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Keywords:

  • Airway hyper-responsiveness;
  • Forced oscillation technique;
  • Histamine challenge;
  • Recurrent airway obstruction

Abstract

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References

Background: The objective of this study was to examine the relationship between chronic cough, pulmonary mechanical function, bronchial hyper-responsiveness, and peripheral airway inflammation in adult performance horses with nonseptic inflammatory airway disease (IAD).

Hypothesis: We hypothesized that horses with a presenting complaint of cough have a higher percentage of inflammatory cells on bronchoalveolar lavage (BAL), greater airway obstruction, and airway hyper-responsiveness (AHR) than do horses without cough.

Animals/Sample Population: Adult performance horses (n = 137) referred for evaluation of signs of IAD including cough or exercise intolerance (university hospital patients) and BAL samples (n = 142) taken in first opinion practice.

Methods: A descriptive, retrospective cohort analysis was performed to evaluate the association between chronic cough and pulmonary mechanical function, AHR, BAL cytology, patient signalment, and comorbid features (multivariable logistic regression).

Results: Cough was significantly more prevalent in horses >7 years, and best characterized by a high BAL neutrophil count (>5%) and nasal discharge. Lung function mechanics, abnormal thoracic auscultation, and exercise intolerance did not retain statistical significance in the logistical regression analysis of cough. Although AHR was not related to neutrophilic airway inflammation (BAL neutrophils >5%), it was significantly associated with BAL mast cells >2%.

Conclusions and Clinical Importance: Our data support that neutrophilic airway inflammation may potentiate cough without further changing respiratory mechanical function in IAD. In contrast, mast cell release increased AHR without affecting the incidence of cough. Cough may be used as an indicator of neutrophilic airway inflammation in the presence of low-grade nonseptic respiratory disease.

Cough is an important clinical sign in horses, having both infectious and noninfectious etiologies. In the absence of clinical or hematological evidence of respiratory infection, cough when accompanied by abnormal bronchoalveolar lavage fluid (BALF) cytology or pulmonary mechanical dysfunction is defined as inflammatory airway disease (IAD).1 There is, however, an important gap in our knowledge concerning the pathophysiology and pathogenesis of cough in horses with IAD. For example, it is unknown whether cough is associated with a specific type of BALF cytologic abnormality. Furthermore it is unknown whether cough is a reflection of airway dysfunction as previously described for equine heaves.2

Based on more proximal sampling of the airways using tracheal wash (TW) cytology, cough has been associated with excess mucus secretion and neutrophilic inflammation.3–5 Although TW neutrophilia is prevalent in coughing horses, it is unknown whether these neutrophils arise from proximal or peripheral airways, and the discordance between TW and BALF makes it difficult to infer the cellular pathogenesis of IAD from these data.6 For example, Robinson et al7 found that neutrophilic TW cytology was common in stabled pleasure horses in Michigan. Because the majority of these clinically normal horses were reported to have >20% tracheal neutrophils, it was unclear whether this characterized a local phenomenon of the trachea or peripheral airways. Although cough and mucus scores >1 were associated with tracheal neutrophilia in a subset of horses in the latter study, the impact on exercise tolerance remained uncertain. In contrast, Holcombe et al8 demonstrated that moderate to severe mucus accumulation, but not increased tracheal neutrophils, was a risk factor for poor racing performance in thoroughbreds, suggesting that TW cytology may not represent deeper processes in the lung that produce mucus. Hence, the current emphasis has shifted toward the use of BALF cytology and pulmonary function to describe features of IAD.1 The shift is supported by data that clearly link clinical signs (including cough and exercise intolerance) to BALF inflammation and pulmonary dysfunction in horses with either IAD or heaves.9–14 For example, BALF neutrophilia is a common finding in horses with heaves, and the onset of cough coincides with the appearance of a threshold number of neutrophils.2 Horses in remission from heaves also exhibit BALF neutrophilia and airway dysfunction (airway obstruction and airway hyper-responsiveness [AHR]).15 Neutrophilia in BALF also is a prevalent finding in horses with clinical signs of IAD,14 and these horses exhibit airway dysfunction. Hence, peripheral airway inflammation and pulmonary dysfunction are closely associated in horses with IAD and heaves, but these features have not been specifically related to cough in horses with IAD.

