AN UNUSUAL CASE OF EPIZOOTIC LYMPHANGITIS IN A MULE
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the parasites multiply by a process of budding and such division forms were not
infrequent in the smears.
Histopathology.—The lesion commences in the lymph follicles of the intes¬
tines where the cells are found with ingested parasites ; these latter then multi-
ply and bring about the rupture of the host cells. The parasites so liberated are
in turn phagocytosed or taken up by other cells which also share the same
fate. In this way an abscess is formed, surrounded by numerous large mono¬
nuclear cells tending to form giant cells. Thus giant cells in various stages of
formation are found around the central mass, which consists mainly of parasites
and debris of degenerated lymphocytes. The organisms are found in great
abundance in the affected tissues both in intra and extra-cellular positions, and
their faint outlines are discernible even in sections stained by haemotoxylin and
eosin (Plate XII, Fig. 5 and Plate XIII, Fig. 6). Individual cells are often loaded
with as many as 10 to 30 parasites and in consequence they are doubled or trebled
in their normal size. A group of parasites in a cell often presents an external
form suggestive of a mulberry. After the abscess has burst, the breaking down
process still continues at the borders and in this way the ulcer gets larger and
deeper and new nodules continue to form at the periphery.
One remarkable feature in this condition is the entire absence of fibrous
tissue formation around the lesions, which might have limited their extension to
the neighbouring tissues. The lesions wherever they occur are structurally
the same. The invaded tissues react by a proliferation of the connective tissue
cells which tend to form giant cells and ingest the parasites, and an inflammatory
abscess is thus formed. Three zones are recognizable in these lesions. The
central zone consists entirely of parasites and the debris of broken down cells,
the intermediate zone of mononuclear cells, and the outer zone of large endothelial
cells and giant cells. In the central zone of the affected lymph glands the para-
sites are found embedded in an amorphous matrix. In sections stained with
haematoxylin and eosin, this central zone presents the appearance of a caseat¬
ing lesion with no evidence of cellular or nuclear elements. However, when
stained for organisms, large masses of cryptococci are seen in this area. The
parasites in the older abscesses are found to have lost their staining properties
considerably. The pus-like material in these abscesses is formed by the death
of the host cells and their subsequent softening, but no actual pus cells have
been seen in the smears from this material. The giant cell reaction in the lesions
would appear to be one of foreign body reaction, and in the absence of pyogenic
organisms and pus cells the extensive necrosis and suppuration in the centre of
the lesions are to be ascribed to the action of the cryptococci. The parasites
liberated in the evacuations of these intestinal ulcers are passed along the gut
when they are phagocytosed and ingested by the cells in the other parts. Exten-
sion of the disease to distal portions of the intestines is thus produced.
The
histological picture, in short, resembles caseating tuberculous lesions, though
caseation is not a feature of equine tuberculosis.