The effect of season, management and endocrinopathies on vitamin D status in horses

Serum 25(OH) Vitamin D Concentrations in Horses: Effects of Age, Gender, Breed, Skin Colour and Season

BACKGROUND

The optimal amount of vitamin D required for the proper functioning of the immune system differs from the amount necessary for bone homeostasis. Furthermore, vitamin D metabolism varies among horses. Nevertheless, there is a dearth of information regarding reference values for vitamin D in horses, particularly in the Turkmen breed. The primary objectives of this study were to determine the concentrations of 25-hydroxyvitamin D (25(OH)Vit D) in Turkmen, Thoroughbred and mixed Turkmen × Thoroughbred horses and to explore the effects of various factors on it and its relationship with serum biochemical variables and signalment data.

METHODS

For the measurement of 25(OH)Vit D, 90 healthy horses (min age: 6 months; max age: 10 years) that were stable in the north of Iran (37°17' N, 55°18' E) were selected for clinical examination and complete blood biochemistry analysis. The horses were categorised into different groups based on sex, season (spring, n = 45; autumn, n = 45), breed (Turkmen, Thoroughbred and mixed Turkmen × Thoroughbred), age (< 2 years, 2-8 years and > 8 years) and skin colour. Sampling was performed only once from each horse.

RESULTS

In this study, the median serum concentration of 25(OH)Vit D in the sampled horses was 17.42 ng/mL (IQR: 9.82-30.85). The serum 25(OH)Vit D concentration was significantly lower in autumn (median: 15.83 ng/mL and IQR: 8.67-54.68) than in spring (median: 18.02 ng/mL and IQR: 13.77-27.54) and was also lower in Turkmen horses (median: 15.83 ng/mL and IQR: 11.63-23.12) than in mixed breed horses (median: 17.72 ng/mL and IQR: 8.94-51.67) (p ≤ 0.05).

CONCLUSION

According to the results of the present study, the season had a significant effect on the serum 25(OH)Vit D concentration, and this difference was also observed between Turkmen horses and mixed-bred horses (p ≤ 0.05). It is unclear whether the time of sun exposure or vitamin D content of diet causes the seasonal difference of 25(OH)Vit D concentration.

Serum Vitamin D Level Is Unchanged in Equine Asthma

Vitamin D deficiency is associated with asthma development and severity of symptoms in humans, but whether the same occurs in horses is unknown. We aimed to determine whether the serum vitamin D levels differ in horses with asthma compared to control animals and, secondarily, to explore clinical, respiratory, and environmental parameters associated with its concentration in equids in a retrospective cross-sectional study. The total serum vitamin D (25(OH)D) was measured by radioimmunoassay in 45 serum samples from the Equine Respiratory Tissue Biobank (15 control animals, 14 horses with mild or moderate asthma (MEA), and 16 horses with severe asthma (SEA)). Descriptive clinical and environmental parameters, bronchoalveolar lavage fluid cytology, and lung function data were extracted. There was no difference in serum 25(OH)D levels between healthy controls, horses with MEA, and horses with SEA (respectively, means of 57.9 ± 11.6, 55.6 ± 20.0, and 64.6 ± 14.5 nmol/L; p = 0.3), suggesting that this micronutrient does not play a major role in equine asthma pathophysiology.

Serum 25(OH)D Analysis in Captive Pachyderms (Loxodonta africana, Elephas maximus, Diceros bicornis, Rhinoceros unicornis, Tapirus indicus) in Europe

This study aimed to detect seasonal and species differences in serum 25-hydroxy vitamin D (25(OH)D) concentrations during summer and winter months in captive pachyderms in Europe. Both elephant species had low 25(OH)D while African elephants did not show a seasonal variation. Asian elephants had significantly higher 25(OH)D compared to their African counterparts but also did not show a seasonal difference. Both rhinoceros species investigated had higher 25(OH)D compared to both elephant species; the Indian rhinoceros had high circulating levels year-round, while the black rhinoceroses showed significantly lower 25(OH)D in winter. Malayan tapirs have very low 25(OH)D, comparable to horses. The higher 25(OH)D of elephants and rhinoceroses could indicate that elephants and rhinoceroses are capable of producing vitamin D. This might indicate that the Indian rhinoceroses are capable of producing enough endogenous vitamin D year-round at latitudes around 52° N, while both elephant species and the black rhinoceros are not. This study also showed that it is likely that both elephant species and rhinoceros species are capable of absorbing cholecalciferol from the digestive tract, according to the existing literature, while tapirs may not.

