Sixty 15-month-old ewes were randomly allocated to six groups of 10 sheep each; the first four groups were infected three times with 20,000, 40,000 and 60,000 T. colubriformis 3rd stage larvae (L3) at 21 day intervals and infections in each group were terminated with Oxfendazole at different times. Group 1, 2 and 3 infections were terminated 15, 7 and 3 days respectively after each infection and 21 days after the third infection for only Group 4. Groups 1-5 were then challenged with 60,000 L3 T. colubriformis. Group 6 was left as uninfected control. All sheep were bled before and after the immunization procedure and then at weekly intervals after the challenge infection. The blastogenic responses of peripheral blood lymphocytes to Con A, PHA, PWM and LPS, as well as responsiveness to L3 and adult T. colubriformis antigens, were evaluated using tritiated thymidine incorporation into DNA. The greatest changes in blastogenic response were observed in Group 2 immunized by successive infections terminated 7 days after infection; the smallest in Groups 1 and 4 immunized by early L5 (immature 15 days) and L5 (mature 21 days) stages of T. colubriformis, respectively. Lymphocyte responsiveness to L3 and adult parasite ES antigens, assessed for all groups on the day challenge infection was given, were variable amongst groups.
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