Eight control and 8 asymptomatic COPD-affected horses were given, on separate occasions, inhalation challenges with extracts of Micropolyspora faeni, Aspergillus fumigatus and Thermoactinomyces vulgaris. All horses were also given nebulised phosphate-buffered saline (PBS) challenges and ‘;natural challenges’ (NCs), i.e. exposure to hay and straw, as control challenges. Responses were assessed by clinical, pulmonary mechanics, arterial blood gas tensions, arterial blood pH and bronchoalveolar lavage fluid cytological examinations.
PBS challenges had no effect on control or COPD-affected horses, while NC induced COPD only in the COPD-affected horses. Pulmonary disease, similar to naturally occurring COPD, was induced, only in the COPD-affected horses, by M. faeni and A. fumigatus challenges, thus implicating these organisms in the aetiology of equine COPD. The role of T. vulgaris in the aetiology of equine COPD could not, however, be determined because the T. vulgaris challenges, in addition to inducing pulmonary disease in 4 COPD-affected horses, induced pulmonary disease in 2 control horses which had been unaffected by NC.
The absence of pulmonary disease in control horses after M. faeni, A. fumigatus and NC challenges suggests that equine COPD is a pulmonary hypersensitivity, rather than a non-specific toxic response.