Studies on the aetiopathogenesis of equine chronic obstructive pulmonary disease (COPD)

Abstract

This thesis describes an investigation into the aetiopathogenesis of equine chronic obstructive pulmonary disease (COPD).To investigate the aetiology of equine COPD, control and asymptomatic COPD affected horses were given nebulised inhalation challenges with extracts of Micropolyspora faeni (MF), Aspergillus fumigatus (AF) and Thermoactinomyces vulgaris (TV). MF and AF challenges induced pulmonary disease, similar to naturally occurring COPD, only in the COPD affected horses, implicating MF and AF in the aetiology of equine COPD. The role of TV in the aetiology of equine COPD could, however, not be assessed as the TV challenges induced pulmonary inflammation in 2 control horses, which had been unaffected by hay and straw challenges i.e. 'natural challenges' (NC), indicating that the experimental TV challenge differed from the TV challenge which occurs during NC. The absence of pulmonary disease in control horses following MF, AF challenges and after NC suggests that equine COPD is a pulmonary hypersensitivity, rather than a non specific toxic response. In this study, bronchoalveolar lavage fluid (BALF) cytology examinations proved to be more useful for detecting pulmonary disease than clinical, pulmonary mechanics, arterial blood gas tensions and arterial pH examinations.The role of oil seed rape (OSR) (Brassica spp.) in the aetiology of equine pulmonary disease was investigated by exposing horses to a field of flowering B.campestrus and by experimental B.napus inhalation challenges. OSR had no detectable effect on control and asymptomatic COPD affected horses, suggesting that OSR is not a major cause of equine respiratory disease. However, the experimental B.napus inhalation challenges exacerbated pulmonary disease in some symptomatic COPD affected horses, presumably via non specific bronchial hyperresponsiveness/toxicity. Intradermal testing using a commercial B.napus pollen extract suggested that none of the horses investigated was hypersensitive to B.napus pollen antigens.As BALF is comprised of lavage fluid and pulmonary epithelial lining fluid (PELF) in variable proportions, quantitative comparisons of the cellular and molecular components in different BALF samples are valid only if the proportions of PELF in the BALF samples are standardised. Two BALF standardisation techniques, namely the urea and albumen dilution techniques, were evaluated in the horse. While both techniques were found to be satisfactory, the urea dilution technique was considered to be the more accurate.Comparison of the cellular and molecular components of BALF collected from 4 different lung segments of control and symptomatic COPD affected horses indicated that, in these horses, BALF components showed regional homogeneity. This suggests that the composition of PELF is uniform throughout the lungs of these horses and that a single BALF sample, collected from any lung lobe, is representative of the entire lung.The role of mast cells/basophils in the pathogenesis of equine COPD was investigated by quantifying histamine, an indicator of mast cell/basophil degranulation, in plasma, BALF and PELF of control and COPD affected horses, before and at 0.5 and 5h after NC. The PELF histamine concentrations of COPD affected horses were significantly increased only at 5h after NC. NC had no significant effect on the PELF histamine concentrations of control horses nor on the plasma and BALF histamine concentrations of either group. As the histamine concentrations of whole BALF lysates were significantly correlated with the numbers of metachromatically staining BALF cells, presumed to be mast cells and/or basophils, these findings support involvement of a late phase, mast cell/basophil mediated, hypersensitivity reaction in the pathogenesis of equine COPD.Quantification of tryptase, an inflammatory mediator which offers potential advantages over histamine as an indicator of mast cell degranulation, in equine serum and BALF, using a commercial radioimmunoassay kit for human tryptase, was unsuccessful.The role of lymphocytes in the pathogenesis of equine COPD was investigated by determining the lymphocyte phenotype distributions of peripheral blood (PB) and BALF from control and COPD affected horses, before and after NC. Prior to NC, asymptomatic COPD affected horses had a significantly higher proportion of BALF B lymphocytes than control horses, suggesting that these cells have a role in the pathogenesis of equine COPD. NC significantly increased the ratios of CD4+, T helper/inducer lymphocytes and significantly reduced the ratios of CD8+, T suppressor/cytotoxic lymphocytes in BALF from COPD affected horses, suggesting that T lymphocytes have an important role in the pathogenesis of equine COPD.Intradermal mould antigen testing was evaluated as a diagnostic technique for equine COPD. The intradermal endpoint litres of control horses for AF, MF and TV were not significantly different from those of COPD affected horses, suggesting that this technique is of limited value in the diagnosis of equine COPD. Furthermore, the lack of correlation between the intradermal endpoint titres for each antigen and the changes in pulmonary mechanics, arterial blood gas tensions and with BALF neutrophil ratios which had followed previous MF, AF and TV inhalation challenges and NC suggests divergence of equine dermal and pulmonary reactivities to these antigens.Local transendoscopic endobronchial antigen challenge, which has proved to be a valuable clinical and research technique in the study of human pulmonary hypersensitivity, was evaluated in the horse. As local endobronchial challenges with phosphate buffered saline, MF extract and mouldy hay extract induced a non specific pulmonary neutrophilia in both control and asymptomatic COPD affected horses and elicited endoscopically visible responses in a proportion of horses from both these groups, this technique was considered to be of limited value as a clinical and research technique in the study of equine COPD.