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 Hg
Inhalation 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
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.