Short communication: Metabolic synthesis of vitamin D2 by the gut microbiome

The origin of vitamin D2 in herbivorous animals was investigated in vivo in sheep and in bovine as well as mouse gastrointestinal tracts. A high concentration of 25-hydroxyvitamin D2 in blood plasma of sheep both in summer and winter appeared to be incompatible with the undetectable level of vitamin D2 in the pasture on which the sheep were grazing. Studies with bovine rumen contents from a cow grazing the same pasture as the sheep, demonstrated an increased concentration of vitamin D2 on anaerobic incubation in a 'Rusitec' artificial rumen, which was further enhanced when cellulose powder was added as a fermentation substrate. The colon contents of mice that were fed from weaning on a vitamin D-free diet were found to contain vitamin D2. The results of these comparative studies in 3 animal species indicated that vitamin D2 was being generated by microbial anaerobic metabolism in the gastrointestinal tract.

Metabolic and Endocrine Insights in Donkeys

Donkey medicine is gaining attention due to their increased use as companion animals, in shows, asinotherapy, etc. The increasing demand and unique aspects call for specialized care, requiring new information (physiology, infectious disorders, pharmacology, etc.). Since obesity is common in this species, hyperlipemia, metabolic syndrome and insulin dysregulation (ID) are common disorders in donkeys, in some cases with high mortality, either directly (multiorgan dysfunction) or indirectly due to poor quality of life (chronic laminitis). Donkeys have long-life expectancy and are often afflicted with pituitary pars intermedia dysfunction (PPID), a neurodegenerative and endocrine disease. Hyperlipemia is diagnosed based on high plasma triglyceride concentration in association with clinical findings and laboratory abnormalities from affected tissues (liver, kidney and pancreas). The measurement of resting serum insulin and plasma ACTH concentrations is the first step in ID and PPID diagnosis. In donkeys with clinical signs of ID (obesity or recurrent laminitis) or PPID (hypertrichosis, regional adiposity, laminitis and weight loss), where these hormones are in the normal or non-diagnostic range (donkey-specific cut-off values and reference ranges need to be established), dynamic tests are recommended (oral sugar test or thyrotropin-releasing hormone, respectively). Equine treatment protocols apply to donkeys, although pharmacological studies for most drugs, except pergolide, are lacking.

Thoroughbred Racehorses in Hong Kong Require Vitamin D Supplementation to Mitigate the Risk of Low Vitamin D Status

There is a paucity of data relating to the vitamin D status of racehorses. We hypothesised that the management of racehorses in Hong Kong (HK) predisposes to low vitamin D status unless they receive dietary supplementation. Serum concentrations of 25-hydroxyvitamin D₂ (25OHD₂), 25-hydroxyvitamin D₃ (25OHD₃) and total 25-hydroxyvitamin D (total 25OHD) for 79 non-grazing HK racehorses were compared with those for 22 racehorses training in the United Kingdom (UK) that grazed for ≥1 h/d, and for which published data exists. A nested group of 41 HK horses was sampled twice to determine the effect of the duration in HK on vitamin D status. The HK horses had significantly lower serum concentrations of total 25OHD and 25OHD₂ than the UK horses; 25OHD₂ was undetectable in 15/79 HK sera and serum concentrations of 25OHD₂ declined with the duration in HK. The main determinants of vitamin D status were assessed using linear regression; the retained variables were the 25OHD₃ concentration and the duration in HK. The inverse relationship between the serum concentrations of 25OHD₂ and 25OHD₃, previously identified in humans, was observed for the first time in horses. In conclusion, HK racehorses have low serum 25OHD₂ and total 25OHD concentrations and rely on D₃ supplementation to maintain adequate vitamin D status. Further study is required to determine the optimal form of dietary vitamin D supplementation for Thoroughbred racehorses.