Authored by: Christina Miller CAHT/RVT, RLAT, BSc
MBD in Herpetoculture
Captive reptiles and amphibians are subject to a somewhat unique situation. Unlike our mammalian companions, they’re highly dependent on their captive environment for their most basic physiology to function, making good husbandry the key to maintaining healthy companions. One of the most common health problems we see in captive herps relates to calcium metabolism:
The infamous “metabolic bone diseases,” often abbreviated as MBD in both lay and professional literature.
I feel this is a suitable topic to start with in what will be a series of health and medical columns, as it’s (in my opinion) one of the saddest syndromes we treat in captive herps. It’s entirely preventable in all but the most obscure of cases, and metabolic bone diseases essentially do not occur in wild animals. Often, early clinical signs are subtle and go unnoticed by most hobbyists. Prevention at the level of diagnosing husbandry errors before they promote a disease process is key (and that applies to all husbandry-related pathologies in captive herps). My goal, is to give our readers a basic understanding of the complexities of this disease process (and others in the future), and how to apply this understanding to your husbandry techniques.
This subject is of course very complex. In fact, “metabolic bone disease” does not actually refer to one pathology, but a series of syndromes that affect bone form and function (Mader 2006). The most prevalent form of metabolic bone disease that we see in captive herps is nutritional secondary hyperparathyroidism (NSHP- because it’s really quite a mouthful when you say it out loud). Before we delve into the meaty medical and husbandry stuff, understanding the bodily processes that are involved in using dietary calcium is the first step. There are a lot of intricate details to calcium metabolism, and just because a herp is getting calcium in their diet, that does not necessarily mean that they can use it appropriately.
Calcium and Reptile Relationships
Back to basics: Calcium in the body
Calcium, simplified, is an element in our diets, important for the health of bones, teeth, and muscles. It’s a major component in the structure of both bones and teeth, contributing to the mineral-like rigidity of these body parts. It’s also heavily involved in cellular signaling and blood clotting (Wedekind et al. 2010). The most prominent effects of problems with calcium metabolism in our captive herps involves first the structural component of the skeleton, and second the ability to maintain cellular signaling.
Calcium in our bones and teeth (about 99% of the body’s calcium) is in flux with the calcium in our blood (the remaining 1%) (Wedekind et al. 2010). While we tend to think of our skeletons as mostly inert and unchanging after we’ve reached adulthood, bones are constantly being broken down and built back up again by a series of cells called osteoclasts and osteoblasts. Osteoblasts create new bone out of calcium, phosphorus, and other components, and once they’ve become trapped in their own mineral matrix they are called osteocytes. Osteocytes can transform to break down bone (now called osteoclasts) so that bone can be remodeled and those components can re-enter the bloodstream (Colville and Bassert 2002).
While this seems like a lot of detail to go into, I assure you understanding the skeleton is not a static, unchanging structure is an important concept.
Many cellular signals rely on calcium moving in and out of cells, usually involving proteins that act as pumps to move calcium across the cell membrane (Alberts et al. 2008). The most relevant cell signaling for our purposes involves muscle contractions, both voluntary (used for conscious movements) and involuntary (such as cardiac muscles responsible for maintaining blood flow). In short, calcium is stimulated to enter the involved cells by an electrical stimulus (called an action potential) from the central nervous system. This initiates a chain reaction in submicroscopic muscle fibre structures that results in muscle contraction (Colville and Bassert 2002). Without adequate levels of calcium in the blood, problems with muscle contraction occur (as well as with the uncountable other cell processes involving calcium).
As mentioned above, calcium moves back and forth between bones and the bloodstream, and these processes are very tightly controlled by multiple organs. First and foremost, the concentration of calcium in the blood is the major driver of all of the following processes. When calcium is used for bone building/remodeling, in signaling molecules (like neurotransmitters), and some cellular processes, this depletes some of the free calcium in the blood (Colville and Bassert 2002). When the ionized calcium concentration drops below a specific concentration, the parathyroid gland (located in the neck of all vertebrates except fish) reacts by secreting parathyroid hormone (Wedekind et al. 2010).
Hypercalcemia in Reptiles
Parathyroid hormone, which we’ll abbreviate as PTH, works several ways to correct hypocalcemia (see Figure 2 for a diagram version of the following):
- PTH induces osteoclastic activity in bone tissue by binding to osteoblasts and causes them to transform into osteoclasts. Osteoclasts initiate bone resorption, which release stored calcium (and phosphorus in the form of phosphate) into the bloodstream.
- PTH acts on the kidneys to recuperate calcium (as well as magnesium) that would have been excreted in the urine, and increases phosphate excretion into the urine. (Calcium and phosphorus have a close relationship, but we’ll explore that topic more, later.)
