with Pui (P<0.05), but the correlations were weak (r2 = 0.26, 035 and 0.40, respectively). Grade of abduction and F2 were positively and significantly correlated (P<0.006, r2 = 0.76). Grade of arytenoid abduction and Pui were not correlated (P = 0.12). CONCLUSIONS Laryngoplasty reduced inspiratory noise in laryngeal hemiplegia-affected horses by 30 days following surgery, but did not return it to baseline values. While upper airway noise and Pui were correlated, this relationship was insufficiently strong to predict Pui from noise in individual animals. The degree of arytenoid abduction was not correlated with Pui, but was positively correlated with noise production. POTENTIAL RELEVANCE Laryngoplasty reduces upper airway noise in horses with laryngeal hemiplegia, but is not as effective as bilateral ventriculocordectomy in this regard, although respiratory noise reduction occurs more rapidly than with bilateral ventriculocordectomy. Residual noise during exercise cannot be used as a predictor of improvement in upper airway function in individual horses following laryngoplasty. The degree of arytenoid abduction obtained following surgery does not affect upper airway flow mechanics. Interestingly, we found that the greater the arytenoid abduction, the louder the respiratory noise.

Regional Pulmonary Veno-occlusion: A Newly Identified Lesion of Equine Exercise-induced Pulmonary Hemorrhage

Exercise-induced pulmonary hemorrhage (EIPH) is common in horses following intense exertion, occurring in up to 75% of racing Thoroughbreds and Standardbreds. In spite of this, the pathogenesis of EIPH is poorly understood. In 7 racing Thoroughbred horses with EIPH, 6 sections were collected from the left and right lung, representing the cranial, middle, and caudal region of the dorsal and ventral lung (84 sites total). Grossly, both right and left lungs had numerous dark brown to blue-black foci along the caudodorsal visceral pleura. Tissue sections were stained with hematoxylineosin, Massons trichrome, and Prussian blue. Verhoeff Van Gieson and immunohistochemistry for α-smooth muscle actin were used to assess the pulmonary vasculature. Histologic scores (HS = 0–3) were assigned to each region/slide for the presence and severity of 5 findings: interstitial fibrosis, hemosiderin accumulation, pleural/interlobular septal thickness, arterial and venous wall thickness, and evidence of angiogenesis (maximum cumulative HS = 15). Thirty-nine of the 84 (46%) sections were histologically normal (HS = 0); 33/84 (39%) were mildly to moderately affected, with small amounts of hemosiderin and fibrosis (HS = 1–9) while 12/84 (14%), primarily from the dorsocaudal lung, had severe vascular remodeling, fibrosis, and hemosiderin accumulation (HS = 10–15). In the latter, veno-occlusive remodeling of the intralobular veins colocalized with hemosiderosis, fibrosis, hypertrophy of vessels within the pleura, and interlobular septa and bronchial neovascularization. We propose that regional veno-occlusive remodeling, especially within the caudodorsal lung fields, contributes to the pathogenesis of EIPH, with the venous remodeling leading to regional vascular congestion and hemorrhage, hemosiderin accumulation, fibrosis, and bronchial angiogenesis.

Ventriculocordectomy reduces respiratory noise in horses with laryngeal hemiplegia

REASONS FOR PERFORMING STUDY Show and performance horse with laryngeal hemiplegia (LH) often present for excessive respiratory noise rather than significant exercise intolerance. Therefore, the goal of surgery in these horses is to reduce respiratory noise but there are no quantitative studies evaluating the effect of any upper-airway surgery in LH-affected horses. OBJECTIVE To determine whether bilateral ventriculocordectomy (VC) reduces respiratory noise in exercising horses with laryngeal hemiplegia. METHODS Six Standardbred horses with normal upper airways were used in this study. Respiratory sounds and inspiratory trans-upper airway pressure (Pui) were measured in all horses before and after induction of LH, and 30, 90 and 120 days after VC. In horses with LH, spectrogram analysis revealed 3 inspiratory sound formants centred at approximately 400, 1700 and 3700 Hz. Inspiratory sound levels (SL) and the sound intensity of the 3 inspiratory formants (F1, F2, F3 respectively) were measured using a computer-based sound analysis programme. RESULTS In LH-affected horses, Pui, inspiratory SL and the sound intensity of F2 and F3 were significantly increased compared to baseline values. At 90 and 120 days after VC the sound intensities of F2 and F3 returned to baseline values. The Pui and SL, were significantly decreased compared to LH values, but remained different from baseline. CONCLUSIONS VC effectively reduces inspiratory noise in LH-affected horses by 90 days following surgery. Inspiratory trans-upper airway pressures are improved 30 days following VC, but do not return to baseline values. POTENTIAL RELEVANCE VC can be recommended as a surgical treatment of LH-affected horses if reduction of respiratory noise is the primary objective of surgery. Further studies are required to determine if variations of the surgical technique used in this study will have similar results.

Pleuropulmonary and cardiovascular consequences of thoracoscopy performed in healthy standing horses

Six healthy, awake, and pharmacologically restrained mature horses were studied in order to define the changes in cardiopulmonary function during and after exploratory thoracoscopy and to determine the presence of postoperative complications occurring 48 hours after thoracoscopy. In a randomised 3 x 3 latin square design with 2 replications, 18 procedures were performed: 6 right (RTH) and 6 left thoracoscopies (LTH) and 6 sham procedures (STH). Prior to each procedure a physical examination and a bronchoalveolar lavage fluid analysis were performed. During thoracoscopy and sham protocols, horses were sedated with a continual drip of detomidine HCl and data were collected at 6 time intervals: T1 (baseline), T2 (10 min detomidine administration), T3 (first 15 min pneumothorax), T4 (5 min recovery from pneumothorax), T5 (second 15 min pneumothorax), and T6 (10 min recovery from the second pneumothorax and detomidine). An endoscopic thoracic examination was conducted during the 2 pneumothorax periods. An identical protocol was followed for sham procedures without surgery or pneumothorax. Data were analysed by ANOVA with time and surgical procedure as main factors. Physical examinations, thoracic radiography and ultrasound, CBC and bronchoalveolar lavage fluid analysis were performed 48 h after thoracoscopy. Heart rate, respiratory rate, and cardiac output decreased following detomidine administration. There was a trend for cardiac output to be lower during thoracoscopy. Mild systemic hypertension was associated with thoracoscopy although there was no effect on pulmonary arterial pressure. Total and pulmonary vascular resistances were increased following detomidine administration. Thoracoscopy caused a further increase in systemic and pulmonary vascular resistances especially during the second pneumothorax. Arterial O2 tension decreased following detomidine administration and was further decreased during the second pneumothorax period. PaO2 values were lower when thoracoscopy was performed on the left rather than the right hemithorax. No significant complications were found during the 48 h follow-up evaluation. A subclinical postoperative pneumothorax occurred in 2 horses, one of which had sustained a lung laceration by the trocar. Thoracoscopy performed in healthy, awake, and pharmacologically restrained horses did not have detrimental cardiopulmonary effects and did not cause postoperative complications within the first 48 h period.