Ostertagia circumcincta infection of lambs, the effect of different intakes of larvae on skeletal development

Six groups of 8 4-month-old cross-bred lambs were used to investigate the effect of daily intakes of Ostertagia larvae on skeletal growth. One group was killed at the onset as control. The remaining groups were offered a complete ruminant ration ad libitum and dosed with zero (group ALC control), 1000 (group 1), 3000 (group 2), 5000 (group 3) and 5000 (group 4) larvae of Ostertagia circumcincta per day for 12 weeks. Group 4 also received a single dose of anthelmintic (fenbendazole) on days 21, 42, 63 and 84 of infection. All sheep were killed after 14 weeks and selected bones removed for histological and chemical evaluation. None of the sheep showed obvious clinical signs of infection and mean worm egg counts were less than 400 e.p.g. Deposition of calcium in the carcase of Ostertagia infected sheep was reduced by 11, 35, 57 and 27 per cent in groups 1, 2, 3 and 4, respectively. Phosphorus deposition was similarly affected. Growth in volume of the tibia, rib and vertebra was reduced in groups 2, 3 and 4, compared to controls (ALC). There was also a reduction in the amount of ash per unit volume of bone in groups 2 and 3, mainly due to a decrease in bone matrix, although in the more severely affected animals mineralization of bone matrix was slightly reduced. Anthelmintric treatment did not prevent retardation of bone growth but it did improve the quality of the skeleton. Intakes of 1000 Ostertagia larvae per day did not significantly affect any of the bone parameters.