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 epidemiology of nematode parasites and on calf productivity was evaluated in a field trial under Northwestern European conditions. Twenty parasite-naive female Friesian calves (principals) aged 5-9 months were used 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 deliver 12 mg ivermectin per day for 135 days, orally on day 0. Each group was grazed on adjacent paddocks, naturally contaminated with parasitic nematode larvae, from 13 May 1991 (day 0) until housing on 30 September (day 140). Body weights of principal calves were recorded and individual blood and faecal samples taken at regular intervals throughout the trial. Pasture nematode contamination was monitored by larval counts on herbage and by worm counts of tracer calves grazed on each paddock from day 126 to day 140. Nematode contamination levels on the control paddock did not rise until the end of the grazing season, as a result of a mid-summer drought period. The period of exposure to a high larval challenge was too short to provoke body weight losses and clinical parasitic gastroenteritis in control calves. Use of the I-SRB resulted in zero faecal egg counts of trichostrongyles during the whole pasture season, thereby preventing a build-up of parasitic gastrointestinal 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 control measures taken to prevent clinical parasitism during the first grazing season.