Clostridial neurotoxins not the cause of equine grass sickness, research suggests

Clostridium botulinum stained with gentian violet. Photo: Centers for Disease Control and Prevention via Wikimedia Commons
Clostridium botulinum stained with gentian violet. Photo: Centers for Disease Control and Prevention via Wikimedia Commons

British researchers have poured cold water on the suggestion that equine grass sickness is caused by neurotoxins released by clostridial bacteria.

Their findings are reported in the Equine Veterinary Journal.

The cause of equine grass sickness remains unknown, although there is some evidence suggesting it may be linked to the toxins released by Clostridium botulinum types C and/or D. Neurotoxins released by these bacteria can cause botulism, which is often deadly in horses.

The University of Edinburgh’s Professor Bruce McGorum and his colleagues investigated several avenues to determine whether there was a link between the effects of these neurotoxins on horses and equine grass sickness.

They set out, firstly, to determine whether botulism causes problems with the autonomic nervous system – that’s the branch of the nervous system that influences the function of internal organs – as well as signs of deterioration in the part of the nervous system related to the gut. This is important, because while equine grass sickness is known to cause both, this had not been definitively assessed for equine botulism.

The study team also investigated how equine grass sickness affected what are known as SNARE proteins within the autonomic nervous system. SNARE proteins help in the operation of cell membranes and are commonly affected by botulism toxins.

The researchers studied stained slides of the cranial cervical ganglion and part of the small intestine, the ileum, from five botulism-affected horses, six horses with equine grass sickness, and six control horses. They used immunohistochemistry techniques to assess concentrations of SNARE proteins.

They found that equine grass sickness, but not botulism, was associated with autonomic and gut-related neurodegeneration and with increased immunoreactivity for SNARE proteins within nerve cells. This was not the case in the botulism-affected horses.

This, they said, suggested that equine grass sickness may not be caused by neurotoxins released by Clostridial bacteria, although the results do not preclude involvement of some of the other non-neurotoxic toxins produced by C. Botulinum.

Further investigation into the causes of equine grass sickness were therefore warranted, they concluded.

The researchers are variously affiliated with Royal (Dick) School of Veterinary Studies and The Roslin Institute, at the University of Edinburgh; Newcastle University; SAC Consulting Veterinary Services in Midlothian; and the Central Veterinary Research Laboratory, in Dubai.

McGorum, who heads the Equine Section at the Royal (Dick) School of Veterinary Studies and the Roslin Institute in Edinburgh, has been leading much of the research into equine grass sickness.

He says researchers are now moving on to determine whether the disease is caused by ingesting mycotoxins produced by pasture fungi.

The study is among four into equine grass sickness that were made freely available by the Equine Veterinary Journal this month.

McGorum, B. C., Scholes, S., Milne, E. M., Eaton, S. L., Wishart, T. M., Poxton, I. R., Moss, S., Wernery, U., Davey, T., Harris, J. B. and Pirie, R. S. (2016), Equine grass sickness, but not botulism, causes autonomic and enteric neurodegeneration and increases soluble N-ethylmaleimide-sensitive factor attachment receptor protein expression within neuronal perikarya. Equine Vet J, 48: 786–791. doi:10.1111/evj.12543

The study can be read here

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