36 AN INTRODUCTION TO THE STUDY OF VIRUSES 
adherent and leave a bleeding surface when torn away; the surface 
beneath is red and finely granulated. The inflammatory process may 
extend to the larynx and on to the trachea and bronchi. Respiration 
and swallowing become very difficult and inspiration is characterized 
by a whistling noise. Finally the appetite is lost, the animal can neither 
eat nor drink and must be fed by hand (McKinley, 19269). 
Fowl-pest, or Newcastle disease, as it is sometimes called in England, 
is a severe and extremely infectious disorder and is essentially an 
infection of the alimentary tract. The onset of the disease is sudden, 
and affected fowls show a disinclination to move about, preferring to 
sit or stand quietly in a secluded spot. Early in the course of the 
illness there is diarrhoea. Gasping for air associated with jerking 
movements of the head downward and backward is a characteristic 
symptom. This is caused by large quantities of tenacious mucus which 
obstruct the posterior nares and pharynx. Many fowls die of sufto- 
cation early in the course of the disease. In the latter stages of infection 
the legs and wings may be completely paralysed and the birds are 
unable to move. Death usually occurs within a week but may be 
more rapid; it is not uncommon for every bird in an infected flock 
to die. Of the fowls that recover from the acute symptoms, a high 
percentage develop paralysis and only a very few fully recover from 
this paralysis. 
The next four diseases to be described are characterized by growth 
stimulation and tumour formation. The best known and most studied 
is a sarcoma of fowls known as the Rous sarcoma, so called after its 
discoverer who described it first in 1910. He showed that the tumour 
was a spindle cell sarcoma and it was both infiltrative and destructive. 
In 1911 Rous further demonstrated that the tumour was extremely 
malignant and possessed a marked tendency to widespread metastasis. 
Not only was the tumour transmissible to barred Plymouth Rock 
fowls by inoculation with tumour material, but also with bacterio- 
logically sterile Berkefeld filtrates of tumour emulsion. Rous also 
discovered that the tumour could be transmitted to susceptible fowls — 
with dead cells killed by desiccation or by 50 per cent glycerol. 
Fowl leukaemia is of interest because of the resemblance which it 
bears to the leukaemia of man and because it is one of the few examples 
of a blood disease which can be transmitted experimentally. Ellermann 
(1921), quoted by Bedson (1930), recognizes three clinical types of 
fowl leukaemia: the lymphatic, the myeloid, and the anaemic. In 
the lymphatic type there are few clinical symptoms; the blood picture 
