Metabolic Bone Disease in Reptiles
A Common, Serious, and Preventable Disease in Pet Reptiles
by Lianne McLeod, DVMfor About.com

 

Metabolic bone disease (MBD) is a well recognized and all too common disease of reptiles. Other terms which may be used include fibrous osteodystrophy, osteomalacia, secondary nutritional hyperparathyroidism, osteoporosis, and rickets. There is no single cause and the disease is not as simple a calcium deficiency. However, 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 generally preventable by providing a proper environment and diet. This is not always easy or inexpensive, but is vital to the health of pet reptiles.

Discussions of this disease often involve iguanas. Because iguanas are very popular and are susceptible to MBD due to their specific diet/light/environment needs, there are a large number of cases seen in iguanas.

Causes

MBD is complex disease. In it's simplest terms, MBD results from an improper calcium to phophorus ratio in the body. Normally this ratio should be around 2:1 calcium:phosphorus (in the range of 1:1 to 2:1). When the calcium level is relatively low the body tries to compensate by taking calcium from wherever it can, for example 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) and blood clotting. The 2:1 ratio of calcium to phosphorus that is ideal in the 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.

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: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 impair conversion of vitamin D to it's active from), small intestinal disease (disrupts absorption), and disease of the thyroid or parathyroid glands (produce hormones which affect calcium metabolism). These are minor contributors - most cases are nutritionally based.
cool temperatures impair digestion and therefore calcium absorption
If you want to learn more about metabolism and the interactions of calcium phosphorus, vitamin D and how they act together, try Melissa Kaplan's "Calcium Metabolism and Metabolic Bone Disease."

Signs and Symptoms:

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 bones of spine
softening and swelling of the jaw (bilateral) - sometimes called "rubber jaw"
receded lower jaw
in turtles, softening of the carapace or plastron (the shell)
tremors
jerky movements-twitching in the muscles of the legs and toes
lameness
anorexia
constipation
fractures of the bones due to bone weakness
lethargy
weakness and even partial paralysis (sometimes unable to lift body off ground)
The disease is distinctive enough that diagnosis is usually made based on the symptoms, physical exam, and discussion of husbandry. Radiographs (X-rays) may be taken to confirm the diagnosis and monitor treatment. Treatment depends on the severity of the disease. For very mild cases a switch to a balanced diet and proper husbandry may be enough, but many cases require intensive calcium and vitamin supplementation under a veterinarian's care.

Prevention

Proper husbandry is more than just the right diet. The following are important in both prevention and treatment:

diet balanced in calcium and phosphorus, protein, energy and other nutrients
exposure to UVA/UVB for diurnal reptiles - need fluorescent bulbs that are rated to provide UVA and UVB (see "Reptile Heat and Light")
proper heat gradients (day and night)
proper light/dark cycles
adequate enclosure/room to exercise

 

MBD Resources

Calcium Metabolism and Metabolic Bone Disease by Melissa Kaplan
Metabolic Bone Disease - Identification and Treatment by Melissa Kaplan
Metabolic Bone Disease by Tricia Power
Metabolic Bone Disease by The Winter Park Veterinary Clinic