Therefore, we undertook a retrospective study in 279 horses with a chief complaint of cough or exercise intolerance and poor performance suggestive of IAD that was confirmed with either BALF cytology or pulmonary function tests. We hypothesized that horses with the complaint of cough have more severe BALF inflammation (ie, higher percentage of inflammatory cells), airway obstruction, and AHR than do horses without cough. The concern over skewing of data toward referrals only led us to examine both referral and nonreferral groups of patients. The referral data (Database I, n = 137) included signalment, history, BALF cytology, and pulmonary function test results of horses referred to the Cummings School of Veterinary Medicine at Tufts University, and the nonreferral data (which did not incorporate pulmonary function tests) included horses seen by veterinarians in 12 practices within North America (Database II, n = 142).

Materials and Methods

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References

Study Overview

All horses included in this study fulfilled the definition of IAD according to the recent Consensus on IAD by the American College of Veterinary Internal Medicine.1 The current study consisted of a retrospective cohort of 137 equine patient referrals to the Pulmonary Function Laboratory (Database I: university hospital patients) and 142 broncho alveolar lavage (BAL) cytology consultations (Database II: field patients) between 1997 and 2003, performed at the Cummings School of Veterinary Medicine at Tufts University (TCSVM). The purpose of this study was to evaluate the association between the complaint of cough and BALF cell types, pulmonary function variables, patient signalment, and comorbid features in the history. In all cases, horse owners were asked if their horse coughed. Only cases where the cough was current and the duration of cough was >3 weeks (defined as “chronic”) were entered. In most horses, the cough was several months to years in duration (intermittent), but the specific length of time was unknown by the owner. Horses were eliminated from the analysis if coughing was classified as “merely occasional,” which included patients with 1–2 coughs restricted to the beginning of exercise or the start of feeding time. Questionnaires for private practice patients were obtained by a variety of primary care veterinarians. This may have prompted somewhat greater variability in historical data collection compared with in-house evaluations that were consistently performed by a small group of clinicians.

The mean age of horses included in the study was 8.6 ± 5.9 and 9.3 ± 5.7 years for Databases I and II, respectively. Major breeds represented included Thoroughbred (31 and 34%, Database I and II, respectively), Quarterhorse (8.8 versus 23.3%), Standardbred (20.4 versus 5.8%), Arabian (5.1 versus 8.7%), Warmblood (16.1 versus 13.6%), draft horses (1.5 versus 4.9%), ponies (1.5 versus 1.9%), and cross-breeds and others (15.3 versus 7.8%). For Database I, the history of turnout and feeding habits was available. The majority of these horses were stabled with access to turnout for >0–12 hours (60/101, 59.4%) or >12 h/d (9/101; 8.9%), and less frequently were reported to lack access to outdoor turnout (no turnout: 32/101, 31.7%). Dry hay was the main forage available to 77% of horses (97/126), whereas 13.5% (17/126) received soaked hay and 9.5% (12/126) received dehydrated, processed hay.

Database I

The medical records of 137 adult horses (mean, 8.6 ± 5.9 years; range, 2–25 years) specifically referred to the TCSVM for a chief complaint of cough and/or poor performance with a final diagnosis of IAD, were examined. All horses with this complaint that received both pulmonary function testing and sampling of BAL fluid during this period were included in the analysis. Horses that presented with a history (based on owner or referring veterinarian) or current clinical signs of recurrent airway obstruction (RAO, heaves) were excluded from the study. Similarly, patients that presented with signs of respiratory infection (defined as fever with increased respiratory secretions) were excluded. Any horses with evidence of sepsis on BAL also were eliminated (acknowledging the weaknesses of BAL as a diagnostic tool for infection). Because TW cytology and CBC were not routinely performed, some horses in this study may have had subclinical respiratory infection (eg, TW neutrophilia, bacterial pathogens in the trachea).

