PILOT FIELD TRIAL OF A RECOMBINANT TAENIA-OVIS VACCINE IN LAMBS EXPOSED TO NATURAL INFECTION

Previous trials of an experimental Taenia ovis vaccine using the recom binant antigen GST-45W(B/X) established that it was possible to achiev e >90% protection against a single artificial challenge of T. ovis egg s. This trial was undertaken to assess vaccine efficacy against artifi cial challenge and natural infection acquired by lambs grazing contami nated pasture. Two hundred Romney lambs were vaccinated at 6 and 12 we eks of age. One hundred control lambs were not vaccinated but were all owed to run with the vaccinated mob. At 15 weeks of age, 10 controls a nd 18 vaccinated lambs were artificially challenged with 2000 T. ovis eggs. The remaining control and vaccinated lambs were allowed to graze contaminated pasture for 3 weeks and were then moved to clean pasture for 5 months. The artificially challenged lambs plus 24 of the field- infected lambs were slaughtered and the carcasses dissected to obtain cyst counts. The remaining field-infected lambs were slaughtered at a commercial processing plant and the carcasses examined by conventional meat inspection. The results showed that the vaccine provided a high level of protection against artificial challenge (92%) and natural inf ection (98%) when assessed by carcass dissection. The data from commer cial meat inspection showed that vaccination provided 89% efficacy aga inst downgrading or condemnation compared to non-vaccinated control la mbs. The average difference in carcass values between vaccinated and n on-vaccinated groups was $4.36, representing a 35% loss in value due t o T. ovis infection in non-vaccinated lambs.