Pasture deworming and (or) subsequent feedlot deworming with fenbendazole. II, Effects on abomasal worm counts and abomasal pathology of yearling steers

Abstract

734 steers, weighing 286 kg, were used during a 118-day grazing period, after which 640 were placed in a feedlot for finishing (average of 121 days, range of 111 to 133 days) to measure the main effects and interactions of 2 pasture deworming treatments (negative control, strategically dewormed with fenbendazole) and 2 feedlot deworming treatments (negative control, dewormed with fenbendazole) on faecal egg counts, abomasal worm counts and abomasal pathology. Seven to 10 abomasa from steers on each of the 4 pasture-feedlot treatments (pasture control-feedlot control; pasture control-dewormed in the feedlot; strategically dewormed on pasture-feedlot control; strategically dewormed on pasture-dewormed in the feedlot) were selected at slaughter to determine total worm counts, and to evaluate gross pathology and histopathology of the abomasal mucosa. Pasture deworming with fenbendazole (FBZ) in a strategic programme on days 0, 28 and 56 reduced adult and L4 and EL4 larval stages of Ostertagia ostertagi and adult and L4 stages of Trichostrongylus axei residing in the abomasum at slaughter. Feedlot deworming with FBZ reduced L4 and EL4 stages of O. ostertagi and adult and L4 stages of T. axei. An interaction between pasture deworming treatment and feedlot deworming treatment showed that pasture deworming had a significant effect on reducing the number of Trichostrongylus adult parasites, which was not further reduced by feedlot deworming. Strategic pasture deworming with FBZ followed by FBZ administration at the beginning of the finishing period reduced total O. ostertagi, T. axei and Haemonchus adult and larval stages by 99.13%, compared with untreated controls. There was no relationship between faecal egg counts and total worm counts. Gross pathology showed that strategic deworming on pasture reduced the mean abomasal lesion score at slaughter. An interaction between pasture treatment and feedlot treatment showed that feedlot deworming of pasture control steers reduced mean histopathology score, while feedlot deworming of steers strategically dewormed on pasture had no impact. Distribution of histopathology scores showed that strategic deworming on pasture resulted in more steers without histopathological changes to the abomasal mucosa and fewer steers with higher histopathology scores.