A PURIFIED STAGE-SPECIFIC 31 KDA ANTIGEN AS A POTENTIAL PROTECTIVE ANTIGEN AGAINST OSTERTAGIA-CIRCUMCINCTA INFECTION IN LAMBS

The 31 kDa antigen of third-stage (L3) Ostertagia circumcincta larvae was evaluated as a potential prophylactic antigen by an analysis of the protective, humoral and cell-mediated responses of lambs immunized with this antigen. Six lambs were immunized by subcutaneous injection with a total of 400-mu-g of the purified 31 kDa antigen in 250-mu-g ml-1 Quil A adjuvant. Five sheep given identical injections but without the 31 kDa antigen were used as controls. All animals were challenged with 4.2 x 10(4) infective L3 0. circumcincta larvae 1 week after the last booster injection. The protection afforded by the 31 kDa antigen was demonstrated by a significant reduction in faecal egg counts (p < 0.05) and total worm counts (p < 0.005) in vaccinated animals. Elevated ELISA antibody levels specific to the 31 kDa antigen were detected in the sera of vaccinated animals as early as 3 weeks after immunization. Specific antibodies were further demonstrated by Western blot 4 days after the first booster immunization at 3 weeks. In control animals no antibodies to the 31 kDa antigen were detected in Western blots throughout the course of the experiment. Immunized lambs had significantly higher in vitro lymphocyte stimulation indices (p < 0.05) than control animals. In haematoxylin and eosin stained sections taken at necropsy from vaccinated sheep, compared with control lambs, there were more larvae present in the crypts of the abomasal epithelium and these larvae were surrounded by lymphocytes and eosinophils. In vaccinated lambs a marked infiltration of lymphocytes in the lamina propria and oedema in the submucosa were also observed. There were significant increases in the numbers of both mast cells (p < 0.05) and globule leucocytes (p < 0.05) in vaccinated lambs compared with controls. It is therefore concluded that the significant protection obtained using the 31 kDa antigen resulted from both humoral and cell-mediated immunogenic responses against this antigen.