Abstract:
1. Experiments are described in which it was possible to recover
blue tongue virus from laboratory cattle exposed on the veld at
Tzaneen. In these cattle no clinical evidence of blue tongue was
manifested.
2. This Tzaneen strain of virus is capable of "breaking the
immunity" of sheep immunized with the Onderstepoort vaccine,
however, without any mortality and without the occurrence of typical
blue tongue symptoms.
3. There is no doubt that some basal immunity persisted in these
vaccinated sheep, even for considerable periods after immunization
and utilizing exceptionally virulent viruses for the various tests.
4. The significant thermal reactions provoked in vaccinated
sheep by this Tzaneen strain were referred to, and contrasted with
the marked reactions and mortality in the blue tongue susceptible
sheep.
5. These marked reactions could not be attributed to the size
of the dose and the method of inoculation.
6. In one instance the virus prepared according to the existing
method of preserving the virus, remained virulent for susceptible
blue tongue sheep for a period of 8,5 months.
7. A passage of 15 generations through blue tongue vaccinated
sheep and 9 generations through blue tongue susceptible sheep failed
to attenuate the virus in respect of its virulency.
8. It was shown that cattle could remain carriers of the virus
without manifesting symptoms for a period of 22 days after injection
and in sheep up to 60 days.
9. The significance of first and second reactions in the same
blue tongue susceptible sheep with the same strain of Tzaneen virus
is referred to.
10. The nature of the most important lesions was considered as
well as their pathogenesis and close relationship to the various forms
of horsesickness in horses.
