52
Vaccination against Ranikhet Disease
production in all except those seriously affected is resumed after 21 days. A number of
not less than 1000 birds forms a suitable basis for a day's work and for convenience
the virus is diluted at 1:500. Thus one ampoule of virus provided vaccine for the day.
Actually, experiments have shown that there is no noticeable difference in the severity
of reaction between an undiluted and a diluted virus. At first only birds over four
months old were vaccinated, but later it was found safe to reduce the age limit to six weeks,
at which time the birds usually leave the brooders.
In all, well over 100,000 have been vaccinated and the results of those for which
records are available are given in Table VII.
TABLE VII
Vaccination with egg-passaged virus
Where no intercurrent disease was
noted
Where intercurrent disease or un-
favourable conditions existed
Total
No. of birds
61,372
2 ,147
83,519
Casualties*
1971
2962
4933
Percentage
3.2
13.3
5.9
*Casualties include deaths and culls from all causes during 11 days from the date of vaccination.
Figures are thus somewhat higher than those observed where only casualties directly attributed to vaccina-
tion are recorded.
The diseases encountered included Ranikhet disease, mild fowl pox, coccidiosis, mild
fowl cholera, and spirochaetosis. In all cases, where flocks were clean and management was
good, results were uniformly good, but where intercurrent disease, other than Ranikhet disease
or unfavourable conditions existed, the casualty rate was higher.
Apart from flocks where for various reasons the general standard of management was
low, it was observed that dietetic errors gave rise to high casualty rates and that heavy rain
or excessive heat during the period of reaction were unfavourable factors. The latter,
however, were not of sufficient importance to prevent the prosecution of vaccination during
the hot weather and monsoon periods, provided the birds had plenty of space.
When Ranikhet disease is present on the farm, the degree of control obtained is largely
dependent of course on the time taken for immunity to develop, and by experiment this was
shown to be about 48 hours after inoculation. In accordance with this, it is found that in
pens already infected, mortality from the disease ceases on the fourth or fifth day after
vaccination. As Ranikhet disease spreads rapidly in susceptible fowls, the vaccine must be
applied quickly if it is to be effective. In one small flock of 210 birds, distributed in an
isolated pen of 32 birds and in three contiguous pens, the results were as follows :
Isolated pen (32 birds)
Contiguous pens No. 1
and 2 (107 birds)
Contiguous pen No. 3
(71 birds)
1 dead and 3 sick at the time of
vaccination
3 dead and 23 sick at the time of
vaccination
None sick
18 were protected
17 were protected
62 were protected
Pen No. 3 was vaccinated first, and the high mortality in pens Nos. 1 and 2 is
attributed at least in part to the fact that, despite precautions to prevent the spread of
infection on instruments during vaccination, infection was spread by contact with the
clothes of the operators. Although preventive vaccination has always been our objective, in
several outbreaks on large farms where separate attendants are available our results
indicate that it is possible to save 70 per cent of birds in pens in which the disease is not
evident, about 50 per cent in pens in which the disease has clearly just started and only
about 10 per cent in pens where the disease has obtained a firm hold.
We have as yet had no opportunity to test the duration of immunity over the full period
of a fowl's normal life. Frequent small-scale tests have been carried out from eight days to
nine months after vaccination and in no case has there been noted any evidence of a break-
down in immunity. Approximately 250 birds, vaccinated at ages between one and seven
months, have been tested nine to fifteen months later and all have proved solidly immune.