1. The technique used for the propagation of the Bekker strain of bluetongue
virus in the developing chick embryo is described in detail.
2. The important role played by the temperature of incubation on the
multiplication of the virus is stressed.
3. A convenient and accurate method of determining the air temperature
of incubation is to take an average of the temperatures of 6 fertile eggs.
4. The importance of an accurately controlled system of incubation by
dispersed forced draughts is stressed for all work of this nature.
5. The temperature of a developing fertile egg between the 8th and the
15th day of incubation is higher than the air temperature of incubation.
The older the embryo, and the higher the temperature of incubation in the
range 32.1°C. to 38.2°C., the greater the difference between the egg and
the air temperature.
6. The apparent virus titre of a given emulsion is dependent on the
temperature of incubation of the eggs used for the titration test. The lower
the temperature the higher the apparent titre.
7. The titre obtained on incubation at 33.6°C. corresponds to the infectivity
of the emulsion for sheep.
8. Using fertile eggs after 8 days' preliminary incubation, virus titre of
an emulsion prepared from embryos incubated at 32.1°C. is consistently
higher than that obtained by incubation at higher temperatures.
9. At 32.1°C. the majority of embryos in injected eggs are dead by the
3rd and 4th days and the remainder invariably are dead on the 5th day.
There is little difference in the titre of virus in the 3rd and 4th day dead
embryos but a significant decrease occurs on the 5th day.
10. At 35.0°C. a significant number of embryos survive for longer than
5 days and there is a rapid decrease in titre after the 3rd day.
11. The longer injected eggs are left at a higher temperature before
transfer to, or transfer from, a lower temperature the lower the virus titre
in the embryos at death.
12. The optimum conditions for maximum multiplication of this strain
of virus is to use an inoculum containing 500 M.I.D.'s of virus, to incubate
at 35.0°C. for 24 hours, then at 32.1°C. and to harvest the dead embryos
on the 3rd and 4th day after injection.
13. The harmful effect of storage at ca. -10°C. is noted.
14. Attention is directed to the apparent poor viability of virus in
certain undiluted embryo emulsions and to a hypothetical interference
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