Categorical variables that were analyzed included sex (male versus female), athletic discipline (racing, nonracing sport horses or other), and the presence or absence of exercise intolerance (ie, poor or decreased performance), mucoid nasal discharge, abnormal lung sounds (crackles or wheezes during baseline or rebreathing examination—all horses were auscultated during rebreathing), and increased respiratory rate at rest (>20 breaths per minute [bpm]). Endoscopy was not performed in all horses, because this represented an additional expense to the owners and was not considered a definitive diagnostic test for IAD. Moreover, BAL was performed without an endoscope, so mucus scores were not collected consistently enough for inclusion.

Database II

Dichotomous variables that were recorded from submission forms and dialogue with contributing practices (n = 12) included the following: age (≤7 versus >7 years), sex (male, female), exercise intolerance (presence or absence), exercise-induced pulmonary hemorrhage (presence or absence), and chronic cough (>3 weeks, presence or absence). Although no horse in Database II was reported to exhibit respiratory infection, any horses that showed sepsis on BAL cytology were excluded from the study. A TW cytology was not available to exclude infection. The certainty with which a diagnosis of IAD could be excluded was lower for Database II in that none of the horses received pulmonary function tests. Therefore, some horses with a normal BAL in Database II may have been removed from the study unnecessarily, biasing the patient selection toward airway inflammation.

BAL

BAL was performed in all referral horses (Database I) on the day of admission, with a commercial cuffed BAL tubea and 2 aliquots of 250 mLs warmed saline, as described previously.16 The 2 samples subsequently were pooled, and slides were prepared by cytocentrifugation. Slides were stained with modified Wright stainb and Toluidine Blue,c the latter for enumeration of mast cells.17 Cells were classified by one of the authors (A.M.H.) as percentage of macrophages, lymphocytes, neutrophils (PMN), eosinophils, and mast cells by counting a minimum of 400 cells (× 1,000 magnification). A diagnosis of IAD was made if the percentage of BAL cells was >2% mast cells, >5% neutrophils, or >1% eosinophils.9 BAL in the field (database II) was performed in the same fashion except that the volume instilled ranged from 300 to 500 mL and air-dried sediment smears were made from 10-mL samples. All slides were stained with Diff Quikc and Toluidine Blued and cells were classified by the same person.

Measurement of Respiratory System Mechanics in Referred Horses (Database I)

Mono sinusoidal, multifrequency forced oscillatory mechanics (FOM, 1–3 Hz) were used to measure total respiratory system resistance (Rrs) and reactance (Xrs) in sedated horses (0.4–0.6 mg/kg xylazine IVd) as described previously.10,18,19 In brief, sinusoidal flow (1–6 Hz) was generated using compressed air (75 psi) released through a proportional pneumatic valvee and superimposed over the horse's spontaneous breathing frequency via a latex-sealed low dead space facemask. Flow at the mask opening was measured with a pneumotachographf and the difference between mask and atmospheric pressures was recorded with a differential pressure transducer.g Total respiratory impedance (Zrs=Xrs+Rrs) was calculated as the ratio of instantaneous pressure at the airway opening to flow (averaged over 10-second periods) with a microcomputer and commercial software.h Coherence was computed for each frequency, and only values with a coherence >0.9 were accepted for analysis. Total respiratory system resistance was derived from Zrs, using the following formula:

  • image(1)

where I is inertance, f is oscillatory frequency, and C is compliance.19 Frequency dependence was expressed as the baseline ratio of Rrs at 1 Hz/Rrs at 2 Hz (decrease in resistance with increased input frequency) as an indicator of heterogeneity of airway constriction in horses with peripheral airway obstruction.18 Resonant frequency (Fres), which increases with bronchoconstriction, was defined as the impulse frequency (Hz) at which impedance measurements (Zrs) entirely comprised resistance (ie, Xrs= 0). By using the components of reactance, inertance (I) and compliance (C), Fres can be calculated according to the following formula20:

  • image(2)

After baseline measurements, values for Rrs were used to monitor the effects of histamine aerosol challenge.