- PTH initiates another cell signaling process in the kidneys to help absorb calcium from the diet. This part involves the vitamin D3 metabolism process, which we’ll touch on soon. Essentially, the intestines receive a signal to let more calcium into the bloodstream from food in the animal’s gut. Note that there is always some passive absorption of dietary calcium, but it alone is not adequate to correct hypocalcemia (Wedekind et al. 2010).
If PTH starts this sequence but blood calcium cannot reach adequate levels, the parathyroid gland continues to be stimulated to produce PTH, resulting in a negative feedback loop that will not stop until the hypocalcemia has been corrected.
If and when blood calcium levels rise too high (hypercalcemia), the thyroid gland, found next to the parathyroid glands will release the hormone calcitonin. Calcitonin causes the opposite of PTH’s functions (refer again to Figure 2 to see this in diagram form):
- Calcitonin inhibits osteoclastic activity, slowing bone resorption. It also stimulates osteoblastic activity, causing calcium to be redeposited into bone tissue.
- Calcitonin inhibits calcium resorption in the kidneys, causing more calcium to be excreted in the animal’s urine. However, it copies PTH in that it also inhibits phosphate resorption, so that more phosphates are also excreted in the urine.
- Finally, it inhibits calcium absorption from food in the intestines (Colville and Bassert 2002).
Phosphorus and Calcium
Phosphorus and calcium are intricately linked in nutrition. Phosphorus, like calcium, makes up a large structural component of teeth and bones, so when osteoclasts initiate bone resorption there is release of phosphorus in addition to calcium into the bloodstream (Colville and Bassert 2002), and recall that PTH will stimulate the kidneys to excrete more phosphates to avoid phosphorus levels from climbing too high. Hyperphosphatemia (high blood levels of phosphorus) is dangerous. Chronic hyperphosphatemia leads to soft tissue mineralization, among other problems (Thrall et al. 2004).
These two nutrients like to stay within certain proportions of each other in the bloodstream; there exists a relationship between calcium and phosphorus concentrations, and the pH of the blood. When too much phosphorus is absorbed from the digestive system into the bloodstream, the calcium in the bloodstream declines in relation to the phosphorus as it’s “forced” to precipitate as calcium phosphate (Thrall et al. 2004). Not only are the total amounts of calcium and phosphorus in the diet important, but their proportions to each other is also critical when it comes to calcium balance. The calcium-to-phosphorus ratio is critical in diet assessment (Wedekind et al. 2010), and we’ll touch more on this topic later.
So, we’ve seen that the body runs a pretty tight ship when it comes to calcium and phosphorus. It has ways to regulate how much calcium is in the bloodstream through those two hormones, PTH and calcitonin. The main way the body replenishes used up calcium (and any nutrients) is through the diet. If an animal is provided with a calcium-deficient diet, then they will be unable to replenish that lost calcium. But, as noted above, there’s another key factor in calcium metabolism that we still need to explore: Vitamin D3.
D3: The “sunshine vitamin”
In response to low blood calcium levels, PTH signals though a series of reactions to the intestines to start absorbing more calcium from food in the animal’s gut. PTH signals to the kidneys to initiate the absorption process, but this is hardly where the story of vitamin D3 begins.
This process is simplified in some species. They eat another animal that already contains pre-formed vitamin D3 (also called cholecalciferol) that is transported to the liver and transformed into calcidiol (a prohormone). When the kidneys are stimulated by PTH, they start to convert calcidiol into calcitriol (a hormone), which is the active form that signals to the intestines to start absorbing dietary calcium (Colville and Bassert 2002). Note that all of these compounds have alternate names in the literature, as well as chemical names. I’ve included an appendix (Appendix 1) for those who are interested in case you happen to see these synonyms in the literature.
In most species, the process is a little more complicated. Many animals synthesize their own vitamin D3 in the skin in response to sunlight, which is why D3 is often called the sunshine vitamin. Previtamin D3, found in the skin, reacts to sunlight and forms D3, or cholecalciferol (Colville and Bassert 2002). The process then continues as above. Sometimes this process is complimentary to what’s absorbed from the diet, but in some species this is their only source of vitamin D3.
With captive herps, this is where many of our problems occur. We know from trial and error in herpetoculture that many species require an aspect of sunlight, ultraviolet-B radiation (UV-B), in order to make their own vitamin D3. Failure to provide UV-B lighting results in the animal not being able to use calcium in their diet, which leads to metabolic disturbances.
