Vaccine effect of intact metacestodes of Taenia crassiceps against T. taeniaeformis infection in rats.

Wistar rats inoculated intraperitoneally with 10 viable metacestodes of Taenia crassiceps without adjuvant once on day 0 showed strong resistance to challenge with 200 eggs of T. taeniaeformis on day 30. When rats were killed one month after challenge, there were 80.4% and 46.1% reductions in the number of cystic and total metacestodes of T. taeniaeformis in the liver, respectively. When five rats were killed 16 months after challenge, they showed almost complete immunity against the challenge, with 99.4% and 91.1% reductions in the number of cystic and total metacestodes, respectively. There were only a few degenerated, pin-point metacestodes of T. taeniaeformis in the liver of all five rats; one harbored one cystic metacestode as well. However, there were no such reductions in rats injected initially with cyst fluid antigens of T. crassiceps with Freund's complete adjuvant. An additional experiment was carried out using 500 eggs of T. taeniaeformis in order to confirm the vaccine effect against higher egg dose. There were 96.6%, 87.9%, 83.9%, and 79.3% reductions in the number of cystic metacestodes in rats initially inoculated with 10 viable, 10 formalized, and 10 frozen metacestodes, and injected with sodium deoxycholate-solubilized metacestode antigens, respectively. It is strongly suggested that rats singly dosed with 10 viable or non-viable, intact metacestodes of T. crassiceps without adjuvant became highly resistant to challenge infection with eggs of T. taeniaeformis, which resulted in almost no cystic metacestode establishment.