Haemonchus contortus is the most pathogenic of the gastrointestinal nematode parasites of sheep with a worldwide distribution. Most successful vaccines currently available for viral, bacterial or protozoal infections accelerate the development of immunity naturally acquired by the host after infection. Natural immunity to gastrointestinal nematode (GIN) parasites develops in ruminants after repeated infections, and most animals are refractive to infection after 1-2 grazing seasons. This indicates that the development of GIN vaccines based on ‘natural’ antigens, is an achievable goal. By focusing on the local antibody response generated in the lymph nodes draining the infected tissue (abomasum or true stomach), we were able to identify and isolate a surface antigen, specific to the infective larval stage (HcsL3). Antibodies generated against HcsL3were able to mediate larval killing by eosinophils in an in vitro assay and vaccination with HcsL3 significantly reduced worm burden after challenge in a small experimental pen trial.
Translation of experimental vaccine studies to the field is not always successful as it has to comply with commercial realities, including fewer vaccinations with licensed adjuvants, and using animals with varying levels of exposure in the field. In the present study, we performed a larger dose-response trial with the HcsL3antigen by vaccinating sheep in the field using a commercially acceptable adjuvant and vaccination protocol. The results suggest that sufficient protection against infection may be achievable with this vaccine regimen under acceptable management practices.