Background Obesity is a growing problem in UK equine population. Achieving weight loss in obese horses and ponies at risk of laminitis is an important but often challenging objective. Methods We hypothesised that supplementing poor winter pasture with a mix of barley straw and hay (50:50) rather than hay alone (group B) would lead to weight loss in grazing equids over winter. For this purpose, a group of 40 horses were fed either the straw mix (group A) or hay alone (group B) over winter. Results Over the study period, all animals in group A (n=25) lost weight with a mean weight change of −27±17 kg, while in group B (n=15) only 3 out of 15 lost weight (20 per cent), and overall, group B gained weight (+6±18 kg). Conclusions This study suggests that straw is a cost-effective and low-energy roughage, which may be a useful alternative to hay alone when trying to induce weight loss in grazing equids over winter. There were no episodes of colic or laminitis during the study period in either group.

Background: Vitamin D deficiency is common in humans and is increasingly linked to the pathogenesis of a multitude of diseases including obesity and metabolic syndrome. The biology of vitamin D in horses is poorly described; the relative contribution of the diet and skin synthesis to circulating concentrations is unclear and associations with the endocrine disease have not been explored.Objectives: To determine the relationship between management, season and endocrine disease and vitamin D status in horses.Study design: Cross‐sectional cohort study.Methods: Plasma concentrations of 25‐hydroxyvitamin D₂ (25(OH)D₂) and D₃ (25(OH)D₃) were measured by liquid chromatography‐tandem mass spectrometry in 34 healthy unsupplemented grazing ponies and 22 stabled Thoroughbreds receiving supplementary vitamin D₃ in feed. A nested group of 18 grazing ponies were sampled on long and short days (>12 and <12 h of light/day) to determine the effect of sunlight exposure. In addition, the relationships between age, sex, adiposity, serum insulin, adrenocorticotropic hormone and vitamin D status were assessed in a mixed group of 107 horses using a linear regression model.Results: All animals had a measurable level of 25(OH)D₂ (median 10.7 nmol/L) whilst 25(OH)D₃ was only detected in Thoroughbreds receiving D₃ supplementation. Thoroughbreds had lower concentrations of 25(OH)D₂ than ponies (7.4 vs. 12.6 nmol/L, p < 0.01). In grazing ponies, 25(OH)D₂ concentrations were significantly higher on long days compared to short days (14.4 vs. 8.7 nmol/L, p < 0.01), whilst 25(OH)D₃ was undetectable. Measures of increased adiposity, but not basal insulin, were associated with higher 25(OH)D₂ concentrations, conversely to humans. Increasing ACTH was associated with lower 25(OH)D₂ (p < 0.01).Main limitations: Vitamin D₂ concentrations were not measured in grass or forage.Conclusions: In horses 25(OH)D₂ is the predominant vitamin D metabolite, and there is an apparent lack of endogenous vitamin D₃ production. The relationship between vitamin D and endocrine disorders in horses does not reflect that of other species and warrants further investigation.

Background: Equine grass sickness (EGS) is a frequently fatal multisystem neuropathy of equids. The aetiology is unknown; proposed causes include toxicoinfection with Clostridium botulinum and a mycotoxicosis. The effect of EGS on the organisation and structural integrity of the skeletal neuromuscular junction (NMJ), the target of botulinum neurotoxins (BoNTs), is unknown.Objectives: To compare the organisation and structural integrity of skeletal NMJs from EGS horses, control horses and one horse with a presumptive diagnosis of botulism.Study design: Blinded, retrospective case control.Methods: NMJs in samples of diaphragm or intercostal muscle from six EGS horses, three control horses and one equine botulism case were compared using electron microscopy, morphometry and confocal light microscopy.Results: A significantly higher percentage of EGS NMJs had abnormal morphology (EGS 72.2%, 95% CI 55.6–84.4; Controls 6.9%, 1.7–23.8; OR 35.1, 8.47–244.8; p < 0.001). EGS NMJs had a significantly lower mean volume fraction occupied by synaptic vesicles (SVs) (EGS 18.7%, 12.6–28.0; Controls 36.3%, 20.8–63.4; p = 0.024). EGS NMJs had evidence of accelerated SV exocytosis and SV depletion, accumulation of neurofilament‐like material in terminal boutons and/or bouton degeneration. NMJs from the botulism horse had dense packing of SVs towards the presynaptic membrane active zone, consistent with BoNT intoxication, but had absence of the abnormalities identified in EGS NMJs.Main limitations: Group sizes were limited by difficulties obtaining suitably processed samples. Ages of control and EGS horses differed. Botulism was diagnosed based on clinical and post mortem findings.Conclusions: EGS is associated with major changes in skeletal NMJ ultrastructure that are inconsistent with the effects of BoNTs. SV depletion may reflect increased exocytosis coupled with reduced repopulation of SVs via anterograde axonal transport and endocytosis, consistent with the action of an excitatory presynaptic toxin and/or neurotransmitter reuptake inhibitor. Skeletal NMJs represent a previously unrecognised target for the toxin that causes EGS.

Background: Hyperinsulinaemia (HI) is an important feature of Equine Metabolic Syndrome (EMS). It has been suggested that reduced hepatic clearance of insulin contributes to HI, particularly in humans affected by metabolic dysfunction-associated steatotic liver disease (MASLD). Hypothesis: In obese horses with HI, insulin clearance is impaired and associated with MASLD. Animals: Tissue samples were collected at post-mortem from clinically well-characterized horses with HI (n = 13; basal insulin > 20 mIU/l) and without HI (control; n = 20). Methods: Retrospective observational study. Molecular drivers of hepatic clearance (CAECAM-1, an insulin chaperone protein and IDE-Insulin Degrading Enzyme) were quantified by RT-qPCR and activity, respectively, in liver tissue. Fixed liver sections stained with hematoxylin and eosin (H&E) were assigned a histological score by two blinded observers using an equine liver disease score and a human MASLD score. Triglyceride (TG) content in liver sections, serum liver enzymes, ACTH, basal insulin, and serum triglycerides were also measured. Results: IDE activity was 2.73 (IQR 4.00 activity/mg of protein) in HI horses and 2.18 (IQR 0.55) in controls (p = 0.07). IDE activity correlated negatively with insulin (rho = 0.561, p = 0.04) but not with liver or serum TG. CEACAM-1 gene expression was higher in the HI group (2.09 ± 1.79 folds) than in controls (0.69 ± 0.62, p = 0.03). Liver disease and MASLD scores were no different between groups, whereas triglyceride liver content was higher in horses with HI (504.83 IQR 217.51 nmol/g) compared to controls (363.58 IQR 67.32 nmol/g, p = 0.04). Conclusions and Clinical Relevance: MASLD is not consistently present in HI horses, but CAECAM-1 expression is higher.

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.

Summary: Adipose tissue is not only required for energy storage but is an essential endocrine organ with a central role in the pathology of obesity. The understanding of its role, both in human and equine medicine, is continually evolving. With obesity being an ever‐growing problem in equine populations, gaining owner compliance is critical when implementing management plans. The aim of this review is to encourage the inclusion of the concept of adiposity in discussions with horse owners on obesity and metabolic syndrome. In doing this, we hope to improve clients' understanding and, therefore, maximise the impact of diagnostic tests, monitoring tools and management.