Vitamin D is a circulating hormone important to homeostasis and normal physiology, including bone development, growth, neuromuscular function, reproduction, cardiovascular health and immune function through stimulation of active intestinal absorption of calcium, promotion of calcium reabsorption by the renal tubules, regulation of parathyroid hormone levels and management of osteoclast activity in the bone (Holick, 2007).

The latest research confirms vitamin D levels are important to overall health in many species, including humans (Watson et al, 2014). Vertebrates can acquire this hormone through their diet or can generate endogenous vitamin D following exposure to ultraviolet B radiation (UVB).

The way these two mechanisms are employed to obtain vitamin D varies greatly among different animal species. Domestic dogs and cats rely entirely on dietary intake of vitamin D, whereas llamas and alpacas obtain vitamin D principally by dermal photoconversion of 7-dehydrocholesterol following exposure to UVB.

Other species, including red-eared sliders (Trachemys scripta elegans), corn snakes (Pantherophis guttata), and Hermann’s tortoises (Testudo hermanni), employ a combination of both methods (Rivas et al, 2014).

The importance of exposure to UVB radiation relative to this function in many mammalian species is unknown. Therefore, at this time, there are often no specific lighting recommendations for many small mammal exotic companion species other than providing a photoperiod of 12 hours to mimic their natural diurnal cycle (Watson et al, 2014).

However, latest research seems to shed some light on this matter and may provide useful information that may be employed in clinical practice in the near future.

Research analysis

Guinea pigs

Watson et al (2014) explored the potential benefit artificial UVB radiation exposure may have on guinea pigs and evaluated the long-term safety of such supplementation.

Guinea pigs are diurnal animals evolved in a high altitude environment and would be expected to have increased exposure to UVB radiation compared to animals living at lower altitudes.

Findings from the present study demonstrated guinea pigs provided with artificial supplementation of UVB light have the ability to generate significant quantities of vitamin D, despite adequate levels being provided in the diet, compared to those without UVB supplementation. However, reference ranges for optimum serum 25-hydroxyvitamin D₃ (25-OHD₃) in wild or captive guinea pigs have yet to be established.

When comparing the vitamin D levels of the guinea pigs in this study to those of rabbits used in a similar study (Emerson et al, 2014), the average 25-OHD₃ level in guinea pigs was higher, despite similar exposure. This could suggest these species may have a higher vitamin D requirement in relation to their natural habitat being at a higher altitude and lower latitude.

The dietary vitamin D requirements for guinea pigs are also unknown, but it is suggested to be 1,000IU/kg diet (nutrient requirements of lab animals). Signs of hypervitaminosis D secondary to increased levels of vitamin D in the diet have been reported in guinea pigs (Jensen et al, 2013).

There have been no reports of hypervitaminosis D secondary to photoconversion of 7-dehydrocholesterol (7-DHC). This is likely because photobiochemical conversion mechanisms are tightly regulated within the body.

In humans, these mechanisms have been elucidated and it has been shown sunlight actually destroys excess previtamin D₃ or vitamin D₃ when exposure is prolonged or excessive.

Since other pathways involving photoconversion in guinea pigs are similar to humans, it is highly likely these control mechanisms exist in guinea pigs as well. However, specific investigation in guinea pigs is warranted and no recommendations can be given clinically based on these current studies.

Chinchillas

Another pilot study also demonstrated that chinchillas (Chinchilla lanigera), which are native to the Andes mountains, have the ability to produce 25-OHD₃ through photobiochemical synthesis following exposure to artificial UVB lights (Rivas et al, 2014).

Most captive pet chinchillas are housed indoors with little opportunity for exposure to natural UVB.

There are no recommendations for UVB supplementation for captive chinchillas and studies on commercial diets manufactured for this species are limited, with no vitamin D research findings published. Unfortunately, there are no established reference values for serum 25-OHD₃ in chinchillas.

