Field vaccination of sheep with a larval-specific antigen of the gastrointestinal nematode, Haemonchus contortus, confers significant protection against an experimental challenge infection

► Sheep were vaccinated on pasture with a larval-specific antigen. ► Only two immunizations using an approved adjuvant were administered. ► Significant protection against infection was achieved in a dose-dependent manner. ► There was a dose-dependent increase in red blood cells in vaccinates. ► Specific antibodies were raised in pasture-reared sheep independent of vaccination.