Metabolic Bone Disease ( or MBD ) in Reptiles

What is MBD?

MBD stands for Metabolic Bone Disease – which is not a singular illness, but an umbrella term used to describe a collection of symptoms associated with an imbalance of calcium and vitamin D3 in the reptile body. There are five main subtypes of MBD, all with very similar symptoms but each with slightly different causes. The specific disorder most often diagnosed in reptiles is known in veterinary terms as secondary nutritional hyperparathyroidism. The other, less common types of MBD are oesteomalacia, fibrous osteodystrophy, rickets and osteoporosis. MBD can be a painful, distressing and sometimes fatal illness in captive reptiles – most noticeably in insectivorous and vegetarian species such as bearded dragons, leopard geckos, green iguanas and tortoise species. Perhaps the saddest thing about this illness is that it is entirely preventable with proper dietary management and correct husbandry practices.

What causes MBD?

Put simply, MBD is caused by a lack of calcium and vitamin D3 in the body. Calcium is an essential mineral that is needed in the reptile body for a number of critical biological functions - these include the transmission of nervous signals to the brain, the contraction and movement of muscles and most importantly, the development and maintenance of the hard skeleton. In order for all of these body systems to work properly, there must be a constant good supply of calcium available in the diet. In order to ensure this, many reptile product manufacturers sell a range of dietary supplements that have been researched and developed to provide extra essential nutrients. These supplements are usually sold as a fine powder containing a balanced formula of calcium, other minerals and vitamins, and are absolutely essential for maintaining the health of pet lizards and tortoises. They can be bought cheaply in large tubs from most pet shops or online reptile suppliers, with a small pinch being lightly dusted over the animals’ food each day.However, it is not simply enough to provide pet reptiles with large amounts of dietary calcium, as another vital component is still needed in order for it to be properly absorbed and metabolised by the body – vitamin D3. The hormonally active form of vitamin D3 needed for calcium metabolism is known as calcitriol (Chemical name: 1,25-dihydroxycholecalciferol), and it is made in special cells in the skin of diurnal (day time active) reptiles such as lizards and tortoises. When these reptiles are exposed to full spectrum ultraviolet light at wavelengths of 270-290 nanometers, the special skin cells produce an inactive form of vitamin D3. This inactive form travels in the bloodstream to the kidneys where it is converted into the compound calcitriol. This ‘active’ form of vitamin D3 reacts with cells in the lining of the reptiles’ digestive system, acting like a key in a lock in order to allow calcium to be absorbed from the food and into the blood. Once in the bloodstream, the calcium is then free to travel to anywhere in the body where it may be needed, such the muscles, nerves or skeleton. If a pet lizard or tortoise is not provided with enough dietary calcium or insufficient UVB lighting, the levels of calcium present in the blood serum will begin to fall, leading to a condition called hypocalcaemia. If hypocalcaemia is allowed to develop and persist over an extended period of days or weeks, the reptiles’ body responds by producing a substance called parathyroid hormone from special glands in the neck, which acts on the rest of the body to try to restore normal blood calcium levels by forcing the production of more calcitriol and stopping calcium excretion in the kidneys. If this fails to stop the blood calcium levels from falling, the parathyroid hormone then triggers a biochemical change that begins to strip calcium molecules away from the skeleton and muscles, causing the skeleton to become weak and distorted. This is the start of metabolic bone disease.

Reptile Husbandry: Preventing MBD

In insect eating lizards such as bearded dragons or leopard geckos, the need for high levels of calcium is met by feeding them on a varied diet of live insects, such as crickets, locusts or mealworms, which have been gut-loaded on a suitable calcium-rich bug-grub and then dusted with a specialist reptile powder supplement. In vegetarian reptiles such as green iguanas and tortoises, owners can ensure their pets have plenty of calcium by feeding a varied diet of leafy greens that are especially high in naturally occurring calcium, such as dandelion leaves, mustard greens and watercress, and the salad mixture then sprinkled with a powder supplement. The use of these supplements depends on the age, size and type of reptile they are being used for, but as a general guide it is common practice to dust the food of young and juvenile reptiles every day and twice to three times per week in adults. Pregnant females will also need a calcium supplement every day as her body will have a much higher demand for calcium in order to calcify the shells of her eggs in oviparous species and to develop the skeletons of the foetuses in viviparous species. Without vitamin D3, calcium molecules in the food cannot be properly absorbed from the gut into the bloodstream, and so are not available for use in the body. This is why it is equally important not only to provide plenty of calcium in the diet, but to also ensure that pet lizards and tortoises are provided with specialist full spectrum ultraviolet (UVB) lighting during the day so that they can produce their own vitamin D in their skin. In modern reptile keeping, these lights are available from all good pet shops and can be purchased either as long strip lights or as smaller compact light bulbs that look similar to normal energy saving bulbs, both of which are suitable for use with smaller lizards such as geckos. For larger lizards such as iguanas and for tortoises, new high wattage mercury vapour lamps are available which provide strong levels of UVB lighting as well as heat for basking can be used. All types of UVB lighting bulbs should be positioned within a distance of 5 – 10 inches of the reptile’s preferred basking spot in order to maximise their exposure to the beneficial ultraviolet lighting, and should always be replaced at least every 6 months. This is because the special coating on the bulbs that creates the UVB radiation wears out over time, so even if an older bulb is still working, it may not be giving off adequate levels of UV light to keep a reptile healthy after this period of time.Some pet food manufacturing companies are now selling a vitamin D3 powder supplement, which is intended for use in the same manner as a standard calcium or vitamin powder, being dusted over food each day. However, more research needs to be carried out in order to determine how effective these vitamin D3 powders are and if they have the same effect as the calcitriol produced in the reptiles’ skin. Until further evidence is available to support the efficacy of these products, they are best use only to compliment, rather than replace, the use of proper full spectrum UVB lighting.

