SOME REPRESENTATIVE VIRUS DISEASES bled 7, 
is normal but the liver may be enlarged up to eight times its normal 
size and mottled in appearance. The spleen is also enlarged. In the 
myeloid type, the birds do show some recognizable symptoms during 
life. They appear ill and off their feed, and their combs are pale. 
The blood picture is definitely pathological with an enormous 
increase in the number of leucocytes. Counts of 200,000 to 600,000 
per cu.mm. are met with, the normal count in the hen being about 
30,000 per cu.mm. 
In the anaemic type the outstanding feature is the yellow colour of 
the comb and other visible epidermal structures. This yellow colour 
is not due to bile-pigment but to the normal lipochrome of the hen, 
and is therefore an exaggeration of a normal process. The under- 
lying pathological condition is an increase in the fatty substances in 
the plasma, the vehicle of the lipochrome (Bedson and Knight, 1924). 
The total blood count is low, one million or less, as against the normal 
figure of three millions. The liver is enlarged and paler and yellower 
in colour than normal. The spleen is enlarged up to four or five times 
its normal size. 
Fowl leukaemia appears to be a very fatal disease and the fowl seems 
to be the only susceptible animal. 
Of these tumour-forming viruses in fowls, Rous (1943) writes as 
follows— 
Viruses are responsible for a wide variety of naturally occurring meso- 
dermal growths in the domestic fowl—fibromas, sarcomas, myxomas, 
osteomas, chondromas, lymphoid tumours, and so forth—as well as for 
leukaemias and an endothelioma. Carcinomas occur frequently in chickens 
but are usually overlooked, and there has been little effort to determine 
their cause, and this little ineffectual. ... The chicken tumours that yield 
virus are typical neoplasms in all respects, even in metabolic peculiarities, 
and like them they appear hither and yon in the fowl community, giving 
no indication of any connexion with one another. The causative viruses 
are as various as the tumours, each engendering growths of the sort from 
which it was originally procured, and growths of such sort only. 
Rabbits are also susceptible to virus diseases which take the form 
of papillomas on the skin and an infectious papillomatosis of rabbits 
has been described by Shope (1933). These papillomas or warts are 
frequent in Kansas, U.S.A., where about one animal in every twelve 
trapped may be found to carry them. The warts vary in size and may 
be quite small, or large onion-shaped or jagged growths. The number 
of warts on individual animals varies, though exceptional cases have 
occurred where the body was entirely covered with warts, enough, 