Histamine Bronchoprovocation in Referred Horses (Database I)

Airway hyper-reactivity was assessed via histamine bronchoprovocation as previously described.10 In short, after baseline lung mechanics, Rrs was measured after nebulization with saline (the solvent for histamine) and incremental concentrations of histamine (2, 4, 8, 16, and maximum 32 mg/mL). The dose of histamine that evoked a doubling of baseline Rrs at 1 Hz (PC100 of Rrs) was determined by log-linear interpolation of the dose-response curve.21

Statistical Analysis

Normality of continuous data distribution was evaluated using the Kolmogorov-Smirnov test. The BAL cytology differentials were the only continuous data that were not normally distributed. Coughing versus noncoughing horses were compared using Student's T-test for normally distributed continuous data, Mann-Whitney U test for BAL cell differentials, and χ2 analysis to compare proportions of dichotomous variables. Continuous data were presented as mean ± standard deviation (SD) or median ± interquartile range based on normality of data distribution. Additionally, univariable and stepwise multivariable logistic regression were used to derive odds ratios (OR) for analysis of variables related to cough, using standard criteria of 0.05 for entry and 0.1 for removal of variables. All clinical and laboratory variables were reported as dichotomous variables in the multivariable forward logistic regression analysis of cough for purposes of modeling and to provide results considered clinically useful. All data were analyzed by commercial software.i

Results

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References

The prevalence of historical and clinical variables for both databases is presented in Table 1. Evaluation of “athletic discipline” was available only for horses in Database I and demonstrated the presence of 63 racehorses, 55 nonracing sport horses, and 19 patients with other occupations (pleasure and school horses). Coughing was reported in 51.8% of horses in Database I and 57.7% in Database II. Exercise intolerance was observed more frequently in Database I (referred horses) than Database II.

Table 1.   Univariable association between cough and dichotomous historical and clinical parameters in horses referred (Database I) or evaluated in private practice (Database II).
Variable (V)Database IDatabase II
Prevalence of V/n (%)OR of cough (95% CI)P-valuePrevalence of V/n (%)OR of cough (95% CI)P-value
  • a

    Significant difference in prevalence between Databases I and II (P < .01).

  • b

    b Data not available.

  • c

    c Negative association.

  • n, number of animals for which data were available; RR, respiratory rate; EIPH, exercise-induced pulmonary hemorrhage; OR, odds ratio; CI, confidence interval.

Age > 7 years62/135 (45.9)4.1 (2–8.6)< .00178/134 (58.2)4.0 (1.9–8.3)< .001
Sex male76/133 (57.1)b.7385/127 (66.9)b.065
Nonracing sport horse55/137 (40.1)6.6 (3–14.9)< .001bbb
Exercise intolerance105/120 (87.5)ab.4150/142 (35.2)a0.21 (0.1–0.45)c< .001
RR > 20 bpm63/136 (46.3)2.5 (1.2–5).009bbb
Nasal discharge53/134 (39.6)4.5 (2.1–9.6)< .001bbb
Abnormal auscultation49/134 (36.6)4.9 (2.2–10.6)< .001bbb
EIPHbbb14/140 (10)b.067
Cough71/137 (51.8)Reference variable82/142 (42.3)Reference variable  

The univariable association between cough and historical and clinical variables is further characterized in Table 1. Coughing horses were significantly older (9.6 ± 6.6; 95% CI, 6.7–12.5 years) than noncoughing patients (4.5 ± 2.6; 95% CI, 3.3–5.8 years). Additionally, horses that coughed were more frequently nonracing sport horses, and had abnormal baseline respiratory rate, nasal discharge, and abnormalities on auscultation. In both databases, coughing horses exhibited significantly (P < .001) higher BAL neutrophil percentages and lower macrophage percentages, and in Database II, lower lymphocyte percentages (see Table 2). Assessment of dichotomous patient groups with normal versus neutrophillic BALF specifically confirmed that coughing was associated with significantly higher pulmonary granulocyte percentages compared with noncoughing horses in both databases (38 versus 9% BAL neutrophils in database I [P < .001]; 18.9 versus 7% in database II [P= .001], if BALF > 5% neutrophils). This association was not observed in horses with noninflammatory BALF (≤ 5% neutrophils).

