Metabolic Bone Disease in Reptiles
Dr. Jenni Bass
MBD is often used loosely and interchangeably with several other syndromes related to abnormal calcium-vitamin D metabolism. Some species or individuals seem more commonly to suffer from one or two forms than from others. Factors involved include age, species, gender, reproductive status and the combination of genetics and environmental conditions.
Secondary Nutritional Hyperparathyroidism: emphasizes the role of malnutrition in the development of disease that leads to a poorly functioning parathyroid gland. The parathyroid gland produces hormones that regulate calcium within the body. The following are some common terms used by veterinarians to describe MBD:
Rickets: inadequate calcification of bones in a young animal. These bones break easily, often without apparent reason, and often without the owner's knowing.
Osteoporosis: loss of bone density or mass in a mature animal. Osteoporotic bones are very brittle.
Fibrous Osteodystrophy: loss of bone mass, which is subsequently replaced by bulky fibrous tissue, in the body's effort to maintain bone strength. This is commonly seen in juvenile or young lizards, and manifests as firm, swollen limbs or a swollen, shortened lower jaw. This is because the shape of the softened bone is distorted by the pull of the muscles.
Hypocalcemia: Low blood calcium has numerous effects, but is most obviously manifested as weakness, paralysis or seizures.
Common signs of MBD in lizards: soft or swollen lower jaw, lameness (often a broken bone), scoliosis (an "S" shaped spine) or kyphosis (a humped spine), the inability to stand upright on all four legs with the body raised from the ground, dragging the body rather than walking normally, failure to grow and persistence of a juvenile appearance (such as the typical "rounded head" of the juvenile iguana) and a general "rubbery" or weak appearance.
Signs in mature lizards as a result of low blood calcium include tremors, muscle twitching, seizures, tetany (severe stiffening of the body and limbs). These signs may be intermittent, but will usually worsen in severity and increase in frequency. An episode is often triggered by an underlying condition such as kidney disease, or an increased need for calcium, as during egg laying.
Signs in tortoises and turtles: in the case of young animals, the shell will fail to grow at the same rate as the body inside. The shell often develops an abnormal shape, with irregular scutes (often called pyramiding of the shell). In the case of young animals, the shell may fail to harden and in the case of mature animals, although they may take a relatively longer time to show signs of MBD, the shell will eventually soften, as calcium is depleted to meet the body's most immediate needs, such as nerve and muscle function. Beaks and toenails may bend and become over grown, rather than wearing as they would when normally calcified.
MBD is uncommon in snakes (see below), but signs are similar to those seen in lizards (excepting the broken legs).