THE EFFECT OF TRUNCATED INFECTIONS WITH OSTERTAGIA-OSTERTAGI ON THE DEVELOPMENT OF ACQUIRED-RESISTANCE IN CALVES

The relative contribution of the third (L(3)), fourth (L(4)) and adult stages of Ostertagia ostertagi to the development of immunity was ass essed in calves which were either continuously infected during 21 week s or subjected to infections truncated by anthelmintic treatment at th e L(3) or L(4) stage. A fourth group remained uninfected (control grou p). Faecal samples and blood samples were collected weekly for faecal egg counts and determination of pepsinogen and antibody levels. Only t he continuously infected animals showed positive egg counts, which fel l towards the end of the primary infection period. Pepsinogen and anti body levels remained high in the continuously infected group until the end of the primary infection period. At that time, they were signific antly higher compared to the control calves, with intermediate values in the truncated infection groups. After the 21 weeks primary infectio n period all animals were dewormed. To evaluate the protection provide d by the different immunisation protocols, all animals were challenged 1 week later with 156000 Ostertagia L(3), spread over 12 consecutive days. The marked reduction in egg counts following challenge infection indicated a certain degree of immunity in the continuously infected c alves, which was confirmed at necropsy by the reduced worm burdens, th e high percentage of inhibited early L(4) larvae, the reduced size of the adult worms and the higher numbers of mucosal mast cells in this g roup, Numbers of globule leucocytes and eosinophils were not significa ntly different from the control group. Infections truncated by anthelm intic treatment elicited poor development of immunity as shown by the egg output after the challenge infection and the percentages of arrest ed larvae and the lengths of adult worms which were intermediate to th ose of the continuously infected calves and control animals.