A FIELD-STUDY OF THE IVERMECTIN SUSTAINED-RELEASE BOLUS IN THE SEASONAL CONTROL OF GASTROINTESTINAL NEMATODE PARASITISM IN 1ST SEASON GRAZING CALVES

The effect of an ivermectin sustained-release bolus (I-SRB) on the epi demiology of nematode parasites and on calf productivity was evaluated in a field trial under Northwestern European conditions. Twenty paras ite-naive female Friesian calves (principals) aged 5-9 months were use d together with six male Friesian tracer calves. Principal calves were allocated by restricted randomization on day 0 body weight to either an untreated control group or a group given one I-SRB, designed to del iver 12 mg ivermectin per day for 135 days, orally on day 0. Each grou p was grazed on adjacent paddocks, naturally contaminated with parasit ic nematode larvae, from 13 May 1991 (day 0) until housing on 30 Septe mber (day 140). Body weights of principal calves were recorded and ind ividual blood and faecal samples taken at regular intervals throughout the trial. Pasture nematode contamination was monitored by larval cou nts on herbage and by worm counts of tracer calves grazed on each padd ock from day 126 to day 140. Nematode contamination levels on the cont rol paddock did not rise until the end of the grazing season, as a res ult of a mid-summer drought period. The period of exposure to a high l arval challenge was too short to provoke body weight losses and clinic al parasitic gastroenteritis in control calves. Use of the I-SRB resul ted in zero faecal egg counts of trichostrongyles during the whole pas ture season, thereby preventing a build-up of parasitic gastrointestin al nematodes on pasture. During the second grazing season no signs of parasitic gastroenteritis were detected in any animal, but an outbreak of parasitic bronchitis (PB) was observed in both experimental groups , indicating that PB can occur in older cattle regardless of the contr ol measures taken to prevent clinical parasitism during the first graz ing season.