IMMUNIZATION OF SHEEP AGAINST SCHISTOSOMA-MATTHEEI USING EITHER IRRADIATED CERCARIAE OR IRRADIATED SCHISTOSOMULA

Irradiated cercariae, irradiated schistosomula or heterologous infections were used to vaccinate sheep against S. mattheei infection. In the 1st experiment 4 doses of 104 S. mattheei cercariae irradiated at 6000 R were administered to sheep by percutaneous infection at 4 wk intervals. This induced a 74% reduction in a challenge infection compared to control sheep while only 13% protection was achieved in a 3rd group of sheep immunized with normal cercariae of the heterologous parasite S. mansoni. No significant differences were seen in histopathology of the liver of any of the sheep but the pathological changes were more severe in the large and small intestines of sheep vaccinated with the heterologous parasite. In the 2nd experiment with irradiated cercariae only 1 or 2 immunizing exposures were used. The degree of protection in the adult worm load (9-11%) was not significant and no significant differences were noticed in the pathology of the vaccinated and control animals. In the 3rd experiment 4 doses of irradiated organisms were used to vaccinate 5 groups of sheep: 3000 R or 6000 R irradiated cercariae were administered by percutaneous infection; 6000 R skin-transformed schistosomula were administered by i.m. injection; the same 6000 R skin-transformed schistosomula were given by i.v. injection and 6000 R syringe transformed schistosomula were administered by i.m. injection. The degree of protection (determined as the reduction in worm burden) achieved by these different procedures was, respectively, 72%, 61%, 77%, 56% and 78%. These results indicate the possibility of making a live vaccine against ovine schistosomiasis and show that effective immunization is not dependent on the presence of a mature worm infection or on cercarial penetration of the skin by the immunizing infection.