Abstract
Equine recurrent airway obstruction (RAO) has been used as a naturally occurring model of human asthma. However, it is unknown whether there is an early-phase response in RAO. The aim of this study was to determine whether exposure to organic dust induces immediate changes in lung function in RAO-affected horses, which could be mediated by airway mast cells. Six RAO-affected horses in remission and six control horses were challenged with hay-straw dust suspension by nebulization. Total respiratory resistance at 1 Hz, measured by forced oscillation, was increased from 0.62 +/- 0.09 cmH(2)O.l(-1).s (mean +/- SE) to 1.23 +/- 0.20 cmH(2)O.l(-1).s 15 min after nebulization in control horses (P = 0.023) but did not change significantly in the RAO group. Total respiratory reactance at 1 Hz (P = 0.005) was significantly lower in the control horses (-0.77 +/- 0.07 cmH(2)O.l(-1).s) than in the RAO group (-0.49 +/- 0.04 cmH(2)O.l(-1).s) 15 min after nebulization. Bronchoalveolar lavage fluid (BALF) histamine concentration was significantly elevated 10 and 20 min postnebulization in control horses but not in RAO horses. Minimum reactance at 1 Hz in the early postnebulization period significantly correlated with both prechallenge BALF mast cell numbers (r = -0.65, P = 0.02) and peak BALF histamine concentration postnebulization (r = -0.61, P = 0.04). In conclusion, RAO horses, unlike human asthmatic patients, do not exhibit an early-phase response. However, healthy control horses do demonstrate a mild but significant early (<20 min) phase response to inhaled organic dust. This response may serve to decrease the subsequent dose of dust inhaled and as such provide a protective mechanism, which may be compromised in RAO horses.
Highlights
HUMAN ASTHMATIC PATIENTS develop an early-phase bronchoconstriction within minutes of exposure to inhaled allergens
The degranulation of basophilic blood cells and pulmonary mast cells in vitro in response to mold spores has been demonstrated to be greater in cells derived from Recurrent airway obstruction (RAO)-affected horses than in those derived from control horses [13, 19]
Exposure to hay and straw for 5 h increased the concentration of histamine in bronchoalveolar lavage (BAL) fluid (BALF) in RAO-affected horses but not in control horses [26]
Summary
HUMAN ASTHMATIC PATIENTS develop an early-phase bronchoconstriction within minutes of exposure to inhaled allergens. The activation of mast cells in response to inhaled allergen results in the release of mast cell mediators, including histamine, tryptase, chymase, cysteinyl leukotrienes, and prostaglandin D2 [8, 14]. These mediators can induce airway smooth muscle contraction and the observed early-phase asthmatic response [8, 14]. Recurrent airway obstruction (RAO) is an equine inflammatory airways disease characterized by the development of a late-phase response after exposure to organic dust, typically from hay and straw.
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