THE EFFECTS OF FIRST-SEASON STRATEGIC AND TACTICAL IVERMECTIN TREATMENTS ON TRICHOSTRONGYLOSIS IN THE FIRST-SEASON AND 2ND-SEASON GRAZING

A 2 year study was conducted to evaluate the effects of first-season s trategic or tactical treatments with ivermectin on trichostrongylosis in heifer calves in the first and second-season grazing, Three groups of each eight Holstein-Friesian calves were turned out in early May on to a permanent pasture naturally contaminated with trichostrongyle lar vae, Two of these groups were given ivermectin either as strategic tre atments (Weeks 3, 8 and 13 after turnout) or as tactical treatments (W eeks 14, 18 and 22 after turnout); the third group served as untreated controls. The strategic ivermectin treatments prevented build-up of h igh herbage infectivity from mid-summer onwards as shown by low tricho strongyle egg outputs, serum pepsinogen levels and serum antibody resp onses. In spite of exposure to continuous high larval challenge in lat e season, the pathogenic effects of worm loads in calves receiving the tactical ivermectin treatment were significantly suppressed. The perf ormance of the strategically treated calves tended to be higher than t hat of the tactically treated calves in the first-season grazing; yet, there was no statistical difference. During the following summer, all three groups were grazed in a single herd together with a new group o f eight first-season calves. No anthelmintic treatments were given to any animals during the season. From late August until the end of the s eason all animals were given weekly experimental challenge infections. Following the challenge infections, the first-season calves developed clinical parasitic gastroenteritis, whereas the second-season heifers showed no symptoms. At post-mortem it was found that worm burdens mai nly consisted of early fourth-stage larvae (L(4)) of Ostertagia ostert agi (>97%). Fewer adult worms were recovered from the untreated animal s than from the treated ones. However, serum anti-parasite IgG1 respon ses and post-mortem worm counts suggested that the untreated heifers h arboured markedly fewer adult O. ostertagi than the previously treated ones, indicating a higher level of immunity against adult worms. Howe ver, this difference did not have any clinical impact in this experime nt.