J.
R.
HADDOW
AND
J.
A.
IDNANI
51
in the case of R2D both fowls died. Larger experiments were then initiated with passages
115 to 126 of R2B and these gave uniformly good results without casualties. The experi-
ments covered over 100 adult birds. In addition, it was shown that, while birds under-
going vaccination were infective to healthy in contacts, such infection was "silent." As
expected, the number of healthy birds infected varied with the ratio of vaccinated :
healthy, e. g. in one experiment where the ratio was 1 : 1 (ten birds in each group),
only two healthy birds were later found to be immune. In another experiment where
the ratio was 4 : 1 (16 vaccinated fowls), all four healthy were silently immunized.
Chickens were also vaccinated and it was found that those under ten days old reacted
violently with an occasional death and a high percentage permanently maimed. From
ten days to a month the death-rate rapidly diminishes but the proportion of paralysis
cases remains rather high. After one month vaccination can be satisfactorily applied
and the casualty rates are nearly comparable to those in adult fowls, provided there is no
intercurrent coccidial infection.
Field trials
Frequent titrations of the amniotic fluid in fowls and chicks have shown that an im-
munizing dose was contained in a 1 × 10 -6 dilution, Also the virus dies rapidly when
diluted or if exposed to heat It was, therefore, considered expedient to carry out the
first field trials using virus stored on ice and freshly diluted at 1/1000 at the time of
inoculation. The dose given was 0.5 c. c. The results of the first field inoculation, which
was carried out on the poultry farm at Izatnagar, are recorded in Table VI. 1492
birds, mostly pullets, were vaccinated, and control pens were kept to study the effect
on growth and egg laying. The breeds included Rhode Island Reds, White Leghorns, local
Indian, and crosses of these breeds.
TABLE VI
Field trials with egg-passaged vaccine on 1492 birds
Days after vacci-
nation
1
2
3
4
5
6
7
8
9
10
11
12
13
14
21
25
Dull
Approx 50 per cent
"
75 per cent
"
75 per cent
"
75 per cent
"
50 per cent
Camatose or ill
4
65
87
29
6
10
2
Lame
1
3
8
10
18
27
23
30
28
32
33
32
22
Paralysed
2
7
10
13
11
12
14
14
12
11
9
6
Destroyed as
useless
1
8
The results show that quite a large percentage of them were mopy and about 6
per cent actually comatose. The symptoms pass off rapidly and usually leave no ill
effects. Mild cases of lameness also tend to recover rapidly unless intercurrent coccidial
infection is present. The common helminthic infections are not affected, nor do they
affect the degree of reaction during vaccination. Except for a few severe cases amount¬
ing to 0.5 per cent, all birds had recovered within a month. As in practice it is not
convenient or economical to nurse ailing birds for long, we consider that all birds which
are definitely lame or paralysed on the 14th day after vaccination should be destroyed for
food. This procedure gives a total casualty rate of about 2.5 per cent, a rate which varies
within small limits according to breed. In heavy placid birds, e. g. Rhode Islands the rate is
lower than in light and more nervous breeds such as White Leghorn or Indian.
Vaccination has no effect on subsequent growth rate either in chicks or pullets, but
it markedly interferes with egg-laying. About the sixth or seventh day after vaccina-
tion, egg-laying decreases and usually entirely ceases by the tenth day. Birds which
have no visible reaction start laying gradually about the 14th day, and full