Table 2.   The association between cough and BAL cytology in horses in Database I (referral) and Database II (first opinion, nonreferral practice).
Median cell count in % (IQR)Database I (n = 137)Database II (n = 142)
CoughNo coughP-valueCoughNo coughP-value
  1. The Mann-Whitney U-test was employed for analysis of these data.

  2. BAL, bronchoalveolar lavage; IQR, interquartile range.

Macrophages38.8 (19.3)47.6 (18.6)< .00139 (39.2)52.5 (26).001
Lymphocytes42 (24)45.6 (19.9).31728.5 (27.9)43 (24).001
Neutrophils9 (19.2)3.2 (4.1)< .0019.0 (52.2)1.5 (5.8)< .001
Mast cells2.8 (2.8)3.2 (3.0).0820.75 (3)1 (4.4).232
Eosinophils0 (0.1)0 (0.3).5910 (0)0 (0).814

Although mean respiratory system resistance (Rrs) in coughing patients was higher at every frequency (1–6 Hz) than in noncoughing horses, these data were not statistically significant (Table 3). Similarly, there was no significant difference in respiratory system reactance (Xrs) or resonant frequency (Fres) between coughing and noncoughing horses with IAD. The frequency dependence of Rrs (RRS—1 Hz/RRS—2 Hz) was significantly (P= .034) higher in patients with cough. The dose of histamine that evoked a doubling of RRS (PC100Rrs), although slightly lower in coughing horses, was not significantly different between patients that coughed (3.89 ± 2.90 mg/mL histamine) and those that did not (4.68 ± 5.38 mg/mL histamine).

Table 3.   Pulmonary function variables in horses with inflammatory airway disease that cough versus those that do not cough, derived from Database I (horses that underwent referral).
VariableUnitsPatients with coughPatients without coughP-value
  1. Values are expressed as mean (SD).

  2. RRS, respiratory system resistance (at 1–6 Hz); SD, standard deviation; XRS, respiratory system reactance (at 1–6 Hz); Fres, resonant frequency.

RRS—1 Hzcm H2O/L/s0.696 (0.289)0.651 (0.345).106
RRS—2 Hzcm H2O/L/s0.570 (0.207)0.564 (0.277).423
RRS—3 Hzcm H2O/L/s0.584 (0.195)0.558 (0.258).210
RRS—4 Hzcm H2O/L/s0.585 (0.204)0.563 (0.218).768
RRS—5 Hzcm H2O/L/s0.605 (0.229)0.554 (0.179).485
RRS—6 Hzcm H2O/L/s0.637 (0.2610.552 (0.319).393
XRS—1 Hzcm H2O/L/s−0.364 (0.184)−0.396 (0.233).572
XRS—2 Hzcm H2O/L/s−0.157 (0.274)−0.083 (0.090).305
XRS—3 Hzcm H2O/L/s0.005 (0.217)0.055 (0.088).115
XRS—4 Hzcm H2O/L/s0.125 (0.203)0.180 (0.102).097
XRS—5 Hzcm H2O/L/s0.147 (0.244)0.191 (0.181).454
XRS—6 Hzcm H2O/L/s0.343 (0.292)0.333 (0.249).899
FresHz3.34 (1.28)3.04 (0.81).341
RRS—1:RRS:2None1.27 (0.34)1.15 (0.19).034
RRS—1:RRS:3None1.23 (1.35)1.16 (0.23).175

A stepwise multivariable logistic regression analysis demonstrated that age >7 years and BAL neutrophil counts >5% retained statistical significance as indicators of cough after adjustment for all covariates in both study groups (Table 4). Nasal discharge also was related to cough (adjusted OR, 3.3; 95% CI, 1.3–8.6) in patient referrals, whereas lung function mechanics, tachypnea, and abnormal thoracic auscultation did not retain statistical significance in the logistic regression analysis of cough. Exercise intolerance was associated with a decreased incidence of cough in private practice patients (adjusted OR, 0.25; 95% CI, 0.1–0.6).

Table 4.   Odds ratios (OR) of BAL and clinical parameters as indicators of chronic cough, after adjustment for covariates (multivariable stepwise logistic regression model of dichotomous parameters).
Potential indicators of coughReferral university patients (database I)Private practice patients (database II)
nOR (95% CI)P-valuenOR (95% CI)P-value
  • a

    Negative association;

  • b

    Nonsignificant (P > .05).

