22           Studies on Newcastle (Ranikhet) Disease Virus

established only after the 7th day and is solid on the 14th and 21st days.
In the previous experiment, it appeared that there was satisfactory immunity

11 days after the vaccination. If this method of preventive treatment is to
prove practicable on any large scale, it will be necessary for the birds sub-
jected to prophylaxis by crystal violet vaccine to be segregated from the
infected surroundings for a period at least of 14 days to obviate the risk of the
birds contracting the infection during the negative phase. It will be further
seen from this experiment that 14 days after vaccination, three fowls with-
stood the infection by pen-contact.

Vaccine does not disseminate infection. From the fact that a healthy fowl
placed along with the vaccinated fowls for a period of 14 days remained un-
affected (Table V), but died when exposed to the infection, the vaccine does
not appear, to be capable of disseminating the disease from the vaccine-
treated birds. At this stage with the limited experiments and observations,
it would not be justifiable to claim that the vaccine is definitely free from
any unmodified virus capable of returning to its virulence under some suitable
conditions. The results, however, warrant a further extensive trial of the
vaccine. So far, the immunization experiments have been restricted to one
strain of virus only, that is, the English strain. It would be interesting to
know if, by this method, other strains of virus, such as the Indian strain,
against which primarily this work was initiated, would yield to similar bio-
logical modification.

Experiment IV. To confirm the promising results so far achieved, im-
munization work with further lots of crystal violet treated egg virus was
undertaken. In this experiment, the virus used had undergone 41 serial
passages through eggs and a larger number of birds and controls employed.
The birds were housed together in a pen with common food and water. Twelve
fowls were inoculated with 5 c.c. of the vaccine subcutaneously, and

12 control fowls of almost the same weight and breed were placed with these
and allowed to mix freely so that the possibility of dissemination of Newcastle
disease consequent on vaccination could be ascertained.

Results. There was no reaction in any of the fowls but subsequent
observations were impossible owing to an outbreak of fowl pox among these
experimental birds. This experiment is, therefore, not recorded in
Table V.

                                                   TABLE V

                    Egg virus crystal violet vaccine for Newcastle disease