METABOLIC BONE DISEASE
The term 'Metabolic Bone Disease' (or 'Disorder') is not an actual disease, but a catch-term for a wide variety of things that can soften or deform a tortoise's shell and skeleton. Some MBD's are due to disease processes, like 'Fibrous osteodystrophy', 'Hypertropic osteopathy', 'Paget's Disease', etc. Most MBD's are due to problems with diet and basic cares. The most common is 'Nutritional Secondary Hyperparathyroidism' or NSHP. Common names for this condition are 'rubber jaw' in lizards, or 'soft shell' in turtles and tortoises. The root cause is usually a combination of too little calcium, too much phosphorous, or too little vitamin D.

The more you study it, the more amazing the calcium cycle is. Calcium, phosphorous, magnesium, iron, water, some dietary vitamin D, vitamin A, and other nutrients and water are taken in the diet, and light with a wavelength around 296nanometers (UVB, almost UVC range) is absorbed through the skin. The liver, kidneys, parathyroid glands, and ultimobranchial glands process chemicals, converting calcium and other ingredients to a form that the cells can absorb and use. It is a fairly intricate balance and all ingredients have to be in place for everything to work correctly. You can offer all the calcium you want, but without the other necessary elements, it won't help at all.

Prevention:
Since NSHP is almost entirely due to poor cares and diet, it can be prevented by providing the tortoise with the proper cares and diet, appropriate to the species. A diet with enough calcium and a good ratio of calcium to phosphorous, in an average ratio of between 1.5:1 and 2:1 is a good foundation. The tortoise should also be provided with unfiltered sunlight as much as possible.

UVB can be difficult even for experienced keepers. The problem is that evidence suggests that most reptiles need at least some exposure to good sunlight, and that no bulb perfectly replaces what the sun does. Many UVB bulbs do not emit enough of the right wavelengths to really help, and as far as we can tell, dietary vitamin D does not help reptiles as much as sunlight does. On the other hand, we really do not know what role dietary vitamin D2 has, or what the real vitamin D/UVB needs really are for most reptiles.

The tortoise also needs proper warmth, plenty of exercise, good rest, an assortment of other vitamins and minerals, etc.- in other words, good general cares and diet.

Signals:
NSHP shows up in tortoises as a softened, leathery, or rubbery shell*, but by the time the shell shows obvious problems, there may be problems in the bones. Things like deformed jaws, weak and/or deformed limbs, splayed walking/dragging, paralysis, and cloacal prolapses are signs something is wrong. Be cautious, though, since a.) some of these symptoms can also signal other issues, and b.) these symptoms may be delayed as the tortoise 'steals' calcium from the shell to protect the skeleton and nervous system.

(* Note: Some turtles and tortoises are supposed to have soft, leathery, or rubbery shells such as the Pancake Tortoise and Soft-shell Turtles.)

Very young tortoises are a bit soft or flexible so they can fit in the egg and so the shell can move as they grow. This often worries keepers, but there is a difference between a young shell and a soft shell. Young tortoise shells are springy, like they are made of a stiff plastic- the plastron may feel a bit springy for some time. Soft shells, however, feel more 'leathery' and do not resist being compressed as much. If the shell just gets softer instead of firming up, you should take action.

Treatment:
As long as the tortoise is still eating, the shell is not too soft, and there are no other worrisome symptoms, the tortoise can probably be treated at home. Do not, however, suddenly start dumping lots of calcium in the poor animal or flooding it with UVB lighting. Instead...
- Work to provide the proper habitat, temps, humidity, and lighting.
- Correct the diet, adding more calcium-rich foods.
- Add a small amount of extra calcium, as a fine powder or liquid, to the diet.
- Add a small pinch or drop of multivitamins once a week or so to help ensure the tortoise is getting the other trace elements it needs.
- Offer good UVB lighting with either natural, unfiltered sunlight or a good UVB bulb. Currently, mercury vapor bulbs (MVB) that are tuned to emit UVB seem to be the most sun-like option, especially if you pair it with a cool-white fluorescent or plant bulb to provide a more balanced color spectrum.

If the shell is very soft; the tortoise is not eating; is showing other symptoms; or is not responding to your cares, you should see a vet. The vet should do blood and other tests, an x-ray, and take a complete history to try to determine if the problem is NSHP or another bone problem. The vet would then determine a course of treatment based on the findings. NSHP is usually treated with vitamin D3 injections and supplemental calcium orally, then follow-ups over two weeks. More severe cases may also need dietary support and other treatment. It is important to follow the vets suggestions as carefully as possible for the best chance at recovery.

Outcome:
Treated intelligently, the tortoise should recover fully, although some shell or skeletal deformity may remain, and there may be residual muscle or nerve problems. The earlier it is caught, the better the chances for a complete recovery. NSHP is rarely fatal by itself, but it can weaken a tortoise and other problems can develop.