Low 25-hydroxyvitamin D concentrations in wild rabbits (Oryctolagus cuniculus) in southern Finland

Diet and endogenous vitamin D synthesis are possible sources of vitamin D in wild rabbits. Higher 25-hydroxyvitamin D (25(OH)D) concentrations have been reported in rabbits after artificial UVB light exposure than in rabbits without this exposure, suggesting that endogenous vitamin D synthesis occurs in the former group. In Finnish pet rabbits, diet was reported as main source of vitamin D, while outdoor access was not. Finland's northern location only enables endogenous synthesis from mid-March to mid-October in people with light skin type. Living conditions during winter are challenging for Finnish wild rabbits. This study aimed to measure serum 25(OH)D concentrations and possible natural seasonal variation of vitamin D concentrations in Finnish wild rabbits.

Post-mortem blood samples (n = 78) were collected between 2013 and 2021 from wild rabbits hunted for reduction of the wild rabbit population. Separated sera were stored at - 80 °C until 25(OH)D concentrations were measured by enzyme immunoassay. Data regarding sex were available from 50 rabbits, 29 (58%) of which were females. Mean 25(OH)D concentration was 3.3 (range 0.3-7.1) ng/ml. 25(OH)D concentration was statistically similar between season (autumn, winter, summer), month or year of sample collection, and sex.

Wild rabbits living in Finland have very low serum 25(OH)D concentrations. This is far below the previously suggested threshold of vitamin D deficiency in rabbits (17 ng/mL) or the mean 25(OH)D concentration reported in Finnish pet rabbits (26.0 ng/mL). Seasonal variation was not observed in 25(OH)D concentrations between winter and summer months. Even though rabbits are crepuscular animals and may spend the mid-day in underground burrows, the very low observed 25(OH)D concentrations raise doubt about whether vitamin D synthesis occurs efficiently in the skin of rabbits and whether the diet of wild rabbits provides adequate amounts of vitamin D. Cutaneous vitamin D synthesis, possible long-term consequences of low 25(OH)D concentrations, and the association of low vitamin D status with other health disorders warrant further investigations in rabbits.