178 ABSTRACTS
Differences occur between the production of dams and daughters which are not due to
the particular mating concerned. It is further concluded, therefore, that nicking may
be a term that is freely used in reference to such differences. Its use in such cases is
erroneous. Nicking was not shown to be a prevailing factor among Jersey cattle, al-
though its possibility should not be denied.
[J. N. W.]
A successful method of immunization against Newcastle disease of fowls.
S. GANAPATHY IYER AND N. DOBSON (1940). Vet. Rec. 52, 52, 889.
T HE authors have been engaged on the investigation of Newcastle disease, commonly
known as Ranikhet disease in India, with particular reference to its control by
means of a suitable vaccine. Experiments were undertaken to determine the effect
of serial transfers of the virus on the chorioallantoic membrane of the developing chick
embryo, using almost the same technique as that described by Burnet (1933 and 1936)
and employing the strain of Newcastle virus that was isolated from the 1933 outbreak
of the disease in Great Britain and had undergone continued passage through fowls at
irregular intervals.
The age of the embryo at inoculation was from 9 to 13 days. The first passage in
eggs was effected by the inoculation of a Berkefeld V filtrate of a 1 per cent emulsion
in saline of liver and spleen, representing fowl passaged virus. Eggs were opened for
examination and collection of the virus on the second or third day after inoculation and
reincubation. Death of the embryo was seldom prolonged beyond the second day.
For the passage subinoculations, from egg to egg, 0.05 c.c. of a 10 per cent suspension
of the infected chorioallantois or the whole embryo, in broth or saline, was used. When
eggs, after inoculation with virus, were incubated for over three days, this often resulted
in partial or complete loss of virulence of the virus. Consequently the period of in-
cubation after inoculation seems to be an important factor. Further, the character of
the lesions is modified by the age of the embryo at the time of inoculation, and the tem-
perature at which the egg is subsequently incubated.
On opening the inoculated egg the chorioallantoic membrane is found to be moist
and oedematous with cloudiness arising from cellular infiltration. There are also irregular
whitish opaque areas on the membrane indicating ectodermal proliferation followed
by, or associated with, vacuolation and necrosis. Small petechial hæmorrhages are
fairly numerous and sharply outlined, particularly on the neck and along the sides of the
ventral aspects of the body. These hæmorrhages were found on the inside of the wings
and the legs, which were highly congested towards the extremities and occasionally on
the chorioallantoic membrane. The best lesions were found following the inoculation
of eggs on the 9th or 10th day of incubation.
The work of Burnet and Ferry (1934) indicated that there was no modification in
the virulence of the Newcastle virus as a result of twelve passages through eggs. But
the present authors' observations showed the necessity for more than twelve passages
before the fowl virus could be attenuated. They found that the virus after cultivation
through eggs for 19 passages remained fully virulent for adult fowls but the fowl sub-
inoculations carried out at the 33rd and the subsequent passages upto the 56th revealed
the fact that the strain of virus had undergone a change, and ceased to cause fatal
results in them and had developed immunizing properties. About two weeks after the
injection of healthy fowls with the egg-passage virus, without any treatment with chemi-
cals (to cause attenuation), they developed immunity which was solid enough to with-
stand subcutaneous injection of 1 c.c. of fowl-passage virus containing at least 100 million
fowl minimal lethal doses and constituting a much nore severe exposure than would
occur under field conditions.
The experiments were repeated and periodic infectivity tests were made and the
results showed that in this line of the same strain of virus even after 14 passages through
eggs, the virus was sufficiently attenuated to be used as a vaccine with safety and re-
liability. The authors are of opinion that this variation in the number of passages
required for proper attenuation may be due to such factors as the individual susceptibility
or age of the embryo at the time of inoculation, the duration of re-incubation of eggs
following inoculation, the occasional necessity for storage of the virus in the refrigerator
between passages, owing to the shortage of suitable eggs for inoculation, and the in-
dividual susceptibility of the test fowl.