48
Vaccination against Ranikhet Disease
Work with this strain is being pursued owing to the advantage that the pigeon virus
could be transported in the living bird and no refrigeration would be necessary as is the
case with other vaccines. In one experiment, healthy fowls were kept in contact with
vaccinated fowls and with pigeons. The contact birds did not contract infection and they
were later found to be fully susceptible. Hence, it seems there would be no danger in
transporting the virus in living pigeons.
IIB. DUCKLING PASSGED VIRUS
One of us (J. A. I.) had previously found that Ranikhet virus could be readily trans-
planted into duck embryos, and it appeared possible that, as adult ducks are known to
be resistant, continued passage of the virus in duck eggs might alter its virulence for fowls.
To this end, 50 passages in duck eggs were accomplished ; at the tenth passage, the virus
was slightly attenuated for fowls, but continued passage produced no further noticeable
attenuation, and as there was no indication that attenuation produced in this way differed
from that obtained in hen's eggs, the experiment was discontinued. It was then considered
that passage in very young ducklings might be more effective. Three separate attempts
were made with ducklings one to ten days old, two with viruses already frequently passaged
in duck eggs—which were as noted slightly attenuated for fowls—and one with the stock R1
strain. These attempts gave more or less similar results. Firstly it was found that though
such ducklings react not all of them die, and, if passage is to be successful, it is necessary
to destroy them as soon as they show symptoms. It is also advisable to sub-inoculate on
eggs in order to obtain greater quantities of virus. In all three attempts we were unable to
go beyond the fifth duckling passage, at which stage the virus had become markedly attenuat-
ed for fowls and also grew very slowly in embryos which survived for 3 to 7 days. In the
latter case, normal growth and embryo-killing powers were recovered after two or three
hens' egg passages, but the virulence for fowls did not return. One of these strains was
then maintained in hens' eggs, and Table IV shows its vaccinating properties.
TABLE IV
Vaccination with duckling-passaged virus
Laboratory
First
field
Second do.
Third
do.
Fourth
do.
experiment
trial
do.
do.
do.
No. of fowls
vaccinated
56
40 (chicks)
782
2023
1000
1474
Result of test on the
14th day
No reaction
do.
48 died, 3 paralysed
15 died, 3 paralysed
22 died or paralysed
46 died or paralysed
Immune.
18 immune, others not tested.
Loss 6.5 per cent.*
Loss 0.9 par cent.
Loss 2.2 per cent.
Loss 3.1 per cent.
* First trial complicated by fowl cholera.
It will be observed that this strain makes an excellent vaccine and it has the advantage
over the egg-passage vaccine that the method of attenuation is fairly precise and can be
repeated as far as we know at will. Chickens can apparently be vaccinated safely, although
the numbers that have been done are not yet large enough to give an adequate picture of its
effects in these young birds. Healthy birds in contact with birds undergoing vaccination may
contract infection,—of five healthy fowls in contact with 25 vaccinated fowls, one was immune
and the infection conveyed was 'silent' in nature.
EGG PASSAGED VIRUS
Iyer and Dobson [1940] and Iyer [1941] reported that they had obtained an aviru-
lent strain of Newcastle disease by passaging the virus serially through developing fowl
embryos. Attenuation apparently occurred somewhat suddenly about the 14th and
33rd passages in the two attempts reported. Iyer (in the press) has described
further work on Indian strains, in which attenuation by simple egg passage was either
unsuccessful (two trials) or only partially so (one trial). In the partially successful ex-
periment, a semi-attenuated strain appeared after 19 passages and again apparently
somewhat suddenly. Subsequent passage produced no further change and the strain then