Читать книгу Blackwell's Five-Minute Veterinary Consult: Reptile and Amphibian - Javier G. Nevarez - Страница 60

Ultraviolet light (UV) is electromagnetic radiation that exists in three main bands: UVA, UVB, and UVC. Each band is characterized by a specific wavelength. UVA has the longest wavelength at 320–400 nm while UVB has a wavelength of 290–320 nm. Both UVA and UVB have low tissue penetration with most being absorbed at the epidermis. UVA has a wavelength of less than 290 nm, the deepest penetration, and is the most harmful. These wavelengths place ultraviolet somewhere between visible light (369–769 nm) and x‐rays (0.01–10 nm). The ozone layer is responsible for filtering the majority of ultraviolet radiation hence protecting living organisms from its harmful exposure. UVC is considered germicidal and can have significant harmful effects to living animals while both UVA and UVB have therapeutic and physiologic applications. Still, as we know from human medicine, too much sun/ultraviolet exposure, has a slew of negative implications ranging from sunburn to premature aging, and cancer.

UVB light is essential for the synthesis of vitamin D3. Although commonly referred to as a vitamin, D3 is actually a hormone. UVB light from the sun stimulates conversion of 7‐dehydrocholesterol to cholecalciferol (D3) in the skin. Cholecalciferol is then transported to the liver via the blood stream and hydroxylated into 25‐hydroxycholecalciferol (calcidiol), which is then transported to the kidneys for hydroxylation into 1,25‐dihydroxycholecalciferol (calcitriol), the active form of D3. The 25‐hydroxycholecalciferol in the liver is the storage form of D3 and is a natural source of D3 for animals fed whole prey. Plants are a source of ergocalciferol or ergosterol (D2), which is not readily bioavailable to most animals and is therefore not an appropriate source of D3 supplementation. Calcidiol, an animal source of D3, is more bioavailable for most animal species, and is suspected to be the same for amphibians.

There are two main sources of UVB, natural sunlight and UVB bulbs. Exposure to natural sunlight is likely the most efficient way for amphibians to synthesize adequate levels of D3. However, there is a high degree of variability in the lifestyle of amphibians from cryptic species that barely get sunlight exposure to those that may display basking behaviors similar to reptiles. This variability makes it nearly impossible to provide blanket recommendations for UVB supplementation for amphibians. Instead, we must rely on knowledge of the natural behaviors of the species, their geographical location, and expected UVB exposures at that location to make extrapolations about the use of UVB light supplementation in captivity. One thing that most can agree on is the fact that amphibians should be exposed to lower levels of UVB than reptiles.