Metabolic bone disease (MBD) is a well recognized and all too common disease often seen in our pet reptiles. Other terms which may be used include fibrous osteodystrophy, osteomalacia, secondary nutritional hyperparathyroidism, osteoporosis, and rickets. There is no single cause of MBD and the disease is not as simple as calcium deficiency. The primary problem is a disruption of calcium metabolism which causes a host of related problems.
MBD is almost always a result of poor husbandry but it is also generally preventable by providing a proper environment and diet specifically for your type of reptile. This is not always easy or inexpensive for a reptile owner to do but is vital to the health of all pet reptiles.
Metabolic bone disease is a complex disease. In it's simplest of terms, metabolic bone disease results from improper calcium to phosphorus ratio in the body. Normally, this ratio should be around two parts calcium and one part phosphorus (2:1). When the calcium level is relatively low the body tries to compensate by taking calcium from wherever it can (such as from the bones). This leads to a softening of the bones making them susceptible to fractures and also leading to a deposition of fibrous tissue as the body tries to strengthen the bone in an absence of available calcium. Calcium also impacts a number of other physiological systems including muscle contraction (including the heart muscles) and blood clotting.
Therefore the 2:1 ratio of calcium to phosphorus is ideal in your reptile's diet but calcium metabolism is not that simple. Vitamin D (especially D3) is also vital to calcium metabolism and because some reptiles do not absorb vitamin D that well (much like humans) they need ultraviolet light exposure to manufacture their own vitamin D. Are you confused yet?
A full discussion of calcium, phosphorus, and vitamin D metabolism is beyond the scope of this article but the basic factors that can skew the calcium to phosphorus ratio include:
- too little calcium or too much phosphorus (i.e. improper ratio) in the diet
- presence of substances in the diet that impair the absorption of calcium (e.g. oxalates)
- a deficiency of vitamin D
- lack of exposure to UVA and UVB (necessary for the reptile to produce it's own vitamin D)
- inadequate protein
- kidney or liver disease (which impairs the conversion of vitamin D to its active form), small intestinal disease (disrupts absorption)
- disease of the thyroid or parathyroid glands (produce hormones which affect calcium metabolism)
- cool temperatures impair digestion and therefore affect calcium absorption
Signs and Symptoms
These vary depending on the severity and length of time over which the condition has developed. Due to the importance of calcium in bone formation and muscle function, most of the signs and symptoms are related to bone and muscle effects. These include:
- Bowed, or swollen legs, or bumps on the long bones of the legs
- Arched spine or bumps along the bones of the spine
- Softening and swelling of the jaw (sometimes called "rubber jaw")
- Receded lower jaw
- In turtles, softening of the carapace or plastron (the shell)
- Muscle tremors and jerky movements or twitching in the muscles of the legs and toes
- Lameness and limping
- Anorexia (not eating)
- Constipation or obstipation (not defecating)
- Fractures (breaks) of the bones due to bone weakness
- Weakness and even partial paralysis (sometimes unable to lift their body off the ground because of how weak they are)
Metabolic bone disease is distinctive enough that a diagnosis is usually made based on the symptoms, physical exam, and a discussion of husbandry. Radiographs (x-rays) may be taken to confirm the diagnosis and monitor treatment and calcium levels in the blood may be tested.
Treatment of MBD depends on the severity of the disease. For very mild cases, a switch to a balanced diet and proper husbandry may be enough. Severe cases require intensive calcium and vitamin supplementation as well as an increase in high-intensity UVB rays under an exotics veterinarian's care.
Proper husbandry is more than just providing the right diet. The following are important in both the prevention and treatment of metabolic bone disease:
- A diet balanced in calcium and phosphorus, protein, energy and other nutrients
- Exposure to UVA/UVB invisible light rays
- Proper heat gradients (day and night)
- Proper light/dark cycles
- Adequate enclosure and/or room to exercise