Control of gastrointestinal nematodes in first-season grazing calves by two strategic treatments with doramectin

Abstract

Twenty first-season Holstein crossbred calves were used in a study conducted to investigate the efficacy of two strategic treatments with doramectin (s.c. 200 μg kg −1 bodyweight). They were allocated in two equal groups: Group D was treated with doramectin at turn-out and 8 weeks later and Group C served as control group. Tracers were used at turn-out, in August and before housing. At monthly intervals all principal animals were weighed, and their faeces were sampled for faecal egg counts and cultured for larval identification. Serum samples were collected for determination of pepsinogen. Animals were slaughtered for worm counts 3 weeks after housing. Clinical signs of parasitic gastroenteritis (PGE) in Group C necessitated a salvage treatment with pyrantel citrate. No signs of PGE were seen in Group D. The faecal egg output remained very low (less than 50 eggs g −1) in Group D, whereas in Group C a mean geometric maximum of 352 eggs g −1 was observed. Pepsinogen levels were below 2000 mU tyrosine in Group D, compared with maximum levels of 4190 mU tyrosine in Group C. The worm counts of the tracers at housing showed an overall reduction of 94% in the total number of gastrointestinal nematodes present on the D pasture, compared with the C pasture. Worm counts of the principal animals showed an 87.8% reduction in the total worm burden. The present results show clearly that doramectin given at turn-out and at Week 8 controls PGE in calves during the first grazing season.