Metabolic Bone Disease (MBD)

In this episode, Dr. Lottie talks about metabolic bone disease in reptiles, one of the most common but also most preventable conditions seen in exotics practice.

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Timeline:
00:00 Intro
00:43 Case
01:28 Aetiology & Pathogenesis
07:02 Clinical Signs
07:48 Diagnosis
09:56 Treatment
12:18 Top Tips
15:47 Key Points
16:51 Outro

Recommended reading

• BSAVA Manual of Reptiles
• BSAVA Manual of Exotic Pets
• Mader’s Reptile and Amphibian Medicine and Surgery
• ReptiFiles website: https://reptifiles.com/

References

• Harcourt-Brown, F. and Chitty, J. (eds.) (2013) BSAVA Manual of Exotic Pet and Wildlife Nursing. Gloucester: British Small Animal Veterinary Association.
• Girling, S.J. and Raiti, P. (eds.) (2004) BSAVA Manual of Reptiles. 2nd edn. Gloucester: British Small Animal Veterinary Association.
• Meredith, A. and Redrobe, S. (eds.) (2010) BSAVA Manual of Exotic Pets. 5th edn. Gloucester: British Small Animal Veterinary Association.
• Mader, D.R. (ed.) (2019) Mader’s Reptile and Amphibian Medicine and Surgery. 3rd edn. St. Louis: Elsevier
• Baines, F.M., Chattell, J., Dale, J., Garrick, D., Gill, I., Goetz, M., Skelton, T. and Swatman, M. (2016) ‘How much UV-B does my reptile need? The UV-Tool, a guide to the selection of UV lighting for reptiles and amphibians in captivity’, Journal of Zoo and Aquarium Research, 4(1), pp. 42–63.
• Allen, M.E. and Oftedal, O.T. (2003) ‘Dietary manipulation of the calcium content of feed insects’, Zoo Biology, 22(3), pp. 269–282.

Current Vet is an educational podcast intended for veterinary students, veterinary professionals, and individuals with an interest in veterinary medicine.
All content provided in this podcast and its associated materials is for educational and informational purposes only. It is not intended as, and must not be considered a substitute for, professional veterinary advice, diagnosis, or treatment.

Any clinical cases discussed in this podcast are fictional, and are designed to reflect typical or likely clinical scenarios for educational purposes. They do not represent specific real-life cases, clients, or animals.

While every effort is made to ensure accuracy and alignment with current evidence at the time of publication, veterinary medicine is a rapidly evolving field, and recommendations may change over time.

Creators and Guests

Host

Dr. Lottie Wilkinson

Creator and host of the Current Vet podcast

What is Current Vet?

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In each episode, Dr. Lottie breaks down clinical conditions, cases, and concepts across species, focusing on pathophysiology, decision-making, diagnostics, and what actually matters in practice. It’s the kind of context that makes your knowledge finally click.

Every month, we’ll also have honest conversations with guests about the incredible variety of veterinary medicine, what you can do with a vet degree and how to think bigger about your career.

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Intro

Hello, hello, and welcome back to Current Vet. This is the podcast that makes veterinary medicine simple. I'm your host, Dr. Lottie, and today we're diving into a condition that is really common in reptile practice. . And it's often tested in exams, and that is metabolic bone disease or MBD in reptiles.

So MBD actually isn't a single disease. It's a bit of an umbrella term, and in most cases, what we're actually dealing with is nutritional secondary hyperparathyroidism. But let's start with our case and then talk about what MBD actually is

Case

So you're in consults and a client brings in Spike, a 10-month-old bearded dragon. The owner says he's been a bit quieter recently and hasn't been eating much. Over the last couple of weeks, they've noticed he's struggling to climb like he used to, and yesterday he seemed to drag his back legs a little bit.

On exam, Spike is a bit quiet and subdued. When you pick him up, his jaw feels a little bit soft and you notice mild swelling in the mandible. And when he walks, there's definite hind limb weakness with an abnormal gait. You gently palpate his limbs and they don't feel quite right

Aetiology & Pathogenesis

So what is MBD? Metabolic bone disease is a syndrome of skeletal demineralization and weakness caused by chronic disruption of calcium and phosphorus and vitamin D metabolism. So in reptiles, the most common underlying cause is nutritional secondary hyperparathyroidism, and this happens when calcium is deficient or phosphorus is excessive, or vitamin D3 is insufficient, or UVB exposure is inadequate, or it's a combination of all four. the result of this is basically that the body cannot maintain normal blood calcium levels, so it compensates by mobilizing calcium stores from the bones
Calcium is really tightly regulated in the body because it is used for neuromuscular function, it's used for cardiac function, and enzyme activity. So if blood calcium drops, the body compensates for that by mobilizing it from stores. When calcium's low, the parathyroid gland releases PTH, which is parathyroid hormone, and that acts to do three key things.

