Metabolic Bone Disease (MBD)

Metabolic Bone Disease (MBD) in Reptiles
Metabolic Bone Disease (MBD) refers to a group of conditions characterized by changes in bone structure resulting from abnormalities in calcium and vitamin D3 metabolism in reptiles. The body relies on UV-B radiation to synthesize vitamin D3. In addition, an inappropriate dietary calcium-to-phosphorus ratio can impair calcium absorption and contribute to the development of MBD in pet reptiles.

Key Contributing Factors
Insufficient dietary calcium or an inappropriate calcium-to-phosphorus ratio: This can reduce calcium availability and absorption.
Vitamin D3 deficiency and inadequate UV-B exposure: UV-B radiation is essential for vitamin D3 synthesis.
Inappropriate environmental temperatures: Unsuitable temperatures can reduce the efficiency of vitamin D metabolism and calcium absorption.
These factors can contribute to nutritional secondary hyperparathyroidism (NSHP), one of the most common forms of MBD in reptiles, particularly in captive bearded dragons, which require appropriate UV-B exposure for normal calcium metabolism.

How Does MBD Develop?
MBD generally develops when the body cannot maintain an appropriate calcium balance in the bloodstream.

When dietary calcium is insufficient or calcium absorption is impaired due to vitamin D3 deficiency or inadequate UV-B exposure, the body responds by increasing the secretion of parathyroid hormone (PTH). PTH stimulates osteoclasts to break down bone tissue, releasing calcium into the bloodstream to maintain blood calcium levels.

However, when calcium is continuously mobilized from the bones, bone strength decreases. This can lead to osteomalacia and fibrous osteodystrophy.

In reptiles, MBD is therefore an umbrella term describing the loss of bone mineral content and structural changes associated with calcium and vitamin D3 deficiencies and the body's parathyroid response.

Clinical Signs and Abnormalities
MBD can range from early-stage disease to severe, life-threatening conditions. Clinical signs and radiographic abnormalities may include:

Soft or pliable bones: Such as jaw pliability, commonly referred to as “rubber jaw,” and softening of the limb bones.
Skeletal deformities: Including bowed limbs, pathological fractures, and spinal abnormalities.
Muscle tremors, twitching, weakness, and abnormal movement.
Severe hypocalcemia: May cause tetany, seizures, and potentially life-threatening complications.
Chronic signs: Such as poor growth, weight loss, reduced appetite, and constipation.
Radiographic findings may include decreased bone density, cortical thinning, and fractures of long bones.

Prevention and Management
Proper care and prevention are essential to maintaining skeletal health in reptiles. Important measures include:

Husbandry management: Providing appropriate environmental conditions, including temperature and UV-B exposure.
Nutritional management: Ensuring a balanced diet with adequate calcium and an appropriate calcium-to-phosphorus ratio.
Supplementation: Providing calcium and other supplements when indicated for the species and individual animal.
Routine health checks: Regular veterinary examinations to identify abnormalities early.
Article by: Thanadon Khant hom, DVM (น.สพ.ธนดล ขันถม)