Russell was said to have prided himself that his terriers never tasted blood. This line of terriers developed by John Russell was well respected for those qualities, and his dogs were often taken on by hunt enthusiasts. Further crossbreeding occurred, with Welsh corgis, Chihuahuas, and other smaller breeds of terrier. Dogs can have a problem with both rear knees, and complications can include arthritis or torn knee ligaments. Severe cases can require surgery. Some are prone to dislocation of the kneecaps, inherited eye diseases, deafness and Legg Perthes—a disease of the hip joints of small dog breeds. Prone to mast cell tumors.