The Animal Health Trust are launching a nationwide trial of a vaccine for Equine Grass Sickness, a debilitating disease which is often fatal.

The vaccine trial will follow 1,100 horses and ponies for two years. In order to be eligible for enrolment the horses and ponies need to be passported, healthy and being kept on premises with a history of EGS cases in the past two years.

Dr Jo Ireland, the AHT’s Equine Grass Sickness Research Co-ordinator, said:

“Half of the horses and ponies on the trial will be vaccinated with a C. botulinum type C toxoid vaccine; the other half of the horses and ponies will receive an inactive placebo injection. We will record, review and compare incidence of EGS between the two groups to determine the efficacy of the vaccine.

“If we see reduced disease incidence in vaccinated horses, this would provide a major breakthrough in the prevention of EGS.”

Almost all cases of EGS occur in horses with access to grazing and it is thought they are exposed to some form of noxious agent in the soil which contaminates the grass they eat.

There is growing scientific evidence to suggest that EGS may be caused by the bacterium Clostridium botulinum (C. botulinum) type C, which is found commonly within soil and is capable of producing a range of toxins, including neurotoxins (toxins that damage the nervous system), to which horses are particularly sensitive.

 As vaccinating horses protects against other similar diseases, such as tetanus and botulism, theoretically it is possible that EGS could be prevented by vaccination. A field vaccine trial is the only way to evaluate whether a vaccine is effective in reducing the risk of EGS.

To find out more about the vaccine trial, or how to enrol horses/ponies, please visit or email