Metabolic Bone Disease(c) 1995 Melissa KaplanMelissK@aol.com
Metabolic bone disease (MBD) is an umbrella term that covers a number of
disorders related to the weakening of the bone or impaired systems function
caused by an imbalance in vitamin D3, calcium and phosphorus. This imbalance
may be caused by a lack of or too much of one of these three essential
elements or the failure to provide one or more of them in a bioavailable
form. Many foods highly touted for their calcium content, such as spinach,
carrots, collards, chards and other thick leafy greens, contain calcium
oxalates that bind calcium. This renders most or all dietary calcium, both
that contained in the foods and that added to the foods as supplements,
unavailable for maintenance and growth.
MBD and calcium metabolism are discussed in great detail in many texts and
so will not be elaborated upon here. Quite simply stated, vitamin D3
(dietary or derived from exposure to ultraviolet B), calcium (dietary and
matter recycled from the bone matrix) and phosphorus (dietary) interact
together to perform a number of functions besides bone growth and
maintenance, incuding muscle contractions and blood coagulation (Wright,
1993). The result is a well-functioning system, with calcium restored to
and, in the case of growing animals, added to the bone matrix. Too much
phosphorus can throw this process off, as can too much or too little vitamin
D3 or too little access to ultraviolet B wavelengths. Many of the leafy
greens recommended as high calcium foods As the dangers of calcium deficiency
become more widely known, there is increased risk that pet owners may add too
much calcium to their reptile's diet. This results in hypercalcemia, a
condition as fraught with peril as is hypocalcemia.
Signs of metabolic bone disease include hard knobs in the long bones of the
legs, bumps along the vertebral column of the back and tail, softening or
hard swelling of the jaw (Fig. 3), and softening of the plastron or carapace.
All of these signs may be felt before they can be seen, making a careful
physical exam important. Visible signs of moderate to severe MBD include
jerky gait when walking, tremors and twitches in the limbs and muscles of the
legs and toes when at rest, and shakiness when being held. Advanced cases of
MBD include all the above signs plus anorexia and fractured bones. Severely
deficient reptiles tend to be lethargic and may only be able to drag
themselves along the ground. Arboreal lizards spend all of their time on the
ground as they lack the strength to grip and climb.
Moderate to severe cases of MBD require the proper diet, temperatures,
and light wavelengths as well as a more powerful calcium supplement than
those found in pet stores. Oral administration of calcium glubionate
(NeoCalglucon(r), 1cc/kg PO bid prn) or injections of calcitonin
(Miacalcin(r)) or calcium gluconate are generally prescribed by
veterinarians. Mader (1993) reports faster recovery with Calcimar(r) (500
IU/kg IM in front leg, repeated once a week for two weeks) is administered to
iguanas who have been returned to normal serum calcium levels. Use of
calcitonin before normal levels have been established, however, may cause
hypocalcemic tetany and death. Mild cases, cases where the signs are felt or
just barely visible, may successfuly be treated by providing the proper
environment and diet. In the case of diurnal lizards and chelonians, proper
environment includes not only the proper temperature ranges and diet, but
daily access to ultraviolent B wavelengths.
There remains much debate as to the necessith and efficacy of natural and
artificially produced ultraviolet B wavelengths in the development of
precursors to vitamin D3 and the metabolism of calcium. Gerhmann (1991)
reports anecdotally that not all lizards require UVB to maintain proper
D3-calcium-phosphorus balance. Bernard, et al. (1991) found that the green
iguana (Iguana iguana) fared much better when exposed to ultraviolet B
wavelengths than they did to vitamin D3 injections or supplements added to
their food. As both ultraviolet A, which acts upon appetite and behavior,
and ultraviolet B wavelengths are more likely to benefit lizards and
chelonians than not, long daily periods of access to such wavelengths should
be considered a necessary part of the care of diurnal lizards and all
chelonians (Alberts, 1994).
Along with proper day and night temperature gradients and a night-time
dark period of sufficient length (based on native habitat), proper diet is
essential to recovery. Herbivores and omnivores should be fed calcium-rich,
nutrient dense foods such as squashes, green beans, alfalfa (from rabbit food
pellets or pulverized hay cubes), parsnips, mustard greens, dandelions,
escarole, and fruits such as figs, papaya, cantaloupe and berries (Barten,
1993; Frye, 1991). The food should be supplemented with additional calcium
and a multivitamin formulated for reptiles or birds, or a crushed Centrum(r)
vitamin tablet formulated for humans (Mader, in press). Omnivores and
carnivores should be fed whole captive bred prey (to reduce the risk of
zoonotic infection from parasites commonly found in wild prey) that have been
raised on nutritious foods or have been gut loaded with nutritious foods for
several days before being fed out.