  • c

    c Not applicable (data not available for this category).

  • BAL, bronchoalveolar lavage; OR, adjusted odds ratio; CI, confidence interval.

Age > 7 years932.9 (1–8).0391345.2 (2.1–12.9)<.001
Exercise intolerance  b1340.25 (0.1–0.6).003a
Nasal discharge933.3 (1.3–8.6).014  c
Neutrophil count > 5%932.9 (1.1–8).0411343.2 (1.3–7.8).013

Discussion

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References

The current study demonstrates for the first time that horses with IAD exhibiting the clinical sign cough (>3 weeks) have significantly greater neutrophilic airway inflammation, whether the BAL was collected in a referral or first opinion practice. Because referral cases by their very nature are a biased population of horses with greater disease severity and chronicity, the degree of BAL neutrophilia was expected to be more severe. However, the extent of neutrophilia was equivalent between databases in the current report, suggesting comparable patient populations. The neutrophil therefore is a key cell type in the pathogenesis of cough in horses with clinical signs of IAD. The median difference in pulmonary neutrophil percentages in coughing (38 and 19% in Databases I and II, respectively) versus noncoughing horses (9 and 7% for Databases I and II) was substantial, if horses with neutrophilic BALF (> 5%) were considered. The large proportion of horses without neutrophilic inflammation (73/137 [53%] and 78/142 [55%] in Databases I and II, respectively) further highlights the importance of cells other than the neutrophil in IAD patients without cough. Those horses had abnormal BAL percentages of mast cells or eosinophils in BALF. These data suggest that multiple subsets of IAD may be present, with coughing and neutrophilia versus exercise intolerance with mastocytosis or eosinophilia as 2 plausible subcategories of IAD that require further exploration. The finding that neutrophils are important in IAD is similar to previous studies that have linked coughing in heaves to neutrophilia.2 The fact that neutrophils were recovered by BAL (rather than TTW) suggests that the origin of these cells is the peripheral airways, as previously suggested by Viel.22

Athletic discipline and age also were important variables associated with cough in this study. In accordance with a previous review,23 nonracing sport horses were 6.6 × more likely to cough than racehorses in the current report. Cough also was a function of age, so it was not surprising to find that the odds for coughing were 4 × higher in older horses, because age and discipline were essentially covariates in this study. In contrast, studies in Australia and England reported that the risk of coughing decreased with age in the narrow age range of racehorses.4,24 We did not observe an association between age and neutrophil percentages in BAL within the younger (<7 years) age group, which suggests that BAL does not reflect the same inflammatory process as TTW in younger horses, despite sharing a common diagnosis (IAD). This confusion has led a recent consensus panel to employ BAL and pulmonary function tests to characterize the IAD phenotype in horses of any age.1

The strong association between age and coughing, as well as the strong correlation between age and percentage BAL neutrophil counts, suggests that age somehow may contribute to the pathogenesis of cough. The most obvious reason might be the increasing length of exposure to particulate matter in older horses, because a strong neutrophil-specific, chemotactic activity in BALF previously has been associated with high levels of dust exposure.25 Chronic inflammation lowers the activation threshold of sensory nerves,26 and therefore may lead to hyper-sensitivity of cough receptors. Gerber et al27 demonstrated that BALF neutrophilia was common in well-performing stabled sport horses and did not differ significantly between 2 age groups (mean age 5 versus 15 years). However, these horses merely demonstrated cytological evidence of IAD without accompanying clinical signs, thus suggesting lower grade airway inflammation without apparent impact on performance. This observation underlines the complexity of the pathogenesis of clinically apparent IAD, which may include environmental and age as well as genetic risk factors.

