EFFICACY TRIALS WITH TISSUE-CULTURE GROWN, INACTIVATED VACCINES AGAINST CHLAMYDIAL ABORTION IN SHEEP

Vaccines containing inactivated semi-purified antigen of ovine abortif acient Chlamydia psittaci were prepared from tissue-culture grown harv ests of two strains. These were evaluated for efficacy against experim entally reproduced enzootic abor tion of ewes (EAE), the protection in dex (PI) of each vaccine being calculated from three variables. Trial 1 tested the adjuvants Marcol 52\Arlacel A, ISCOM matrix, Novasomes an d Alhydrogel, each administered with a total antigen dose of 4 mu g ch lamydial protein (cp), against challenge with the two homologous vacci ne strains. The first three formulations yielded Pls of 39-42%, while that of Alhydrogel was 23%. Marked granuloma and/or abscess formation occurred in the Marcol 52/Arlacel A group but in none of the others, a nd only this group seroconverted following vaccination In Trial 2, ISC OM matrix ix containing 16 mu g cp per dose was administered once or t wice. Two challenges were used, one involving the two vaccine strains and the other these two strains plus five others, but no significant d ifferences were seen between them. The double dose schedule, which was better than the single dose and also several Novasomes formulations t ested concurrently, gave Pls of 64 and 77% against the two challenges (combined value 71.5%). Trial 3 studied duration of immunity over two lambing seasons using three different administration schedules of 16 m u g cp in ISCOM matrix, namely single dose in year I, single dose in y ear 2 (Group I), double dose then booster (Group 2), and double dose w ithout booster (Group 3). Pls against the homologous bivalent challeng e were 79-80% for Groups I and 3 and 9l% for Group 2. Combining Trials 2 and 3 data, double injection of 16 mu g cp in ISCOM matrix reduced the proportions of ewes aborting from 31.5 to 4. 8% and of those excre ting chlamydine at parturition from 74 to 2 7%, while the proportion o f dead lambs fell from 29.9 to 6.5%, producing an overall PI of 74%. T his vaccine appeared virtually innocuous, but seroconversion following its use was detectable only by ELISA. The findings suggested the effi cacy of inactivated EAE vaccines to be antigen dose-dependent, and sho wed that good protection against heterologous challenge was achieved w hen 16 mu g cp (containing approximately 30% major outer membrane prot ein) was combined with ISCOM matrix and administered twice or more.