Do snakes get MBD?

Carnivorous snakes that feed on whole carcass prey, such as defrosted mice, rats and rabbits are much less likely to suffer with MBD than lizards or tortoises. This is because the skeletons and muscles of their mammalian prey contain enough dietary calcium to meet their needs, while the livers of these warm blooded animals already contains calcitriol - the bioactive form of vitamin D3 needed to metabolise the calcium. When a snake such as a python eats and digests a rat for example, the calcium and calcitriol from the rat is released and absorbed into the pythons body. As a result, good quality whole prey bought from reliable suppliers and stored properly should contain everything needed to keep snakes such as corn snakes, royal pythons and boas healthy, and as a result they should not need any extra calcium supplements or UVB lighting. However, there are a few exceptions to this rule. There are a handful of more specialist snake species in the reptile keeping hobby that do not feed on defrosted whole prey, such as the insectivorous rough green tree snake and the various species of garter snakes that prefer a diet of fish. These species will need some adjustments in their feeding regime, often requiring small amounts of additional calcium and vitamin supplements to be given in the diet to ensure they have all the nutrients they need.

What are the symptoms of MBD?

The first symptoms of MBD developing include;

• Depression: Affected reptiles will become quiet, depressed and listless, often refusing to move at all or sleeping excessively. The skin of brightly coloured animals such as iguanas or chameleons may also change and become dark and drab.
• Swelling of the legs: As the bones and joints of the legs become thinner and weaker, the reptile will experience pain and difficulty in walking, and their movement may become more awkward, stiff or uncoordinated. Lizards such as bearded dragons and geckos may be unable to hold their weight and instead will drag themselves along the floor. Arboreal lizards may begin to spend more time sitting on the floor of their vivarium as climbing and holding on to branches becomes impossible.
• Swelling of the jaw: As the jaw bones become soft they often become inflamed and painful, causing the reptile to stop eating and lose weight rapidly.

At this stage the symptoms of MBD are often noticed by the reptile’s owner and veterinary help sought. If caught early, treatment is straightforward and most of the symptoms are reversible. However, if after a period of 1 – 2 weeks, the animal has not been assessed and treated by a vet, the more serious, secondary symptoms of MBD will develop, including;

• Fits, muscle tremors and seizures: The lack of calcium in the blood will eventually cause a break down in the correct functioning of the nervous and muscular systems, resulting in uncontrollable shivering and twitching of the muscles, as well as a complete loss of balance and coordination.
• Distortion, dislocations and fractures of the long bones: Softening of the skeleton as calcium is stripped away causes the long bones of the radius, ulna and femur to become brittle, often causing them to suffer from folding fractures or permanent bending and twisting. The joints of the hips, shoulders and elbows may also become dislocated.
• Deformity of the skull and jaws: The upper jaw and dome of the skull may become contracted, causing the tongue and lower jaw to protrude and putting pressure on the brain.
• Twisting and distortion of the spinal vertebrae: Causing the backbone to appear kinked, hunched or lumpy, and sometimes causing paralysis as the spinal cord is crushed or pinched.
• Stomach bloat and gut rot: As the muscles that control the digestive tract shut down, any food remaining in the stomach or intestines will begin to ferment and rot, resulting in an accumulation of gas and toxins.
• In tortoises, the carapace and plastron of the shell will become soft, twisted and flattened, putting pressure on the internal organs.

Eventually after a period of several weeks, the reptile suffering from advanced stage MBD will succumb to its symptoms and die, often as a result of a heart attack as the cardiac muscle is unable to beat properly without adequate calcium, or from asphyxiation as the rib cage collapses, so making breathing impossible.

Diagnosis and Treatment of MBD

If caught and identified in the early stages, MBD is generally fairly straight forward to correct, requiring a regimen of careful supportive nursing along with the use of specialist calcium replacement and rehydration therapies. If a reptile is presented to a veterinary surgeon with a clinical history of poor dietary and husbandry management and showing the symptoms of MBD, the first step is usually to take a blood sample to assess the basic biochemical parameters such as blood serum calcium levels and levels of parathyroid hormone present in the body. Full body X-rays are also often carried out on the patient in order to identify the areas of the skeleton most severely affected and any fractures or dislocations that may also require attention. Once a diagnosis of MBD is confirmed, the reptile should be moved into an intensive care unit (ICU) or quarantine setup – usually this is a simple vivarium furnished with little more than newspaper and a waterbowl in order to prevent the animal from trying to climb or move around too much and further injuring itself. High output full spectrum UVB lighting and heating are provided in order to encourage the synthesis of vitamin D3 and proper thermoregulation, and the reptile started on a course of rehydration therapy, usually administered as a IV drip in larger reptiles and as regular lukewarm baths in smaller lizards. Once properly hydrated, any fractured or dislocated bones are corrected and set using casts or supportive dressings, and blood calcium levels slowly restored to within a normal range using an injectable isotonic calcium solution administered by the vet each day. Very weak reptiles may need to be carefully tube fed on a calcium-rich liquid diet for several weeks before they regain enough strength to feed themselves on soft foods. Eventually, with enough careful treatment and supportive care, many reptiles will recover and their bones will eventually become re-mineralised and harden, although they may suffer from lasting permanent deformities such as twisted legs, kinked spines or deformed jaws for the rest of their lives. Unfortunately, in extreme cases of neglect or the abandonment of reptiles, the symptoms of MBD may be deemed too severe for the animal to survive treatment – in this cause the veterinary surgeon may recommend human euthanasia to prevent further suffering.