Studies on the therapy of equine chronic obstructive pulmonary disease

Abstract

The review of the literature indicated that although there have been several recent studies into the aetiology and pathology of equine chronic obstructive pulmonary disease (CORD) as well as into the functional disturbances occurring in this disease, very few objective studies have been carried out into the management and therapy of affected horses. The effects of environmental control and several therapeutic agents in COPD affected horses are therefore investigated in this thesis.The use of a controlled environment, i.e. minimising exposure to the aetiological antigens which are contained in hay and straw, by bedding horses on peat or shredded paper and feeding a complete cubed diet, allowed symptomatic COPD affected horses to become asymptomatic within 4 to 32 days (mean (- S.D.) 9.0 - 4.8 days). When asymptomatic, their respiratory function values did not differ significantly from those of normal horses. This indicates that the pathophysiological changes occurring in equine COPD are reversible and that most COPD affected horses are capable of regaining normal pulmonary function when contact with the aetiological antigens is minimised.Symptomatic COPD affected horses are those showing clinical signs of COPD with abnormal pulmonary function values, as described by McPherson et al., 1978 , i.e. maximum change in intrathoracic pressure (max.APpl) > 6 mm Hg and partial pressure of arterial oxygen (PaO₂)< 82 mm Hg. Asymptomatic COPD affected horses are those previously shown to be affected with COPD by the above-mentioned criteria which, at the time of the present examination, are clinically normal and their pulmonary function values within normal ranges, i.e. max.ΔPpl <6 mm Hg and PaO₂ > 82 mm HgInhalation or intravenous administration of bronchodilator drugs (atropine, isoprenaline, terbutaline, clenbuterol and etamiphylline camsylate) to symptomatic COPD affected horses brought about a temporary, marked improvement in clinical signs, accompanied by significant decreases in max.ΔPpl and significant increases in PaO₂. These findings show that airway spasm is involved in the pathogenesis of equine COPD. Although the therapeutic use of parenterally administered bronchodilator drugs in this disease is subject to many limitations including partial effectiveness, short duration of action and side effects, this form of therapy could be of value as a temporary measure in the treatment of acute or severe attacks .Studies into the efficacy of orally administered bronchodilator drugs for equine COPD proved disappointing. When horses were housed in the natural antigen challenge environment, i.e. exposed to poor quality hay and straw bedding which was dusty and visibly contaminated with moulds, and treated with oral clenbuterol or etamiphylline camsylate, they remained symptomatic. Apart from the significant decreases in respiratory rate and max.ΔPpl recorded on 2 days during the clenbuterol trials, there were no significant changes in their pulmonary function values from those recorded when horses were untreated and housed in similar conditions. In addition, neither drug significantly hastened the remission of clinical signs which normally occurred when symptomatic COPD affected horses were housed in the controlled environment.In contrast to the results of oral bronchodilator treatment, studies on the prophylactic treatment of asymptomatic COPD affected horses with inhaled sodium cromoglycate proved hopeful. In preliminary studies, prophylactic sodium cromoglycate inhalation in 2 affected horses prevented the exacerbation of respiratory disease, normally observed in COPD affected horses 4 to 8 hours after experimental Micropolyspora faeni inhalation challenge. These studies were followed by a clinical trial with 56 COPD affected horses in which it was shown that a linear response existed between the number of successive days treatment with this drug and the duration of remission of COPD, while horses, were exposed to natural challenge. The protective period was 3.6 ± 1.1 days (mean ± S.D.) after a single sodium cromoglycate treatment, 8.0 ± 3.4 days after 2 days treatment, 11.9 ± 2.9 days after 3 days treatment and 24.3 ± 13.4 days after 4 days treatment.In a 28 day trial, two successive days sodium cromoglycate treatment administered at weekly intervals was effective in preventing the onset of CDPD in 6 out of 8 affected horses housed in the natural challenge environment. These experiments show that prophylactic treatment of asymptomatic COPD affected horses with inhaled sodium cromoglycate is an effective method of controlling this disease. As sodium cromoglycate is believed to act by stabilising mast cell membranes, these results suggest that pulmonary mast cell degranulation is involved in the pathogenesis of equine COPD.Sodium cromoglycate treatment would be useful when unavoidable exposure to the aetiological antigens is anticipated, for instance, during transportation or when horses are moved temporarily away from the home environment. Long term intermittent sodium cromoglycate treatment could facilitate the management of a horse kept at livery or in large stables where the provision of special environmental control measures to individual animals may prove difficult to institute.