December, 1958]
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a year. Closer observation, however, necessitates consideration with regard to
certain points, especially (a) secondary lesion in vaccinates and (b) spread of the
active virus in the in-contacts leading to 'silent' immunity or secondary lesions in
such groups.
In the first group of baby chicks inoculated with 8 per cent vaccine, there was,
persistence of lesions in 50 per cent of the cases, with 9 per cent secondary lesions
while with the 4 per cent suspension of virus, 4 per cent of such chicks vaccinated
developed secondary lesions. The latter concentration appears to be comparatively
more suitable for satisfactory takes.
Twenty-eight per cent of the in-contact chicks developed lesions on the face
and beaks, which suggests the persistence of the virus on the vaccinated birds.
This indicates the existence of two factors which have to be considered before
selecting this particular strain of virus as an immunizing agent. In general
(a) virus antigen used for prophylaxis should not spread the infection and
(b) the vaccinated animals/birds should not be a source of infection or prove carriers
to disseminate the virus to other susceptible birds/animals.
Even though a virulent virus could be antigenic by inducing a reliable grade
of immunity, the " invasiveness " of such a virus portion into healthy in-contacts
by producing actual lesions, offsets this advantage, since its use in routine vaccina-
tions could become hazardous due to a gradual exaltation of its virulence. This
may ultimately lead to a large-scale infection of baby chicks, resulting in heavy
losses, which has been the actual experience of some workers.
In the present series of observations, it was noticed that the egg-propagated
pigeon-pox virus was capable of conferring a short term immunity in day-old chicks
for about eight weeks without producing secondary lesions in the in-contacts. This
would suggest the safety of the pigeon pox virus as an immunizing agent. The
threshold of immunity conferred by the P. P. V. however can be reinforced by a
second dose of F. P. V. at about the eighth week. Further, some workers including
Kamarov and Kligler [1936] consider the use of F. P. V. in late-hatched chicks
unsafe even at the older age of six weeks, owing to severe post-vaccination reac-
tions, and suggest the use of P. P. V. for the primary vaccination, followed two
months later by F. P. V. Hence it follows, this would be a safe procedure to be
adopted in early chick vaccination.
In our experiments we have found the stab method of vaccination in the wing
web as effective for P. P. V. as for F. P. V. in day-old chicks, as it helps best in
controlling the extent of reactions in unfeathered baby chicks where the feather
follicle method of vaccination is difficult of application.
SUMMARY
Two groups of day-old chicks were successfully vaccinated, using egg-propa-
gated field strains of fowl and pigeon-pox viruses respectively.
The F. P. vaccine was prepared from infected membranes in concentrations
of 4 and 8 per cent in 25 per cent glycerine broth and pH 7.4 and used by the stab
method on the web of the wing in two groups of chicks respectively. It produced
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