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Attempts to immunize sheep against Teladorsagia circumcincta using fourth‐stage larval extracts

A ConcanavilinA (ConA)‐binding fraction of a detergent‐soluble membrane extract from Teladorsagia circumcincta (formerly Ostertagia circumcincta) fourth‐stage larvae was isolated, and two vaccine trials were conducted with this preparation in groups of 7 worm‐free sheep. All groups were challenged with a total of 5000 T. circumcincta larvae from 1 week after the final immunization and protection assessed by comparing the egg and worm counts, and length of developing worms, of the immunized groups with their respective controls. Immunization with the ConA‐binding antigen induced high‐titre serum antibody responses in both trials. However, no significant reduction in either egg count or worm burdens was observed in the vaccinated groups in either trial. It was concluded that detergent‐soluble, ConA‐binding extracts prepared from T. circumcincta fourth‐stage larvae did not contain significantly protective antigens, despite the fact that an extract prepared in a similar manner from Ostertagia ostertagi had previously significantly protected calves against homologous challenge.