Avian Leucosis Complex in India
285
PATHOLOGY
The following description is based on a study of
naterial from 154 natural cases of the disease from
ll over the country.
Gross lesions. Frequently the most striking
hange noticed post-mortem was a uniform enlarge
ment of the liver and spleen due to a soft, greyish-
yellow infiltrating tissue (Plate XV, fig. 2). The
liver was swollen to several times its normal size,
weighing in some cases as much as 1¼ lb. (normal
weight 2 oz.). The borders were rounded and there
were small grey areas on the surface and dipping
Into the organ producing a greyish roughened hob-
hailed appearance.
The liver was friable, resulting occasionally in
upture and internal haemorrhage. In many such
cases, the kidneys (Plate XVI, fig. 5) were also
affected. The bone-marrow was constantly involved,
being swollen, pale-grey or brick-red in colour and
abnormally solid. Anaemic changes were indicated
by pallor of the comb, skin and visible mucous
membranes.
The commonest manifestation, however, was the
occurrence of tumour-like masses in the internal
organs. In order of frequency in which the viscera
were affected was liver, ovary, mesentery, spleen,
pancreas, kidney, intestine, gizzard, proventriculus,
voluntary muscle and skin. The tumours were
well embedded, white or pink-grey, friable and
varied in size from that of a pea to that of a hen's
egg (Plate XV, fig. 3 and Plate XVI, fig. 4). Often
the affection was localized, but sometimes the in-
volvement of adjacent organs rendered it difficult to
demarcate the boundaries of different structures.
In the nervous form of the disease, characterized
by paralytic symptoms during life, changes were seen
mostly in the sciatic, vagus, brachial and lumbo-
sacral nerves. Instead of having a healthy glistening
white colour, they were greyish yellow, oedematous
and enlarged to several times their original thickness
(Plate XVI, fig. 6). The normal transverse striations
were often indistinct. Growths, millet-seed in size,
were occasionally seen on the surface of such nerves.
The posterior root ganglia of the cord were found
symetrically enlarged in a few cases.
In the ocular form shown by blindness during life,
the iris was thickened, oedematous and depigmented,
the pupil being distorted.
Thickening of the long bones, particularly the
shanks, was seen in two localities in fowls which also
showed other forms of the diease (Plate XVI, fig. 7).
Histology. The enlargement of the liver was
usually due to massive infiltration of the stroma
with immature blood cells which predominated in
the peripheral blood and abounded in the bone
marrow, liver, spleen, kidneys, etc. In the liver,
which is the commonest site of infiltration, both
intravascular and perivascular accumulations were
seen. The visceral tumours were usually composed
of closely packed small and large basic-staining non-
granular mono-nuclear cells (lymphocytes and lym-
phoblasts). The invading cells infiltrated the
parenchyma, and in some cases ultimately replaced
it. There were no degenerative changes in the
normal elements and these remained intact until
entirely replaced by the infiltrating cells. In the
liver the distribution of these cells varied from mild
periportal accumulations to massive infiltrations,
completely masking the normal tissue (Plate XVII,
figs. 8 and 9). The lesions in the nerves consisted
essentially of similar cellular infiltrations among the
nerve fibres (Plate XVII, figs. 10 and 11). The brain,
however, was unaffected even in the nervous form.
In the eyes, the iris showed lymphoidal cell infiltra-
tions ; the cornea was normal in structure, the retina
usually unaltered and the optic nerve showed only
slight cellular infiltration.
DISCUSSION
The consensus of expert opinion [Bullis, 1944]
groups the several manifestations of this disease
under the term 'avian leucosis complex'. As the
disease is characterized by definite tissue changes and
as its transmission is difficult or uncertain [Stubbs,
1938], histopathological evidence alone is considered
sufficient for diagnosis. In this country, the clinical
syndrome, gross lesions and histological features of
the prevailing disease were seen to be typical of avian
leucosis complex and comparable strictly with the
disease existing in Great Britain. All the different
manifestations were encountered, but a striking
feature was the preponderance of the visceral form
of the disease compared with the neural and ocular
forms which are more commonly met with in other
countries. The reasons for this are not known.
All the common breeds were seen to be affected,
but, as the survey was confined to organized poultry
farms maintaining chiefly fowls of exotic breeds,
information on the incidence in indigenous breeds
is scanty. This is an urgent question for investiga-
tion and warrants an extension of the survey to
village flocks.
It was observed in the course of this survey that
the advent of the disease may be hardly noticed in a
flock and the danger of its presence rarely appreciated,
as only a small percentage of birds seemed to be
affected at a time. The appearance of other epidemic-
diseases, such as Ranikhet disease, is a spectacular
event, calling for urgent measures, whereas in this
disease accurate mortality records alone reveal the
true measure of its import. In one of the flocks
the disease has been held to be responsible for
25.6 per cent of losses in mature fowls during a
period of three years. In some flocks it may be far
worse, as the following statement from the Report
U. S. Department of Agriculture [Winton, 1943]