Incidence of hydronephrosis among several production colonies of outbred Sprague-Dawley rats

The incidence of hydronephrosis was determined in nine production colonies of Crl:CD (SD) BR rats (CD rats). Kidneys from 3909 rats were examined and 79 (2.0%) had unilateral or bilateral hydronephrosis. A colony with a relatively high incidence of hydronephrosis (4.1%) was chosen for further study. In unselected rats from this colony the incidence in females (87/1882, 4.6%) was not significantly different from that in males (79/1862, 4.2%). In the same group of rats, hydronephrosis was observed most frequently in the right kidney only (2.3%), followed by bilateral involvement (1.2%) and the left kidney only (0.8%). By selection and inbreeding, the incidence of hydronephrosis was increased dramatically (to 33.6%) in two generations. Hydronephrosis in the CD rat appears to be a highly heritable trait, most likely involving more than one gene.