Metabolic bone disease
INTRODUCTION
Calcium deficiency leading to metabolic bone disease is the most common problem seen in sick reptiles presented to animal hospitals. Most causes result from the patience being fed a diet containing no calcium whatsoever, but because calcium metabolism is so complex, treatment involves much more than just adding calcium to the diet.
Calcium is a mineral required for normal neuromuscular function, blood clotting, activation of enzymes, heart contraction, and bone structure. Healthy intestine, liver, and vitamin D are all necessary for calcium absorption through the intestinal lining. Likewise, certain substances in the diet can combine with calcium with calcium in the gut to form insoluble, and therefore nonabsorbable, complexes. These include high in fat levels, oxalate, and acetic acid.
Phosphorus is a mineral that helps maintain normal acid-base equilibrium and is a component of both enzymes and bone. The amount of phosphorus ratio (Ca:P). Normal is 1 to 2 parts calcium to 1 part phosphorus (1:1 to 2:1), both in the diet and in the body. Too much phosphorus in the diet reacts with calcium to form insoluble calcium phosphate, which ties up calcium in the gut, while phosphorus continues to be absorbed. This leads to Ca:P ratios less that 1:1 and a relative calcium deficiency.
Vitamin D is considered to be more like a hormone than a vitamin. The source of vitamin D is either dietary or through the action of ultraviolet light on a precursor molecule in the skin. Both vitamin D's are converted to an intermediate form in the liver, then to its active form in the kidneys. From there it acts on the small intestine to facilitate calcium absorption and on bone to assist normal growth and mineralization. Thus healthy liver and kidneys are necessary to activate vitamin D.
Dietary protein deficiency, especially in young growing animals, can result in osteoporosis, a thinning of the bone that is independent of Ca:P levels. This can occur in cases such as an iguana on an all-vegetable diet or an animal that is refusing to eat at all.
When insufficient levels of calcium exist in the blood, parathyroid hormone is secreted by the parathyroid glands which causes calcium reabsorption from bone, resulting in normal blood levels of calcium at the expense of bone. The bones soften and lose density, making them more prone to fracture. Growth is stunted and in some cases excessive amounts of non-calcified scar tissue are deposited around the bones in an apparent attempt to support structural weakness. This later condition is called fibrous osteodystrophy and is commonly seen as swollen legs and jaws in iguanas.
In summary, metabolic bone disease may be caused by:
SYMPTOMS AND TREATMENT
Symptoms of metabolic bone disease vary with species, age, duration, degree, and type of disease. Skeletal deformities include bowed legs, kinked backs, soft or curled shells, and irregular fibrous swellings of the mandibles and leg bones. Lameness can result from soft jaws and loose teeth. Fractured vertebrae can cause paralysis. Stunted growth is common. Tetany, a spastic contraction of muscles, can result. An animal that appears fat is not always well nourished.
Treatment involves correction in the diet, which is not always easy. Many animals and some owners are set in their ways and reluctant to change. Patience and ingenuity are often required. When supplementing calcium, the Ca:P ratio must be considered. Oral or injectable vitamin D in the form of either vitamin D2 or vitamin D3. While many mammals can utilize D2, reptiles require D3 and probably metabolize D2 too rapidly to maintain adequate blood levels. Thus when purchasing vitamins, even "reptile vitamins", the ingredients should be checked to make sure that vitamin D2 was used. Birds also require D3, so bird vitamins may be used. Vitamins for humans, dogs, and cats, however, usually contain D2. It should be remembered that vitamin D can be overdosed leading to toxicity. This is one case where the rule "if a little is good a lot must be better" does not hold. Ultraviolet light may be provided in captivity by using full-spectrum fluorescent tubes, such as Vita-lite.
It must be realized that requirements for diet and environment vary with the species, sex, size, and age of the specimen and that specific requirements for any one species are generally not known. It is known that young, growing animals have a relatively higher calcium requirement than fully grown conspecifics. It is probably that turtles, because of their shell, need relatively more calcium. And basking species, such as iguanas, probably need more ultraviolet light that burrowing species. Furthermore, over supplementation with calcium, vitamin D, protein (as when the diet consists entirely of cat food), and ultraviolet light can cause pathologic mineralization of the viscera, arteries, and areas of smooth muscle, usually in iguanas, tortoises, and turtles. Clearly, more research into herpetological nutrition is needed.
Copyright: Chicago Herpetological Society 1989.
Reprinted and distributed by the San Diego Herpetological Society with the permission of the Chicago Herpetological Society.