Renal calculi in apparently normal sheep

The incidence of renal calculi in sheep was determined and the effects of season, sex and difference between left and right kidneys were considered. Examinations for calculi were made by x-ray photography and stereomicroscopy. The seasonal study extended from June 1972 to June 1973. The lowest incidence of calculi, occurred from November to February (25 to 35%) and the highest, in June 1972 (61%) and May 1973 (63%); in other months it generally ranged from 40 to 50%. These differences were highly significant (P less than 0.001). The incidence of calculi greater than 2 mm diameter was 0% and 3% in January and February and, except for 18% in June 1972 values were about 8%. Subepithelial deposits ranged from 2% (April) to 25% (June 1972) about a mean of 11%. Calculi were found most frequently in the middle third of the kidney and near the fornices. Of kidneys with microcalculi (less than 0.05 mm diameter) visual assessment showed 64% contained silica, 29% oxalate, 15% carbonate and 5% material of undetermined nature. Difference in the incidence of calculi between ewes and wethers and between left and right kidneys were not significant and the distribution of calculi within these kidneys was similar to that in the seasonal study. These results are discussed in relation to urinary tract obstruction in grazing sheep and to possible factors affecting the retention in and movement of calculi from the kidney.