Calcium Absorption in Reptiles
Linking physiology to disease
The above processes are a lot to digest (no pun intended). Let’s break down some common scenarios that occur in captivity, and relate pathology to physiology:
- There is not enough available calcium in the diet. This occurs either when the animal’s diet simply contains insufficient amounts of calcium, or the calcium in the diet has a poor bioavailability.The first scenario is simple: Not enough calcium. This occurs when an animal is being fed primarily calcium-poor prey items or plants. Insectivores, for example, tend to suffer when their insect prey is not gut-loaded and/or dusted within calcium, as most insects have a naturally low calcium content (Donoghue 2006).
The second scenario involving calcium bioavailability can be quite variable, and to understand this we need to take a detour into some general food science. The bioavailability of a nutrient (be it calcium, a carbohydrate, zinc, a protein, etc.) is evaluated by how much of that nutrient is absorbed from a food and is transported unchanged into the bloodstream to be used by the body (LPI 2014). For example, calcium carbonate, the most common form of calcium in reptile and amphibians powdered supplements, is less bioavailable than calcium citrate, another form of calcium supplement (Hanzlik et al. 2005). This means that if the same amount of each supplement were ingested by two separate animals, the animal who ate the calcium citrate would absorb more useable calcium compared to the animal that ate the calcium carbonate. This of course does not mean that calcium carbonate is a poor choice for supplements, but this aspect of any nutrient is important when assessing diets.Furthermore, the presence of other foods in the diet can reduce how much calcium is being absorbed into the bloodstream. Calcium antinutrients are a loosely-defined group of naturally-occurring chemicals found in many common fruits and vegetables. They bind to calcium in the digestive tract, and stop it from being absorbed by the body. Common examples include oxalic acid and phytic acid (sometimes referred to as oxalates and phytates), which are chemicals found in many vegetables that are commonly eaten by both people and pet herbivorous or omnivorous reptiles. Both compounds will bind to dietary calcium and form compounds that will not be absorbed, but will be excreted instead (Charles 1992, Donoghue 2006, Guéguen and Pointillart 2000). A person’s diet tends to be quite varied, so these antinutrients will likely have a negligible effect. In a reptile that is fed a limited variety of foods, these antinutrients are encountered more commonly and can have a serious impact on how much calcium is absorbed from the diet.In either scenario, there is just not enough calcium being transported from the digestive tract through the bloodstream to the target organs. The body uses the calcium that is absorbed and bioavailable, but will run short when trying to reach its target blood concentration of calcium, and will resort to depleting more and more calcium from the bone “storehouse.” This eventually results in a notable loss of bone density.
- The dietary ratio of calcium to phosphorus (Ca:P) is incorrect. Reptiles and amphibians generally need a dietary Ca:P of 1.5-2.0:1.0 (one-and-a-half to twice as much calcium compared to phosphorus). Recall that when too much phosphorus relative to calcium is absorbed it will cause a drop in blood calcium, which initiates the sequence of PTH trying to bring the calcium concentration back to a normal level.
- The animal does not ingest or cannot make enough of its own vitamin D3. Recall that this vitamin is essential in the process of absorbing calcium from the diet to correct hypocalcemia. Many species of reptile and amphibian rely heavily on sunlight to produce their own vitamin D3 rather than obtain all of their needed D3 from their diet (Ferguson et al. 2003), and some species cannot absorb dietary D3 and completely rely on endogenous vitamin D3 production, including the Green Iguana, Iguana iguana (Allen and Oftedal 2003, Bernard et al. 1991).True dietary vitamin D3 deficiencies are rare. Most carnivores tend to be fed whole prey, which is (or was) a healthy animal in itself that was meeting its own D3 quotas. Carnivores that are only fed select portions of a whole animal, such as those being fed only muscle meat, are more susceptible to this kind of problem (Donoghue 2006).The most common scenario for vitamin D3 deficiency involves failing to provide broad spectrum lighting that includes UV-B radiation, or not using these devices appropriately.
Let’s revisit NSHP, or nutritional secondary hyperparathyroidism, and why this form of metabolic bone disease applies to most husbandry errors. We’ve seen above that the parathyroid gland is heavily involved in regulating blood calcium levels; hyperparathyroidism refers to excessive activity of this gland. This syndrome occurs secondary to nutritional factors, such as calcium, phosphorus, and vitamin D3 intake, which is why this disease is considered nutritional.
Clearly, metabolic bone diseases are anything but simple. Next, we will explore how these nutritional and husbandry problems develop into disease, how veterinarians will treat it, and what we can do to prevent it.
Appendix 1: Synonyms for forms of vitamin D3
|Compound||Synonyms (not an exhaustive list)|
|Vitamin D3||Cholecalciferol, calciol|
|Previtamin D3||Provitamin D3, 7-Dehydrocholesterol|
|Calcidiol||Calcifediol, hydroxycholecalciferol, 25-hydroxyvitamin D3|
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