The importance of the third (L3), fourth (L4 and adult stages of Mecistocirrus digitatus in inducing resistance to reinfection was examined. Three groups of 5 calves (A, B, C) were immunized for 2 consecutive days with a daily dose of 20000 M. digitatus infectious larvae. Group A was treated with ivermectin 7 days post-infection (exposed to L3), group B was treated after 26 days (exposed to L3and L4) and group C after 60 days (exposed to L3, L4 and adults). Thirty days post-treatment, animals were challenged with 30000 M. digitatus L3. Five previously uninfected control animals (group D) also received 30000 L3. All animals were necropsied 75 days post-challenge. Increases in exposure to the immunizing infection resulted in significant increases in the pre-patent period post-challenge; 54 days in the controls, compared with 63, 70 and 72 days for groups A, B and C, respectively. Only adult worms were recovered at necropsy, and the mean number of worms significantly increased with increasing exposure to the immunizing infection: 355 in the controls, compared to 481,
937 and 1174 in groups A, B and C, respectively. No significant changes in worm length were observed. Infection with M. digitatus significantly affects a subsequent challenge infection – stimulation of the immune system by exposure to L3 extends the pre-patent period, and suppression by later stages (L4, adult) leads to higher worm burdens.