Metabolic Bone Disease (MBD)

Metabolic Bone Disease (MBD)
Metabolic Bone Disease (MBD) is a condition or disease characterized by structural changes in the bones caused by abnormal calcium and vitamin D3 metabolism in reptiles. Reptiles rely on UV-B radiation to synthesize vitamin D3. Additionally, an inappropriate dietary calcium-to-phosphorus ratio prevents optimal calcium absorption, serving as a primary cause of MBD in exotic pets.

Key Contributing Factors:

Dietary Imbalance: A lack of calcium in the diet or an improper calcium-to-phosphorus (Ca:P) ratio, which decreases calcium absorption.
Vitamin D3 & UV-B Deficiency: A lack of vitamin D3 and insufficient exposure to UV-B radiation, which are essential for vitamin D3 synthesis.
Improper Temperatures: Inappropriate environmental temperatures, which reduce the efficiency of vitamin D synthesis and calcium absorption.
These factors often lead to Nutritional Secondary Hyperparathyroidism (NSHP), the most common form of MBD in reptiles. This is particularly prevalent in commonly kept bearded dragons, which heavily rely on UV-B for proper calcium metabolism.

Typically, MBD occurs when the body fails to maintain a proper calcium balance in the bloodstream. When dietary calcium is insufficient or its absorption is impaired due to a lack of vitamin D3 or UV-B, the body responds by increasing the production of parathyroid hormone (PTH). PTH then stimulates osteoclasts to resorb calcium from the bones into the bloodstream as a compensatory mechanism to maintain serum calcium levels. However, continuous calcium depletion weakens the bones, resulting in conditions such as osteomalacia and fibrous osteodystrophy.

In reptiles, MBD broadly encompasses bone calcium loss and structural alterations resulting from calcium and vitamin D3 deficiencies, alongside the corresponding response of the parathyroid system.


Symptoms and Clinical Abnormalities
The symptoms of MBD range from early to severe stages and can be observed both clinically and radiographically:

Bone Softening: Soft or flexible bones, presenting as jaw pliability ("rubber jaw") or softened limb bones.
Skeletal Deformities: Bowed legs, pathologic fractures, and spinal abnormalities.
Neuromuscular Issues: Muscle tremors, twitching, weakness, and abnormal movements.
Severe Complications: Shock, muscle spasms (tetany), and seizures in cases of severe hypocalcemia.
Chronic Signs: Stunted growth, weight loss, anorexia (loss of appetite), and constipation.
Additionally, radiographic changes often reveal decreased bone density, cortical thinning, and fractures in the long bones.


Therefore, Husbandry management (managing the environment), Nutritional management (providing proper diet), Supplementation (adding necessary vitamins/minerals), and maintaining a Health check routine (regular veterinary check-ups) are strictly essential for prevention and care.

Article by: Dr. Thanadon Khanthom, DVM