CHRONIC OBSTRUCTIVE LUNG DISEASE IN HORSES 
J. R. Gillespie and W. S. Tyler* 
Tyler and co-workers showed the morphology of the 
lungs of the horse resembled those of man more closely 
than those of dogs, cats, swine, monkeys (Rhesus), 
cattle, sheep, rabbits, rats or guinea pigs. This group 
confirmed earlier studies that showed the horse had 
naturally occurring pulmonary emphysema and chronic 
bronchitis which were similar to these diseases in man. 
The incidence of these diseases in the horse population 
is not known. Certain feed and environmental conditions 
have historically been reported as being important in 
these diseases. 
We found horses with chronic obstructive lung disease 
have greater than normal pulmonary arterial pressure 
and less than normal lung compliance and pulmonary 
diffusion capacity. These animals have an increased 
pulmonary resistance and initial studies indicate an 
impairment to maximum expiratory flow rate at various 
lung volumes. 
Quantitative electron microscopic studies have shown 
a decrease in pulmonary capillaries/cm of alveolar wall 
as well as other cell population changes in the emphyse- 
matous horse lung. 
Most recent studies of scanning electron micrographs 
of sections of emphysematous horse lungs have shown 
new "scale-like" lesions heretofore not seen in emphy- 
sematous lungs. 
INTRODUCTION 
We reviewed the comparative research on 
chronic respiratory disease in the horse in 
1968. Herein we intend to update that review. 
The role of certain phenomena that are com- 
• University of California, Davis. 
mon to all living tissue versus special phe- 
nomena that are peculiar to individual species 
pinpoints the importance of animal selection 
for experimental studies. We postulated that 
structural arrangements of lungs are important 
in determining their susceptibility to certain 
diseases. We took note that horses were reported 
to have chronic bronchitis and alveolar emphy- 
sema in a form similar to that which occurs in 
man. 3* We confirmed earlier studies that 
showed the similarity of these diseases in horses 
and human beings.^ 
DISCUSSION 
Tyler and co-workers ^ studied gross and sub- 
gross anatomical features of lungs from nine 
mammal species. They concluded that of these 
species, the horses had a lung most like that 
of man. A summary of their results is shown 
in Table I. 
We examined the ultrastructure of the nor- 
mal and emphysematous lungs of more than 
sixty horses with the conventional electron mi- 
croscope. The general morphology of the blood/ 
air pathway and the individual cell types were 
found to be similar to those of other species.^ 
We determined the pulmonary capillary and cell 
density of normal and emphysematous lungs 
Table I. — Subgross Morphology of Horse Lung Compared with Seven Other Species 
Species 
Lobules 
Pleura 
Typical Distal Airway 
Structures Supplied by the 
Bronchial Artery 
Horse 
Man 
Incompletely separated 
Extensive interlobular 
connective tissue 
Thick 
Terminal bronchiole 
Respiratory bronchioles 
are rare and poorly 
developed 
Bronchi, vasa vasorum of PA & PV 
Pleura and interalveolar tissue 
Some interalveolar septa 
Cattle 
Sheep 
Swine 
Completely separated 
Extensive interlobular 
connective tissue 
Thick 
Terminal bronchiole 
Respiratory bronchioles 
are rare 
Bronchi, vasa vasorum of PA & PV 
Pleura and interlobular tissue 
Dogs 
Cats 
R. Monkey 
Very poorly defined 
Little interlobular 
connective tissue 
Thin 
Respiratory bronchiole 
Terminal bronchioles 
are short 
Bronchi, vasa vasorum of PA & PV 
223 
