FIELD-EVALUATION OF A TOPICAL DORAMECTIN FORMULATION FOR THE CHEMOPROPHYLAXIS OF PARASITIC BRONCHITIS IN CALVES

A study was conducted to evaluate the efficacy of two topical treatmen ts with doramectin on the season-long control of lungworm and gastroin testinal infections in first grazing season (FGS) calves. At the start of the study, 20 FGS calves were randomly allocated into two treatmen t groups of 10 animals each. Calves in the D-group were treated with d oramectin pour-on on days 0 and 56, at a dosage of 500 mu g kg(-1) BW; calves in the C-group were designated as controls. A permanent pastur e was divided in two blocks and these were randomly allocated to the t reatment groups. Throughout the study, tracers (n = 32) were grazed on each paddock at 3-week intervals. Clinical signs of parasitic bronchi tis (PB) were observed in the C-group in duly and this necessitated tw o salvage treatments with levamisole. From day 28, post-turnout lungwo rm larvae were recovered from faeces of the C-calves until housing. No signs of PB were observed in the D-group during the entire grazing se ason. Shedding of lungworm larvae in the principals of the D-group did not occur until 112 days post-turnout. From the data obtained from th e tracer calves, it appeared that larvae had overwintered on both past ures. On the C-pasture, the number of lungworms recovered from the tra cer calves gradually increased to a peak in September, whereas on the D-pasture, the increase was observed only at the end of the pasture se ason. Both strongyle faecal eeg counts and pepsinogen levels were rela tively low in both groups throughout the present study. At the end of the grazing period (day 161), the principals were housed and treated w ith oxfendazole. During the housing period: all principal animals (D- and C-groups) and a third group of four helminth free animals (N-group ) received a challenge infection with Dictyocaulus viviparus. It appea red that the different exposure to the parasite during the grazing sea son resulted in different establishment rates, i.e., group C < group D < group N. The present results show that overwintering of lungworm la rvae occurs in Belgium and that in such conditions, doramectin pour-on given at turnout and at 8 weeks controls PB in calves during the firs t grazing season. (C) 1998 Elsevier Science B.V.