So it increases bone resorption, which releases calcium from bone. It increases renal calcium reabsorption, so less calcium is gonna be excreted in urine, and it promotes activation of vitamin D. If there is chronic increases in PTH levels,
Bone is continuously being resorbed

UVB and vitamin D3 are also key parts of this. So . unlike a lot of mammals, reptiles rely really heavily on UVB light to synthesize vitamin D3. So UVB radiation hits the skin. It converts precursors into vitamin D3, which is also known as cholecalciferol, and then vitamin D3 is then activated in the liver and kidneys, , boosts intestinal absorption of calcium.

So no UVB means there is no active vitamin D3, which means there is less calcium absorption from the gut. So even if the diet contains calcium, the body may not be absorbing it properly, and then it literally can't use it Final key piece is the calcium phosphorus ratio. So the ideal - dietary calcium phosphorus ratio is two to one of calcium to phosphorus.

And many insect-based diets like crickets that we feed our reptiles are actually high in phosphorus and low in calcium. And this is unfortunate because high phosphorus can bind calcium, which reduces its bioavailability and then worsens hypocalcemia levels.
So that's gonna further push that cascade and increase calcium reabsorption from bones again. So put that all together, we have low calcium or poor absorption means increased parathyroid hormone.

There is chronic bone resorption and the bone becomes really soft, weak, deformed, and eventually basically useless So when we were looking at Spike, when we felt that soft jaw or the slightly strange limbs, that is known as rubber jaw, which is a hallmark sign of advanced MVD

Just to touch on a few of the causes. So NBD is almost always a disease of husbandry treating it is not gonna be just about treating the animal, it's gonna be correcting the owner's entire management system and all their practices at home. So we need to be really confident on knowing what the big contributors are and how we can change and help our owners to improve their husbandry practices.

So first up is poor UVB light, and this is a really common cause. so things like not having a UVB bulb at all. Owners may not realize that it's actually essential for their reptiles. They might have an incorrect bulb type, or they have the correct bulb and the bulb is too far away to be doing any good for the reptile.

UVB may be being blocked by glass in front of the bulb, or the bulb hasn't been replaced because the UV output actually drops over time, so we need to be checking how long these bulbs have been left in Next, we have a poor diet, so insect-only diets without any calcium supplementation, if they aren't gut-loading insects, if there is no calcium dusting on their diets, or if there was just an imbalanced calcium-phosphorus ratio

And we can have a lack of vitamin D3

And this can be because of the lack of UVB again, or it may be that there's no diet supplementation

And then finally, we also have poor temperature regulation at home. So reptiles are ectotherms. They rely on the external environment to regulate their body temperature. If they are too cold, digestion can slow down, and then calcium absorption is going to decrease. So even having a good diet is not going to be utilized properly by the body

Clinical Signs

So what clinical signs are we gonna see in MBD? Early cases are often missed, and these are gonna be pretty subtle signs. So we might see just some lethargy, reduced appetite, a little bit of weakness, and maybe reluctance to move or climb. Progressive signs as the condition worsens, we might see limb deformities, that soft jaw, deformed spine, pathological fractures as the bone gets really thin and weak, and we can also see muscle tremors. And then finally, in really severe cases, they go into hypercalcemic tetani. They - can have seizures, and they can be paralyzed

Diagnosis

Okay, let's go back to Spike. So you're really suspicious of MVD, but you need to confirm it. So how are we gonna make sure we've got the correct diagnosis?

First of all, we're gonna take a really good history from the owner, so making sure to ask about the UVB, the bulb, the type, age of the bulb, how far away it is from the reptile within the cage, what they're feeding, exactly what it is and how often, if there's any supplementation, making sure to ask about calcium supplements, vitamin D3 supplements, and then also about the temperature within the enclosure.

Often these findings, and then obviously looking at the reptile, is strongly supportive enough to give you your diagnosis. To back it up though, we can do radiographs, and this is gonna be one of the best things to do and one of the most widely available things to do in GP. So findings will include a reduced bone opacity, thin cortices in the bones, pathological fractures maybe, and we may also see spinal deformities in a little bit more advanced cases.

But basically, bones look really thin and almost ghost-like because there's just not much substance left to them. We can also do bloods.

