(1) A brief description of Theiler's classical method of bluetongue vaccine
production is given and various modifications introduced from time to time are
(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.