Observations on bluetongue in cattle and sheep

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.