We might see a low calcium level, but actually we can also see normal calcium or even slightly raised calcium sometimes because of how much calcium is being mobilized from the bone. So .. it's not definitive, and having a normal calcium level actually doesn't rule out nutritional

secondary hyperparathyroidism. We may see elevated phosphorus, and we may see a, a change in the calcium-phosphorus ratio. But as we said, levels here are actually so variable that findings aren't going to be Generally that useful for diagnosis

Treatment

Let's move on to treatment. So the treatment is - it's gonna vary depending on the severity of the case. So in emergency cases where we have things like seizures or tetani, - we're
Going to give calcium gluconate to try and boost those calcium levels. We need to make sure that we're monitoring the heart closely

As calcium can cause bradycardia

We then have the long-term management changes that are gonna be really crucial for stabilizing the condition,

And then preventing it from occurring again

So first, we need to make sure that there is really good calcium supplementation. Oral calcium can be given for long-term management, but in severe cases, like if we're needing to stabilize, if they're in severe hypocalcemic crises, injectable calcium can also be given. We need to be correcting the husbandry approaches, so making sure they have a proper source of UVB, the correct type, the correct distance, and exposure to unfiltered sunlight can also help in these cases

If a bulb isn't immediately available. We need to make sure that the temperature and heat gradients are all correct within their environment, and that we are transitioning to a diet with suitable vitamin contents, good supplementation. So for example, the calcium dusting or feeding gut-loaded insects and making sure that it is a balanced , nutritious diet.
If the reptile is also dehydrated or systemically unwell, then some fluid therapy can also be helpful

The key point really here though with MBD and treatment is that we can't just medicate our way out of the condition.

If the husbandry isn't being fixed, the underlying problem isn't being fixed,

So that is really the key to treatment here is fixing those husbandry practices and making sure we can support our owners and inform them on good practice for that

In some cases, especially advanced cases

Bone deformities can actually be permanent and the reptile can develop long-term disability

Top Tips

I'm just gonna go through a couple of tips for consulting these cases, how we can approach these with the owners, because this is the thing that is gonna make the biggest difference for cases of MVD. So my first tip is gonna be be very specific with your questions. Don't just ask, "Do you have a light bulb?"

Ask what exact bulb, what type of UV light, how old is it, how far away from the animal? Is there anything blocking , the light? All of these kind of things are all really key to know because the owner might know that they need a source of light, but they may not know that they need a source of UVB specifically.

So asking those specific questions can help. Ask owners to bring photos, like photos of their enclosure, photos of where they are living is really useful, and they can often help you identify the problem really quickly that the owner may not be aware of and may not think to mention, or , you may notice something completely different is something else to address that you just wouldn't be aware of otherwise Thirdly, we are there to educate.

We are not there to blame. Most owners are actually trying to do their best, so the way we approach it with our advice and our conversation is really crucial

We can motivate our clients and encourage them and make them want to do better for their pets. But I think blaming them and putting them down about it and making them feel stupid or like they should have known better is not gonna get us anywhere. So educate, don't blame. And then finally, prioritize the top three fixes.

So there are a lot of management changes that we can do to approach this condition, but owners are gonna feel really overwhelmed, especially if they're unaware that they're doing anything wrong in the first place, and then you suddenly give them 20 different things that they need to go out and buy, different changes that they need to make.

It's gonna be a lot. So focus on the top, the top big hitters. So we have UVB, making sure they've got a reliable source, basically a fresh UVB bulb. They have a good diet, so focusing on that, and then also focusing on the temperature in the environment. Those are gonna be the big things, and then kind of everything else can come later once those have been addressed.

80, 90% of the problem is gonna be fixed if we address those key things. The final few percent can come after those big priorities have been addressed

I have a couple of recommendations for anyone wanting to build confidence with their reptile medicine, with their exotic medicine. So the BSAVA has great manuals covering reptile care, - called the Manual of Exotic Pet and Wildlife Nursing. They also have the Manual of Reptiles and the Manual of Exotic Pets.

They're really great reads. They're really good summaries, so definitely go out and read those if you can get them. There's also a website called ReptiFiles, which has a huge range of care guides and husbandry guidelines for a load of different exotic pet species. So these are really great to have maybe on your consult screen while talking to the owner or for giving out as a handout to our exotic pet owners.

I'll link those both in the show notes so you can go and visit them too.

Key Points

So what are the key points that you need to know about metabolic bone disease before we finish? Firstly, metabolic bone disease is usually caused by nutritional secondary hyperparathyroidism, driven by -- poor calcium intake and poor UVB exposure.
UVB is absolutely critical for vitamin D3 synthesis and for calcium absorption. Clinical signs range from slight lethargy and inappetence all the way to pathological fractures and potentially hypocalcemic seizures. A really thorough history and then radiographs are going to be the best diagnostic tools, and then treatment focuses on supplementing calcium, supplementing vitamin D, and then fixing the husbandry approaches.
And most importantly, we can't treat MBD without correcting their environment.

Outro

And that is it for metabolic bone disease in reptiles. Thank you for listening to Current Vet. If you found this helpful, follow or subscribe wherever you listen and share it with someone who wants to feel a little bit more comfortable with exotics in practice.
You'll find today's resources in the show notes. If you have any requests for future topics or future episodes, let us know in the comments below or find us on TikTok and Instagram at Veterinary Vista. We'll see you next time