Recruitment for historic EGS vaccine trial begins


Animal Health Trust launches a vaccine trial for Equine Grass Sickness

Leading veterinary charity, the Animal Health Trust (AHT) is launching a ground-breaking nationwide trial of a vaccine for the prevention of a frequently fatal disease affecting horses; Equine Grass Sickness (EGS).

Following the successful completion of a small-scale pilot study in 2013, and with generous funding from a number of sources including Neogen Corporation, Animal Welfare Foundation; Horserace Betting Levy Board; Racing Foundation; Moredun Foundation Equine Grass Sickness Fund; Hong Kong Jockey Club; Dodson and Horrell; South Essex Insurance Brokers; British Horse Society and EB Moller Charitable Trust, the AHT is now in a position to start recruiting horses to be enrolled in the trial. The trial will be conducted by the AHT in collaboration with the Universities of Edinburgh, Liverpool and Surrey.

EGS is a debilitating and often fatal disease affecting horses, ponies and donkeys. EGS occurs predominantly in northern Europe and Britain has the highest incidence worldwide.  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 ingested as a contaminant of grass.

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.

The current theory is that EGS is a toxico-infectious form of botulism caused by C. botulinum type C, with the disease occurring when a combination of risk factors triggers the production of toxins within the horse’s intestinal tract. 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.

 This vaccine trial will follow 1,100 horses and ponies for two years. Only healthy horses and ponies with a valid passport kept on premises with a history of EGS cases in the previous two years will be eligible to be enrolled.

 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.”

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