Equine grass sickness is associated with major abnormalities in the ultrastructure of skeletal neuromuscular junctions

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.

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.