The immunizing potency of a live vaccine prepared from a strain of Besnoitia besnoiti isolated from blue wildebeest and stored frozen as a low passage stabilate was studied in adult male rabbits. The vaccine consisted of parasites that were grown in primary lamb kidney or an established line of green monkey kidney cell (Vero) cultures and suspended in serum-free Hanks' or Eagle's medium respectively. Freshly prepared doses containing from 1 x 10 to 2 x 10⁵ parasites of the 14th to 34th culture passages were injected subcutaneously. The blue wildebeest strain was pathogenic in rabbits at all the dosage levels tested and 24, 6 per cent died as result of vaccination. The survivors were challenged one to six months after vaccination by subcutaneous inoculation with a bovine strain of B. besnoiti grown in lamb or Vero cells from a low passage level frozen stabilate. Immunized rabbits were protected against challenge doses that were 10 to 100 000 times greater than those of the vaccine. Immunized rabbits did not show any evidence of infection, whereas all the controls developed typical reactions (patent parasitaemia, scrotal oedema, orchitis) and 58 per cent died. It was therefore not possible to calculate an ID₅₀ for vaccinated and control rabbits in this investigation, but one experiment revealed that more than 5 logs of protection had been induced by the vaccine. Rabbits injected with Vero cells only were not protected
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