Use of methacholine bronchoprovocation test for the diagnosis of asymptomatic severe equine asthma

Reasons for performing study: A group of experts in the field of equine respiratory disorders has recently proposed to rename Inflammatory Airway Disease and Recurrent Airway Obstruction as mild to moderate and severe equine asthma (SEA) respectively, in reference of the human asthma. One of the characteristics of asthma, in equine and human patients, is the absence of clinical signs during clinical remission. Methacholine bronchoprovocation test (BPT), which is a standard diagnostic test for detection of asymptomatic human asthma, has been used in some research protocols in horses but no study has examined its potential value for clinical diagnostic.
Objectives: To assess whether the methacholine bronchoprovocation test is an applicable and repeatable clinical test to distinguish control horses and SEA horses in clinical remission.
Methods: Twelve horses (6 with SEA history and 6 controls). Seven of them (3 SEA and 4 control horses) underwent BPT twice at a 24-hour-interval to determine the repeatability of the test. Then all horses were tested before and after a 7-days period in a stall environment. Simultaneously, usual ancillary pulmonary examinations were performed.
Results: The BPT had a good feasibility and a significant repeatability. Before the stay in stalls, SEA and controls horses could not be differentiated by the BPT. After a 7-days period of straw and hay exposure, the bronchial hyperresponsiveness was increased in both SEA and control horses, while the other clinical or functional parameters were not significantly affected. The 7-days period in stalls resulted in a significant difference in bronchoreactivity between SEA and control horses.
Conclusions: Methacholine BPT does not permit to differentiate control and asymptomatic SEA horses in clinical remission, unless the horses have been kept in stalls. Potential relevance: These results suggest that the mechanisms underlying the bronchial hyperresponsiveness in asthma differ between human and equine patients. Nevertheless, the BPT may be used to detect asymptomatic severe asthmatic horses after a light environmental challenge. Further researches are needed to assess the clinical interest of BPT for subclinical mild-moderate equine asthma patients.