6
Studies on Newcastle (Ranikhet) Disease Virus
Period of incubation
In natural outbreaks, it is hardly possible to estimate the exact period of
incubation owing to the rapidity with which the infectious agent becomes dis-
seminated and to the highly fatal nature of the disease. It would appear that
two to five days would be normally required for the first symptom of the dis-
ease to be manifested among birds exposed to infection under field conditions.
When healthy susceptible birds are given free access even for one day to com-
mon feeding and watering along with experimentally infected birds, all the
birds invariably start to die of the disease from about the fifth day onwards
and the deaths continue to occur up to two or three weeks or until the last
bird succumbs. Thus, it is evident that perpetuation of infection in infected
premises is possible, especially where unsatisfactory hygienic conditions
prevail. Doyle [1927] found that four days was the average period of incuba-
tion when fresh virus was used.
Chicks are very highly susceptible and the disease runs a much shorter
course than it does in adults (Plate II, fig. 3).
Suggestions regarding method of natural infection
In nature the infection, in all probability, takes place by ingestion and
cohabitation. Healthy fowls, left in contaminated coops, either empty conse-
quent on the removal of the dead bird previously housed therein, or placed
along with the infected birds, usually contracted the infection and died. The
author has observed that transmission of the disease was seldom successful by
an exchange of respiration.
Pathology
The post-mortem lesions of over two thousand fowls have been examined
There do not appear to be any characteristic lesions of the disease. When
the disease first occurred in India, in most of the cases which were presumably
peracute cases, there were practically no discernible lesions though the fowls
were dying in large numbers. Certain carcases looked absolutely normal.
Continued examination of a large number of fowls from different sources
disclosed small sharply outlined haemorrhages on the gizzard fat, pericardium,
heart muscle, and the mucous membrane of the proventriculus (Plate III,
fig. 1). They were seldom seen, however, on the inside of the breast bone
in which site they are usually observed in fowl plague. Catarrhal enteri-
tis is frequently seen in the duodenum (Plate III, fig 3). In experimental
infection, these lesions were consistently observed. The inside of the proven-
triculus is often semi-dry and coated with thick mucus. The nature of the
lesions in the duodenum invariably depends on the time the bird takes to die,
as also on the severity of the infection. Sometimes the mucous membrane is
studded with pin-head-sized sub-mucous haemorrhagic spots (Plate III, fig 3),
or it may be superficially congested. In some birds diffuse reddish ulcers with
irregular margin and coated with bran-like deposits are seen [Kylasamier,
1931]; such lesions occasionally appear along the entire length of the intes-
tines (Plate III, fig 3). The ileo-caecal valve seldom remains normal (Plate III,
fig. 2). The mucous membrane is ulcerated and coated with necrotic
tissue. In the rectum, numerous haemorrhagic-looking ulcers are seen with a
coating of greyish deposit. Other systemic changes are those of fever. The