Materials and Methods

The project was performed in accordance with the regulations established by the Institutional Animal Care and Use Committee at Louisiana State University. Snakes—Twelve adult male corn snakes were used in the study. The snakes were obtained from a commercial herpetoculturist. The snakes, which were housed individually in 47.3-L plastic containers^a (26.7 × 15.8 cm × 10.8 cm), were acclimated to the laboratory for 7 days prior to the start of the study. The environmental temperature was maintained at 30.0° to 31.1°C (86° to 88°F). The snakes were not fed during the study period.

Procedures—After the initial 7-day acclimation period, a blood sample (0.5 mL) was collected from ventral tail vein or heart of each snake. This time point was designated as day 0. Blood samples were collected into tubes containing lithium heparin.^b The samples were centrifuged within 60 minutes of collection. Plasma samples were placed on frozen gel packs and submitted to a veterinary diagnostic laboratory^c for measurement of plasma 25-hydroxyvitamin D₃ concentration by use of a radioimmunoassay.

After the blood collection, snakes were allocated to 1 of 2 groups by use of random number generator. During the 4-week study period, corn snakes allocated to group 1 (n = 6) were not provided supplemental lighting, whereas those in group 2 (6) were provided supplemental lighting.^d This lighting was generated from 2 bulbs placed 25.4 cm (10 inches) apart at a distance of 15.8 cm (6.2 inches) above the basking area. Light was provided for 12 continuous hours each day.

For group 2, UVA and UVB radiations generated by the coil bulbs were measured by use of a radiometerphotometer.^e Levels of UVA and UVB radiation were measured on days 0, 7, 14, and 21. With the exception of day 0, when the reading was recorded immediately after the lights were activated, the measurements were made at the same time of the day in each successive week. Measurements were made from 5 points in each enclosure: at 2.54 cm (1 inch) from each bulb surface, at 15.8 cm (6.2 inches) directly beneath each bulb at the basking surface, and at the basking surface at a location below the midpoint between the 2 bulbs. The UVA and UVB radiation levels were measured in triplicate at each location by the same author (MKR). The arithmetic mean value of the 9 measurements obtained at the basking surface was used to determine the mean UV exposure in each enclosure. The arithmetic mean value of the 3 measurements obtained at each individual bulb surface was used for the statistical analysis.

The snakes were also weighed at the start and end of the study. Weight measurements were rounded to the nearest 0.1 g.

At the end of the 4-week study (day 28), another blood sample was collected for measurement of plasma 25-hydroxyvitamin D₃ concentrations. Sample collection, processing, and analysis were similar to the techniques used for the samples collected at the start of the study.

Statistical analysis—The data were evaluated by use of a Kolmogorov-Smirnov test and were normally distributed. The mean, SD, and range (minimum and maximum) are reported. A paired sample t test was used to determine whether plasma 25-hydroxyvitamin D₃ concentration and body weight changed in individual snakes during the study period. An unpaired t test was used to compare the mean differences for 25-hydroxyvitamin D₃ concentration and weight between treatment groups. A repeated-measures ANOVA was used to assess the quantity of UVA and UVB radiation generated at the bulb surfaces and the mean amount of radiation reaching the basking surface over the 4-week study. Commercially available software^f was used for all analyses, and a value of P ≤ 0.05 was considered significant.

Results

Between the start and end of the study period, the plasma 25-hydroxyvitamin D₃ concentration in group 2 increased significantly (63.0 ± 36.96 nmol/L and 196 ± 16.73 nmol/L, respectively; P = 0.003; Table 1). Group 1 snakes had no significant (P = 0.987) increase in 25-hydroxyvitamin D₃ concentration. At the end of the study (day 28), plasma concentration of 25-hydroxyvitamin D₃ in groups 1 and 2 differed significantly (P < 0.001).

Table 1—

Mean ± SD (range) plasma 25-hydroxyvitamin D₃ concentration in corn snakes that did not (group 1; n = 6) or did (group 2; 6) receive UV radiation via supplemental lighting at the start (day 0) and end (day 28) of a 4-week period.

View Table

Body weight in groups 1 and 2 did not differ significantly at the start (P = 0.991) or end of the study (P = 0.944). Therefore, weights were pooled and evaluated over time. At day 0, mean ± SD weight was 340.8 ± 19.90 g; at day 28, mean weight was 323.3 ± 18.68 g. This reduction in body weight was not significant (P = 0.528).