Exercise intolerance was highly prevalent in horses referred for respiratory dysfunction (87.5% incidence in Database I versus 35.2% in Database II). The difference in prevalence between these studies may be biased because of greater intensity of questioning in the referral group. Exercise intolerance previously has been related to AHR10 and airway inflammation.28 However, there was no difference between coughing and noncoughing horses with regard to exercise intolerance, and the factors that determine coughing and exercise intolerance may be distinct. As in past studies, exercise intolerance was associated with excessive percentages of mast cells in BAL10 in contrast to cough that is associated with neutrophilia. However, the mechanism of this apparent dichotomy (ie, mastocytosis with exercise intolerance, neutrophilia with cough) is unknown. This may represent 2 different pathogeneses, different stages of the same disease (early versus late), or transient events within the life of individuals. Longitudinal studies and studies that address the immunopathogenesis of each “subtype” of IAD will be important to resolve this finding.

Clinical signs of respiratory disease, including tachypnea at rest, nasal discharge, and abnormal thoracic auscultation (Database I), were approximately twice as prevalent in nonracing sport horses than racehorses. Greater aerobic demands may unmask exercise intolerance to a greater degree and earlier in the progression of IAD in racehorses, resulting in less-obvious clinical signs.23

Despite the strong association between cough and airway inflammation, there was little evidence to suggest that pulmonary function was more deranged in coughing versus noncoughing horses. However, horses displaying cough exhibited higher frequency dependence of RRS (ie, increased ratio of RRS at 1 Hz to RRS at 2 Hz) in the univariable analysis. Frequency dependence is caused by a nonuniform pattern of airway obstruction in the peripheral airways.29 Lower frequency oscillations are required to interrogate the peripheral airways, and the increase in RRS specifically at lower frequencies can be localized to peripheral airway dysfunction. Thus inflammation and nonuniform airway caliber both are found in coughing horses. The link between these findings could be increased constriction of airways or obstruction with secretions. Whether frequency dependence of RRS was associated with worsening of performance or gas exchange in these horses was not determined in this study, but warrants further investigation.

In horses with IAD, coughing was not associated with exacerbated airway responsiveness, but the horses in both study groups exhibited greater responsiveness to histamine than found in normal horses without IAD.10,21 This supports the notion that AHR is not causally linked to coughing. However, BAL mast cell percentage was correlated with airway responsiveness to histamine, in agreement with a past study from this laboratory.10 In contrast, neutrophilia of BAL was not associated with airway hyper-reactivity. Hence, there is further suggestion by this study that IAD may involve different disease pathogeneses such as neutrophil-mediated cough and mast cell-mediated AHR and exercise intolerance, but this hypothesis awaits further study.

In conclusion, coughing was greater in older nonracing sport horses (>7 years) and was best characterized by clinical signs of nasal discharge, and cytologically by increased BALF neutrophil percentage. Airway mechanical dysfunction did not feature more prominently in coughing versus noncoughing horses, although there was evidence of nonuniformity in airway function specifically in patients with a cough. This finding may suggest that neutrophilic inflammation is responsible for increased irritation of airways evoking a cough, but coughing was not associated with greater bronchoconstriction or airway responsiveness than in noncoughing IAD horses. The data presented here further substantiate that IAD has multiple presentations (eg, cough and neutrophilia, mastocytosis, and exercise intolerance) that may represent different pathogeneses that warrant further consideration. Future studies might address the role that neutrophils play in altering the threshold of irritation of tracheobronchial sensory receptors and the effects that treatments decreasing neutrophilic inflammation may have on coughing in horses with IAD.

Footnotes

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References

aBivona Medical Technologies, Gary, IL

bDiff-Quik, Loveland, CO

cToluidine Blue, Polyscientific, Bayshore, NY

dRompun, Bayer, Shawnee, KS

eProportional valve No. 602 00001, Joucomatic, Rueil, France

fFleisch, No 4, OEM Medical, Lenoir, NC

gDP45-28, Validyne Engineering, Northridge, CA

hOn The Nose (ONT), Scientific Solutions, Eden Mills, ON, Canada

iSPSS v. 13.0, SPSS Corp, Chicago, IL

Acknowledgment

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References

We thank Dr Belinda Macordes for technical assistance in data collection.

References

  1. Top of page
  2. Abstract
  3. Materials and Methods
  4. Results
  5. Discussion
  6. Footnotes
  7. Acknowledgment
  8. References
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