Nutritional metabolic bone disease is a common disease of reptiles in Denver because of the increased interest of reptiles as pets, and the inability to maintain the proper environment for some reptile species in Colorado’s dry climate. It is made up of several metabolic disruptions that affect the skeletal system of captive reptiles and develops due to husbandry and dietary mismanagement.
Physiology of bone health
Bone health is composed of several elements that work together to maintain homeostasis of the skeletal system. These elements are comprised of internal and external components.
Calcium
This mineral is responsible for many biological functions including bone formation, muscle contraction, cell membrane permeability, and blood clotting. Most calcium is stored in bone, however there is a small percentage that is found in the blood.
Phosphorus
This mineral is used to form and maintain bone. It must be fixed in a calcium: phosphorus ratio of 2:1 in order to protect the integrity of the skeletal system.
Ultimobranchial gland
In reptiles, this gland is believed to produce a hormone called calcitonin that prevents high calcium levels in blood. It does this by targeting the kidneys to excrete excess minerals, the intestines to absorb less calcium, and the bones to store more. The goal is to inhibit calcium reabsorption from bone.
Parathyroid gland
The parathyroid gland produces parathyroid hormone in response to low calcium levels and targets bone to release calcium from its stores. The goal is to maintain appropriate calcium levels in blood.
Metabolism of calcium
Under natural conditions, the body synthesizes its own vitamin D when it's exposed to sunlight. The ultraviolet radiation reacts with the body's skin to convert cholesterol into vitamin D. This is the inactive form of vitamin D3 and initially makes its way to the liver where it is converted into a hormone. The hormone is sent to the kidneys, converted into the active form of vitamin D3, and released into the intestinal tract. Vitamin D3 facilitates the absorption of calcium across the mucosal lining of the intestinal wall where it is then utilized.
Pathophysiology of metabolic bone disease
Nutritional MBD is characterized by a decline in skeletal health. It is not a bone disease; it is a result of low calcium levels. The problem is a disruption in calcium metabolism. Regardless of where the problem originates, the low calcium levels produce a state of hypocalcemia, or low calcium levels in blood. This triggers the production of parathyroid hormone. In response to the hormone, the skeletal system releases its calcium stores and the kidneys reabsorb calcium and excrete excess phosphorus. Overtime, the mineralization of bone is compromised, metabolic pathways continue to fade, and calcium levels continue to decline.
Part 2 of this series on MBD will discuss causes of the disease..
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