March, 1954]
S.
DATTA
5
and some of the immunity tests carried out recently on buffaloes at Haringhata
and sheep and goats at Hissar have shown that adequate immunity was not pro-
bably established or that there has been a breakdown in the immunity when the
challenge was done after one year of vaccination.
The avianised vaccine
At a result of work carried out during the last war by Shope and his co-workers
in Grosse ' Isle in America it was shown that rinderpest virus can be adapted to
grow on the membranes of the developing chick embryo after which it can be pro-
pagated in the yolk sac from where it finally extends to the tissues of the embryo.
By this process, serial passages in the yolk sac cause the attenuation of the virus
and a stage is reached when it is rendered safe enough to be used as a vaccine.
Sufficiently high titres are obtained to render vaccine production much cheaper
than by any other method.
The chief drawback of the avianised strains is that attenuation continues with
further serial passages in the yolk sac so much so that complete attenuation may
result, rendering the strain unsuitable for vaccination. The fixation of the virus
which has been attained with the goat-adapted and rabbit-adapted strains
has not so far been achieved with any of the avianised strains and one has to be
vigilant about the passage which would be effective in conferring immunity, earlier
passages being too virulent and later passages non-antigenic and unless suitable
storage facilities are available, the stage at which the strain is safe as well as anti-
genic may be missed.
Moreover, the strain has not been tried in India on the various types of cattle
required to be protected as well as other species of animals such as buffaloes, sheep
and goats and the nature and duration of immunity conferred by it have still to
be assessed. Unless this is done, and it will entail work of a considerable magni-
tude, involving some considerable time, it will not be wise to adopt this strain in
our eradication campaign for which a product conferring a long and lasting immunity
is required.
A further important consideration is the extremely fragile nature of the egg-
adapted strain which will necessitate its handling in the field with the utmost care
involving the use of ice at every stage of operation which may not render it of
practical utility in any mass vaccination campaign under Indian conditions.
The two main advantages of the avianised strain are its cheapness and complete
freedom from other viruses and bacteria which in the other two types of vaccine
may be acquired from the donor animals if such happen to harbour them in a
latent form.
The freeze-drying equipment
The advantages of freeze-drying live virus vaccines are now established beyond
doubt. The dried product, be it goat tissue vaccine, lapinised vaccine or avianised
vaccine has much better keeping qualities. Normally, the wet seed material of
goat tissue vaccine remains in the refrigerator, viable for about three months whereas
the dry powder kept under vacuum will remain potent and maintain its titre for
at least one year when stored in the refrigerator and for years if kept at a tem-
perature of —20ºC. In spite of the initial fall in the titre during the process