Metabolic Bone Disease (Simplified)
What is MBD?
Although the term ‘metabolic bone disease’ is applied as though to describe one disease, there are in fact several different diseases or disorders pertaining to the condition of the skeleton defined under this term. These include: rickets, secondary nutritional hyperparathyroidism, cage paralysis and osteoporosis to name a few. MBD is one of the most common disorders in captive reptiles which frequently results from poor nutritional management and is entirely preventable. If not controlled, in severe cases death may result. The condition is commonly attributed to vitamin D deficiency although it can also occur as a result of insufficient calcium or phosphorus in the diet.
Symptoms of MBD
MBD can be identified by a range of symptoms:
- Bowing of the legs - Swollen joints
- Swollen face - Well-muscled in appearance
- Rubbery bowed jaw - Soft Shell
- Kinks along the spine and tail - Muscle Tremors
These physical characteristics are a result calcium being transferred (resorbed) from the bone fluid into the blood leaving the bones weak and pliable. The exerted forces of the muscles can then change the shape of the bone or shell, making limbs look bowed or the shell appear lumpy. Fibrous tissues are laid down on the bone to maintain strength causing the limbs to appear well muscled, however bones are still prone to fracturing. Muscle tissue is also degraded along with nerve impulses resulting in poor coordination, muscle tremors and lethargy. Death can be result of insufficient calcium to maintain the heart muscles leading to cardiac failure. The animal may also starve to death with reduced coordination and degraded jaw effecting the animals' ability to hunt or eat.
Causes of MBD
MBD is often a result of inadequate calcium in the diet. Calcium (Ca) works with phosphorus (P) at a ratio of 2:1, any alteration to this balance would result in poor calcium homeostasis, both excesses and deficiencies must be avoided.
Herbivorous species are particularly susceptible to MBD as some foods fed to herbivores contain oxalates which reduce the bioavailability Calcium in the gut. Furthermore, although plants are relatively high in vitamin D2 this form of vitamin D is less efficient than D3 which is an important vitamin for Calcium absorption.
As insects suffer from an inverse ratio of Ca:P meeting dietary requirements are a problem for insectivorous species too.
There is much focus on supplementation, however, species rely on different means to obtain different nutrients such as basking under UV-B light, supplementation may not always be appropriate.