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The Science of Reptile Husbandry: UVB: An Essential Environmental Parameter

Amongst the reptile-keeping community, keepers are largely in agreement that diurnal and basking reptiles or heliothermic reptiles, should be given access to high quality UVB exposure, for example our Ackie monitors (pages 20 – 22) who have a 12% desert UVB tube, where it is deemed to be neglectful and keepers would be shunned for not providing UVB for this species. However, whether predominantly nocturnal or fossorial animals, those which are active at night or live underground so are shaded from UVB or amphibians whose skin should remain moist and not desiccate in bright lights, require UVB remains a subject of debate. In this issue, we shall explore what UVB is and why it is an important parameter to provide in your reptile husbandry. In the next issue, we shall continue this topic and explore not only which animals require UVB but the duration and intensity of their UVB requirements based on scientifically published research investigating their natural ecology.


Above: Salvador, the Reptile Academy's resident ambassador animal for Giant Black and White Argentine Tegus, Salvator merianae, who joined us in 2017 from the RSPCA. Salvador developed severe MBD leading to fractures in both front legs. Through veterinary intervention and post-treatment husbandry and care, Salvador has completely recovered and exhibits very strong bone density, living a very fulfilling life.


What actually is UVB?


The science versus the simple. Ultra-violet-B (UVB) is the wavelength of light in the non-visible region of the electromagnetic spectrum. Whilst the UV region measures 100-400nm, the UVA region represents part of this at 315- 400nm and UVB represents 280-315nm corresponding to a high photon energy level of 3.94–4.43 eV (electron volts).[1] In simple terms, this is a type of light energy that we cannot see and is very high in energy. This energy is powerful enough to chemically interact with molecules and cause reactions that normal visible light, that we see every day from the sun, cannot.


Why do reptiles and amphibians need UVB? – An Essential Reaction.


UVB is essential for the biological process of vitamin D3 metabolism in the skin. As mentioned above, UVB is high energy light and exposure catalyses the reaction of pro-vitamin D3 into pre-vitamin D3, which occurs at 290-315nm, just within the UVB range. Heat then converts or isomerases pre-vitamin D3 into synthesised vitamin D3.


Below: The electromagnetic spectrum showing the division of different wavelengths of light from the sun. To the left we see UV-B at approximately 280-315nm. Invisible to the human eye, UV-B can be detected my many reptile and amphibian species and is essential in calcium metabolism.

Above: Deficits in UV-B exposure coupled with lack of dietary calcium and other malnutrition issues can cause MBD. The body of the animal tries to find emergency calcium (for digestion, the heart, muscles, neurological activity etc) and depletes the bones to achieve this. Through perpetual poor husbandry, the bones become very weak which can cause fractures.


Vitamin D3 is then transported by vitamin D binding protein from the skin to the liver via the bloodstream. In the liver, it is converted to a hormone called calcitriol.[2][3] Calcitriol is a hugely important hormone, so remember this for later. It controls the transcription (expression) of as many as 2000 genes that influence functions such as growth, insulin production and the immune system[4] as well as maintaining a healthy cardiovascular system. One of the most important functions as we understand, however, is calcium metabolism.


A lack of calcitriol due to lack of UVB prevents efficient uptake of calcium from the digestive tract. This can cause a calcium crash, hypocalcaemia, in which the above functions, as well as the skeletal system is highly at risk. The most prevalent disease encountered in animals with an impaired calcium metabolism due to UVB exposure and vitamin D deficiency is metabolic bone disease (pictured above) which can be fatal and, in most cases, requires expensive veterinary treatment such as surgery.


