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 20,000 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 L3 and L4) and group C after 60 days (exposed to L3, L4 and adults). Thirty days posttreatment, animals were challenged with 30,000 M. digitatus L3. Five previously uninfected control animals (group D) also received 30,000 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.
|Number of pages||5|
|Publication status||Published - 1998|