156
Studies on Newcastle
TABLE I
Virulence of egg-cultured virus
Virus
Line ii . . . .
Do.
.
.
.
.
.
Do.
.
.
.
.
.
Do.
.
.
.
.
.
Sub-line ii (B)
Passages
1—18
19—28
29—35
36—43
34—78
Fowl inoculation
Total
number
inoculated
14
27
14
18
81
Number
died
13*
1†
6§
18‡
23||
Number
survived
1
26
8
—
58
Immunity tests
Number
survived
1
26
8
—
54
Number
died
—
—
—
—
4
Remarks
*Died
Ranikhel
disease
†Died other cause
§Died 8 to 22 days
after
inoculation
‡ Died
Ranikhet
disease
|| See discussion
DISCUSSION
Lines i and iii did not prove amenable to attenua-
tion after 69 and 32 passages respectively in
eggs. Line ii showed no change until the 19th
passage, when it appeared to have become attenua-
ted suddenly. Though the virus retained suffi-
cient virulence to kill chick embryos, it did no
apparent harm when injected into fowls but render-
ed them immune to test doses of different strains
of virulent Ranikhet-disease virus. These re-
sults corroborate those of Iyer and Dobson [1940]
who obtained an attenuated virus capable of
immunizing birds after 33 passages in one case
and 14 in the other. The cause of variations in
the number of passages required for attenuation
is, as yet, unknown and needs further study. The
attenuated virus (Line ii) obtained after the 19th
passage retained its antigenic properties for nine
more passages and immunized 26 of 27 birds.
One bird died from other causes prior to being
tested for immunity. From the 29th passage
another change was noticed in the behaviour of
the virus, viz. six out of 14 fowls inoculated
with passages 29 to 35 died 8 to 22 days after
inoculation, whereas ordinarily death occurs five
days after inoculation with the virulent virus.
None of these birds showed the typical clinical or
post-mortem appearance of Ranikhet disease. It
was noticed in individual passages that out of two
fowls, receiving a similar inoculum, one succumbed
and the other survived. This may have been
due to the resuscitation of various latent diseases.
However, from the purely experimental aspect
it is necessary to accept these deaths as either
directly or indirectly attributable to the test i n-
oculation.
In passages 36 to 43 all the inoculated fowl s
either died or were killed after developing Rani
khet disease. A complicating factor in the i n-
terpretation of these results was the spontaneous
occurrence of Ranikhet disease in the healthy
stock from which our experimental birds were
obtained. Further passages were therefore dis¬
continued for some time and fresh sub-lines started
later from previous passage materials stored in
the refrigerator. The following results were
obtained with sub-line ii (B) picked up from passage
33. Out of a total number of 81 fowls inoculated
between passages 34 to 78, 58 or 71.7 per cent
survived and 23 or 28.3 per cent died. These
results were essentially similar to those obtained
in passages 29 to 35 (supra) indicating the neces-
sity of more work before this attenuated virus
could be successfully employed as a vaccine. All
the survivors proved solidly immune to virulent
virus, except four fowls which developed paralysis
after the attenuated virus inoculation and ulti¬
mately died during the immunity test. This
mortality, however, was evidently the result of
the primary inoculation.
SUMMARY
1. Details are given of the behaviour of three
Indian strains of Ranikhet disease virus during