THE EFFECT OF DIFFERENT INFECTION LEVELS ON ACQUIRED-RESISTANCE TO GASTROINTESTINAL NEMATODES IN ARTIFICIALLY INFECTED CATTLE

The objective of this study was to evaluate the effect of different in fection levels of Ostertagia ostertagi and Cooperia oncophora in a sim ulated 'first grazing season' on the resistance of calves to an artifi cial challenge infection. The infection levels were determined by the infection schedules and the chemoprophylaxis used. Thirty six 7-11-mon th old Holstein-Friesian bull calves were randomly divided into four g roups. The animals of group B received an ivermectin sustained release bolus (ISRB) on day 0. The calves of group D were treated on days 0 a nd 56 with a subcutaneous injection of doramectin (0.2 mg kg(-1) BW). Group C was the untreated control group. The calves of group N were us ed as helminth-naive controls, while the animals of groups B, C and D were trickle infected for 24 weeks. The infection schedules were desig ned to simulate the expected infection pattern for each treatment grou p under set-stocked conditions in temperate climate areas. After the l ast infection, all animals were treated with oxfendazole. One week lat er, all animals received a challenge infection of 50,000 O. osteragi L 3 and 100,000 C. oncophora L3, spread over 10 consecutive days. During the primary infection period the faecal egg output and the serum peps inogen and antibody levels reflected the different levels of host-para site contact between the groups (group C > group D > group B > group N ). After the challenge infection, faecal egg counts, total Ostertagia burden, size of the adult worms and abomasal globule leucocyte counts all indicated a positive relationship between the level of Ostertagia infection during the primary infection period and the level of acquire d resistance. A reduction of host-parasite contact: during the primary infection period, as a consequence of the infection schedule and the chemoprophylaxis used, resulted in a diminished level of resistance to the artificial challenge infection with O. ostertagi. Faecal cultures and small intestine worm counts indicated that all previously infecte d groups had acquired a high degree of resistance to the Cooperia chal lenge infection. (C) 1998 Elsevier Science B.V.