The effect on the skeleton of growing sheep of daily infection with small numbers of Ostertagia circumcincta larvae was investigated in 2 experiments. In the first, groups of 6 sheep aged 2 1 2 to 3 months were given either 1000, 3000 or 5000 larvae a day for 8 weeks. In the second, 4000 larvae a day were given for 14 weeks to 3- to 4-month-old sheep; controls were slaughtered initially, fed ad-libitum or pair-fed. In both experiments tibia, lumbar vertebrae, rib and costochondral junction were examined histologically and chemically and the concentrations of certain serum constituents measured. Although histology generally revealed few bone abnormalities other than osteoporosis in those sheep in which parasitism caused greatest depression of appetite, chemical analyses showed that infected sheep deposited less than half the bone ash of normal sheep. This was due to a reduction in bone size, although the density of bone matrix and its degree of mineralization were also reduced. These changes are tentatively ascribed to an induced energy or protein deficiency.
