MEDIAL HYPERPLASIA OF 
THE PULMONARY ARTERIES OF CATS 
T. W. Swerczek' 
Medial hyperplasia of the pulmonary arteries 
(MHPA) of cats is a common finding, especially in 
older animals. The lesion is found naturally in approxi- 
mately one-third of the cats. A majority of older cats 
have mild to marked lesions of MHPA. 
The exact mechanism which triggers medial hyper- 
plasia and hypertrophy of the smooth muscle cells of 
the vascular medial layer is unknow^n. However, the le- 
sion appears to be associated by the presence and mi- 
gration of nematode parasites in the lungs. MHPA was 
induced experimentally in 100% of the cats infected 
with Aelurostrovgylus abstrusus and Toxocara cati 
which are natural parasites of cats. The lesion was also 
induced by T. canis and Ascaris suum. 
In cats infected with T. cati, lesions of MHPA ap- 
peared 3 weeks post-infection and the arteries were 
very thickened by 6 weeks. The increased thickness of 
the pulmonary arteries was due primarily to hyperplasia 
of the medial myocjrtes. During the development of the 
lesion, myocytes were hypertrophied and the cytoplasm 
frequently contained pink droplets when stained with 
hematoxylin and eosin. A proliferative endarteritis and 
a periarteritis characterized by the presence of many 
eosinophil leukocytes were common findings in the 
acute disease but were less frequently observed in the 
chronically infected eats. 
One of the causes of MHPA appears to be related to 
the migration of nematode parasites through the lungs. 
Since the lesions in the acute stages are suggestive of an 
allergic response, it is postulated that MHPA is initi- 
ated by local anaphylaxis induced by a parasitic anti- 
gen-antibody reaction. Other non-nematode antigens 
may act similarily; however, none have been identified. 
Since MHPA can be experimentally induced in 
100% of the cats infected with T. cati and A. abstru- 
sus, the cat is an unique model for studying the mecha- 
nisms of arterial hyperplasia and pulmonary hemody- 
namics. 
INTRODUCTION 
Medial hyperplasia of the pulmonary arteries 
(MHPA) is commonly observed in cats. The le- 
sion is more frequently seen in older cats but is 
also present in the young. No apparent breed or 
• Department of Verterinary Science University of Kentucky 
Lexington, Kentucky. 
sex predilection or obvious clinical disturbances 
have been associated with the lesion. 
MHPA was first reported by Campbell in cats 
exposed to low oxygen pressure.^ Others have 
verified the presence of the lesion in cats.^-^ 
Neumann et al. observed MHPA in cats that 
were given metrazol and insulin by injection.^ 
Kell et al. noted MHPA following excitation of 
the brain with electrical impulses and with met- 
razol by injection.^" Recently, MHPA could 
not be induced with metrazol" or with 
serotonin.^- It was concluded that the earlier 
workers used cats that already had the lesion. 
The first association between Aelurostrong- 
ylus abstrusus and MHPA was made in 1938 
by Pritchett.^^ This relationship was subse- 
quently reported by others."-^^ Hamilton^*' 
and Swerczek^i experimentally reproduced 
MHPA with A. abstrusus. Swerczek et al. re- 
ported that MHPA could be induced experimen- 
tally with Toxocara cati as well as other nema- 
todes that migrate through the lungs of cats.^^ 
Rohovsky et al. found MHPA in 6 of 95 
germfree cats.^^ Rogers et al. found the lesion 
in conventional and specific-pathogen-free 
cats. 2^ However, the arterial thickness of the 
pulmonary arteries illustrated in the latter 
paper was very mild when compared with that 
of the experimentally induced lesion. Until re- 
cently, the cause of MHPA was unknown and it 
could not be experimentally reproduced. The 
purpose of this paper is to describe how MHPA 
can be induced experimentally and to describe 
the possible pathogenesis of MHPA. 
In the author's experience, all cats that were 
naturally infected with A. abstrusus had 
MHPA. However, the lesion was frequently ob- 
served in cats that had no evidence of previous 
exposure to A. abstrusus. It was postulated that 
other nematodes that migrate through the pul- 
237 
