The use of egg attenuated bluetongue virus in the production of a polyvalent vaccine for sheep. A. Propagation of the virus in sheep

Abstract

(1) A brief description of Theiler's classical method of bluetongue vaccine production is given and various modifications introduced from time to time are recorded. (2) In an attempt to adapt the recognized technique to the routine mass production of a polyvalent vaccine using fully attenuated egg adapted (A) strains of virus three difficulties were encountered : (a) As a result of even a single passage through susceptible sheep the adapted (A) strains were transformed at least partially into apparently original or sheep strains (O) so that quantitative control could not be carried out in eggs. (b) During the course of change from the A to the pseudo-O phase at least some strains show a marked change in antigenic structure. (c) Since the majority of sheep infected with egg attenuated virus show no clinical reaction it is impossible to eliminate animals included by accident amongst the virus donors. In the final mixture specific antibodies in the serum of such immune sheep neutralize the virus propagated in the susceptible donors, thus making the vaccine inert. This has proved an insurmountable difficulty. (3) Though passage through sheep causes a change from the A to the pseudo-O type of virus there is no reversion to original virulence.