Although the amount of UVB at the bulb surface appeared to decrease over the course of the study, this reduction was not significant (P = 0.06; Table 2). There was a significant (P = 0.006) difference in the amount of UVA measured at the bulb surface throughout the study (Table 3). The amounts of UVA and UVB radiation reaching the basking surface were measured in triplicate directly under each of the 2 bulbs and also at the location below the midpoint between the 2 bulbs. These 9 values were averaged to calculate the mean UVA and UVB exposure for each snake. There was no change (P = 0.240) in UVB exposure during the course of the study. The mean UVA exposure did not differ (P = 0.686) throughout the study.

Table 2—

Amounts of UVB radiation (mean ± SD [range]) provided by 2 coil fluorescent bulbs in each of 6 enclosures housing a corn snake as measured on days 0, 7, 14, and 21. Assessments were made near the bulb surface* and at the basking surface.†

View Table

Table 3—

Amounts of UVA radiation (mean ± SD [range]) provided by 2 coil fluorescent bulbs in each of 6 enclosures housing a corn snake as measured on days 0, 7, 14, and 21. Assessments were made near the bulb surface* and at the basking surface.†

View Table

Discussion

The synthesis of vitamin D is the result of the photosynthetic conversion of 7-dehydrocholesterol to previtamin D₃ in the skin of vertebrates after exposure to UVB.^1,13 Previtamin D₃ is an unstable molecule that undergoes temperature-dependent isomerization to vitamin D₃.¹³ The newly formed vitamin is transported to the liver where it is hydroxylated to 25-hydroxyvitamin D₃.^3,11 This represents the storage form of the hormone, which is bound to protein and circulated systemically.¹¹ The kidneys are responsible for the final conversion of 25-hydroxyvitamin D₃ to 1,25-dihydroxyvitamin D₃, which is the active form of the hormone.^1,3,11

Vitamin D₃ can be obtained directly through the exposure of the skin to UVB radiation (wavelength, 290 to 320 nm) or via consumption of prey that has already performed the biosynthesis. The need for appropriate plasma concentrations of vitamin D₃ is so important that the skin of some nocturnal species of lizards, such as the Mediterranean house gecko (Hemidactylus turcicus), has developed the ability to synthesize the hormone under minimal light conditions, whereas other species modify their basking behaviors to compensate for variations in dietary amounts of vitamin D₃.^14,15 Not all animals have the same capacity to biosynthesize this hormone or extract it from their diet. Some carnivorous mammals, such as cats, are unable to biosynthesize any vitamin D and must rely totally on dietary intake⁷; by contrast, some lizards are reliant primarily on biosynthesis of the hormone.¹⁶ Because snakes are carnivores, the general assumption among herpetoculturists and veterinarians was that these animals also derived vitamin D from their diet and that UVB radiation–induced synthesis was likely of no importance. The findings of the present study suggest otherwise.

In the present study, plasma 25-hydroxyvitamin D₃ concentration significantly increased in snakes that were exposed to supplemental lighting during a 4-week period, whereas control snakes that were not exposed failed to develop any change in plasma 25-hydroxyvitamin D₃ concentration. There was no significant increase in the weight of snakes exposed to UV radiation or the weight of snakes in the control group.

The coil fluorescent bulbs selected for use in our study were recently included in the selection of reptile full-spectrum lights available through the pet trade. Historically, fluorescent tubes have been the most popular lights for reptile husbandry.^17,18 Results of previous research¹⁸ indicated that these bulbs provide adequate UVB radiation in the 290- to 320-nm wavelength range. In the experience of one of our group, the coil bulbs that we used provide a greater quantity of UVB radiation at the bulb surface and at distances of 6 and 12 inches from the surface than that provided by the more commonly used types of full-spectrum fluorescent tubes. It was for this reason that these bulbs were selected.

The UVB radiation generated by the bulbs used in our study did not decease over time. This is most likely because of the short duration of the study. There was a difference in the amount of UVA radiation released, although most of the difference was attributable to readings obtained from 2 bulbs during the third week of the study. It is possible that the measurements at those bulb surfaces were measured incorrectly. However, the same author (MKR) measured the UVA and UVB radiation levels throughout the study. This was done to minimize the likelihood of error. Interestingly, the amount of UVA radiation at the basking surface did not decrease during the study period.

Corn snakes have the potential for living for long periods in captivity and are popular as pets. It is therefore important to consider the effect of UV radiation on a snake's human caregivers. As newer, more powerful bulbs are developed, they may begin to pose health risks. To the authors' knowledge, this risk has yet to be evaluated.

The findings of the present study have provided important new information regarding the biosynthesis of vitamin D in corn snakes. For appropriate husbandry recommendations to be developed, further studies are required to determine the health and developmental consequences of provision or withholding of supplemental UVB radiation (wavelength, 290 to 320 nm) to these animals.