Metabolic Bone Disease & Calcium Resorption: A Natural Homeostatic Mechanism Out of Control


The prevention of Metabolic Bone Disease (MBD) is the main reason your reptile or amphibian requires artificial UVB in captivity. MBD causes osteoporosis or weakening of the bones as the body begins to digest structural calcium from the bones to provide free calcium for healthy organ function, rather than from the diet. Eventually the bones become so degraded that animals develop skeletal deformities, fractures and eventually terminal organ failure. Many animals do not show visible signs of MBD until the disease has significantly developed. If you think your animal may have MBD, always seek expertise from an experienced exotics veterinarian where an X-Ray can be used to diagnose MBD. See Salvador’s X-Rays showing MBD on pages 18-19 of Volume 1, Issue 1.


Calcium resorption is a natural, seasonal undersupply process that goes into overdrive under these poor husbandry conditions. In the wild, winter months are associated with a lack of sunlight (limiting UVB exposure and thermal basking) and food (dietary calcium) – impairing D3 synthesis and calcium metabolism. Many reptiles and amphibians hibernate or brumate to conserve energy, but this results in almost no UVB exposure or food intake for several months leading to hypocalcaemia. To maintain muscular, organ and immune function, a process called osteoclast mediated bone resorption releases calcium into the blood from the bones as an emergency supply. [5] This process is induced by the parathyroid hormone, PTH and is inhibited by the ever-important calcitriol. PTH also acts on the kidneys to recuperate calcium, which is why kidney failure is another sign of severe MBD. After the hibernation period in spring to autumn where food is more abundant and there is increased UVB exposure and daytime temperatures, UVB-catalysed and thermal-dependent metabolism of vitamin D3 leads to an increase in calcitriol and uptake of calcium from the gut. This calcitriol impairs osteoclast-mediated bone resorption driven by the parathyroid gland, halting the degradation of bone for calcium.


In captivity, perpetual undernourishment and underexposure to UVB, leading to hypercalcemia, results in unsustainable exploitation of the bone resorption homeostatic mechanism, resulting in MBD, the only disease present in captive reptiles for which there is no evidence of its existence in wild animals. Therefore, contrary to popular belief that some reptiles and amphibians do not require UVB, we encourage the use of UVB for all reptiles and amphibians, replicating natural environmental parameters. However, we acknowledge that the individual needs of each species will differ and many factors are involved (intensity, duration, lifecycle stage, coverage and hides) and in the next issue we will explore these differences, based on published research investigating the natural ecology of wild reptiles and amphibians as well as the other advantages of providing UVB such as the bacteria-killing effect. [6] and general physiological health.[7]



References

References UVB: An Essential Environmental Parameter 1. Lamb, S., 2020. [online] Exoticanimalveterinarycenter.com. Available at: [Accessed 16 May 2020]. 2. MacLaughlin, J. A., Anderson, R. R., Holick, M. F. 1982. Spectral character of sunlight modulates photosynthesis of previtamin D3 and its photoisomers in human skin. Science 216:1001- 1003 3. Webb, A. R., DeCosta, B. R., Holick, M. F. (1989) Sunlight regulates the cutaneous production of vitamin D3 by causing its photodegradation. J. Clin. Endrocronol. Metab. 86:882-887 4. Hossein-nezhad, A., & Holick, M. F. (2013). Vitamin D for health: a global perspective. Mayo Clinic proceedings, 88(7), 720–755. 5. Wedekind, KJ, L Kats, S Yu, I Paetau-Robinson, CS Cowell. 2010. Micronutrients: Minerals and Vitamins. In: Small Animal Clinical Nutrition (5th edition). Hand, MS, CD Thatcher, RL Remillard, P Roudebush, BJ Novotny (editors). Mark Morris Institute, Topeka, Kansas, USA. 6. Argyraki, A., Markvart, M., Stavnsbjerg, C. et al. UV light assisted antibiotics for eradication of in vitro biofilms. Sci Rep 8, 16360 (2018). 7. Colville, T and JM Bassert. 2002. Clinical Anatomy & Physiology for Veterinary Technicians. Mosby, Inc., St. Louis, Missouri, USA.

Written by Jake Weeks


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