1. A strain of virus (University Farm strain) was adapted to propagation
in the developing chick embryo by incubation of infected eggs containing
8 day embryos at 33.6°C. but not at 32.1°C., 35.0°C. or 38.2°C.
2. After 3 serial passages at 33.6°C. it was possible to continue propagation
at 32.1°C. and 35.0°C. but not at 38.2°C.
3. Using death of the embryos as an index of multiplication of egg-adapted
virus there was little difference in the results obtained from incubation
at 32.1°C. or 33.6°C. except that multiplication was slightly retarded
at the lower temperature. At 35.0°C. the number of survivors beyond the
4th day of incubation was significantly increased.
4. There was little variation in the titre of emulsions produced from
dead embryos at either of the temperatures after adaptation to eggs by serial
5. The highest titre emulsions (not less than 10⁻⁵) together with the highest death rate on the 3rd day were produced from eggs incubated for
24 hours at 35.0°C. and then transferred to 32.1°C.
6. The virulent strain of virus was attenuated by serial egg to egg
passage. At 32.1°C. attenuation took place rapidly after approximately
20 passages, at 33.6°C. at approximately the same rate, but at 35.0°C. it was delayed until about the 100th subculture.
7. Whether the attenuated virus produces a clinical reaction or not a
solid immunity is produced against the homologous strain of virus.
8. The application of the results to the production of large quantities
of vaccine for the mass immunization of sheep in the field is discussed.
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