Furthermore, chinchillas are nocturnal animals, so the significance of the dermal biosynthesis of vitamin D may be less compared to diurnal animals.

However, because chinchillas have retained the dermal biosynthetic pathway for vitamin D, as this study demonstrated, it is suggested they may have a UVB exposure requirement for maintenance of normal physiologic processes.

The study suggests the possibility that furnishing artificial light sources to produce UVB for chinchillas housed entirely indoors could have health benefits and dietary vitamin D should not be considered a suitable replacement for endogenous sources in animals that have evolved reliance on photochemical synthesis.

Rabbits

Another pet mammalian herbivore that has been found to increase serum vitamin D level in response to UVB exposure is the rabbit (Oryctolagus cuniculus).

Rabbits have a unique calcium metabolism that is largely independent of vitamin D as they can passively absorb calcium from the intestine maintaining a serum concentration, which is 30% to 50% higher than that of other mammals (Eckermann-Ross, 2008).

This efficient mechanism is thought to be related to the increased calcium demand required to sustain the continuous growth of their teeth. Hypovitaminosis has been suggested as a key factor for the development of nutritional osteodystrophy and acquired dental disease in this species (Harcourt-Brown and Baker, 2001).

In a study, Emerson et al (2014) found UVB radiation emitted by artificial lights resulted in a significant increase in serum 25-OHD₃ concentrations in domestic rabbits. Again, the significance of such results is difficult to gauge, as there are no reference intervals for 25-OHD₃ in this species. Therefore, it is difficult to gather whether the untreated control animals were actually to be considered vitamin D deficient.

However, vitamin D concentrations can also increase after dietary supplementation, leading to the conclusion that this species may rely on both mechanisms (photobiosynthesis and dietary intake) for provision of this essential hormone.

Significance of the findings

Vertebrates with hypovitaminosis D are susceptible to diseases that can affect the musculoskeletal, cardiovascular and immune systems.

Chronic vitamin D deficiency during growth and adulthood has been found to contribute to dental disease in vertebrates.

As with pet rabbits and guinea pigs, dental disease is a very common disease presentation of chinchillas to veterinary clinics. Guinea pigs, rabbits and chinchillas all possess continuously growing incisor and cheek teeth to compensate for constant wear, yet captive conditions appear to predispose to tooth overgrowth.

Hypovitaminosis has been suggested as a key factor for the development of nutritional osteodystrophy and acquired dental disease in rabbits (Harcourt-Brown and Baker, 2001) and previous studies have also indicated chinchillas with dental disease have significant differences from healthy chinchillas in plasma calcium and phosphorous levels (Muszczynski et al, 2010).

Calcium and phosphorus are key elements in the formation of bones and teeth and vitamin D is responsible for the metabolism of these minerals by regulating serum levels. It is well known that chronic vitamin D deficiency will result in abnormally low levels of serum calcium and phosphorous, and subsequent poor mineralisation of the skeleton.

Acquired dental disease is a common finding in captive, indoor rabbits and chinchillas, and it is possible chronic hypovitaminosis D may play a role in nutritional osteodystrophy of the skull.

Additionally, cardiovascular disease is not uncommon in chinchillas (Pignon et al, 2012) and the role of vitamin D in heart health of this species should be further investigated.

Risks of UVB exposure

While the benefits of UVB exposure for captive animals appear to be strong, it is not without risk. UVB radiation has been associated with an increased risk of developing skin neoplasia, potential structural damage to the cornea (keratitis, corneal oedema), lens (cataracts) or retina (blindness), and short-term damage such as photodermatitis and erythema (Gallagher and Lee, 2006). For these reasons, the safety of UVB supplementation in small mammals should be further investigated.

If UVB were to be supplemented in captive small mammals housed indoors, it is important to remember acrylic or glass windows are an effective barrier to UVB radiation (Tuchinda et al, 2006) and future studies would be required to determine the minimum effective UVB levels to promote photoconversion and lessen the likelihood of overexposure.