THE PRESENCE OF AN EARLY L(4) LARVAE POPULATION IN RELATION TO THE IMMUNE-RESPONSE OF CALVES AGAINST OSTERTAGIA-OSTERTAGI

The influence of different levels of infection with Ostertagia osterta gi on the development of a protective immune response in calves was in vestigated. Four groups of calves were infected with either 5000 (Grou p A), 10 000 (Group B), 20 000 (Group C) or 40 000 (Group D) infective larvae (0. ostertagi L3) weekly until treatment began. Group E functi oned as controls. All animals were treated with oxfendazole (9 mg ml-1 ) at Week 17 (Groups A, B and E) or Week 18 (Groups C and D). Sixteen days post-treatment all calves received a challenge infection of 150 0 00 0. ostertagi L3 spread over 10 consecutive days. Faeces and blood w ere collected weekly for egg counts and to assess levels of pepsinogen , gastrin and IgG1 and IgG2 Ostertagia antibodies. All calves were nec ropsied 31 days post-challenge for worm counts. Egg counts and pepsino gen levels were proportional to the infection level during the first f ew weeks of the experiment. Only in the high-dosed Group D was a gastr in response evoked. Ostertagia IgG1 antibodies increased between Day 2 5 and Day 95, and in the non-infected control group an antibody rise w as observed from Day 67 onwards. All measured parameters except Ostert agia antibodies showed a gradual decrease from Day 70 until the day of treatment. At necropsy there was no significant difference between th e groups in the total worm populations. Only the composition of the wo rm populations differed, with 35% early L4 (EL4) larvae in the previou sly infected Groups A, B, C and D and only 5% in the control Group E. The results indicate a slow immune response against a ostertagi in cat tle and question the possible role of the EL4 stage in developing immu nity.