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PRESS RELEASE: Results of a nationwide field trial for a vaccine for the prevention of equine grass sickness

Equine Grass Sickness Fund logo - white outline of a horse and rider on a green background.

After years of initial planning and preliminary studies, a unique nationwide field trial for a vaccine for the prevention of Equine Grass Sickness (EGS) has been completed.

The trial was co-ordinated by the Animal Health Trust, in collaboration with the veterinary schools of the Universities of Edinburgh, Liverpool and Surrey, with some logistical and other support provided by EGS-dedicated charity, the Moredun Foundation Equine Grass Sickness Fund. It involved over 1,000 horses and ponies residing on 120 premises across the UK which had been previously affected by a high incidence of EGS cases.

The trial aimed to determine the effectiveness of a C. botulinum type C vaccination in preventing EGS, by comparing incidence between groups of vaccinated and placebo-treated horses and ponies.

Scientific evidence suggests that EGS may be associated with the bacterium Clostridium botulinum (C. botulinum) type C, which is found commonly within soil and is capable of producing a range of toxins.  It is possible to successfully prevent other similar diseases (such as tetanus and botulism) by vaccination, which suggests it could be possible to prevent EGS by vaccination.

Experimental challenge studies are the most commonly used research method to test the efficacy of vaccines for disease prevention.  However it is not possible to experimentally reproduce EGS and therefore a field vaccine trial was the only available method of evaluating the effect of vaccination. 

The results of the field trial found both the C. botulinum type C vaccine and placebo injections were shown to be safe, with a low frequency of local injection site reactions being reported during the trial.

The majority of horses and ponies in the vaccine group had a significant immune response following the primary vaccination course; C. botulinum type C antibody levels after the primary course of injections were on average 2.5 times higher than those before the first vaccination. The horses and ponies in the placebo treatment group showed little change in their antibody levels following their primary course of injections, which was expected. 

However, the overall incidence of EGS during the four-year field trial was considerably lower than anticipated, with just nine confirmed cases occurring amongst the enrolled horses and ponies over the entire trial period. Compared to the placebo-treated group, the risk of EGS was not significantly reduced in the vaccine group.  Meaning that, unfortunately, the trial failed to provide evidence of an effect of vaccination in the prevention of EGS.

However, consistent with previous research studies, both young animal age and low C. botulinum type C antibody levels were significantly associated with an increased risk of EGS. For the first time, findings from this trial confirmed that low C. botulinum type C antibody levels were found in horses and ponies affected by EGS before the onset of the disease, and the results have highlighted the key role a horse or pony’s immune response has in their risk of developing EGS. Therefore further research to fully elucidate the nature of the association between C. botulinum type C and EGS would be warranted.

Dr. Richard Newton at the Animal Health Trust commented on the research:

 “Although the EGS field trial did not demonstrate a significant protective effect of the C. botulinum type C vaccine against EGS, this truly unique research has still achieved a number of things.  We now have a greater understanding of equine grass sickness and the trial provided further evidence of vaccine safety under conditions of field use.” 

Dr. Jo Ireland from the University of Liverpool added:

 “We are so grateful to all the veterinary practices, horse owners and supporters who helped make this research possible. The significant amount of data that has been collated during this nationwide field trial will be a very valuable resource for subsequent research studies to benefit future generations of horses and ponies.”


Release written by the Animal Health Trust.


For further information, please contact:

Dr Jo Ireland: [email protected]

Kate Thomson (Administrator), The Moredun Foundation Equine Grass Sickness Fund. Tel:  0131 445 6257, Email:  [email protected]


Additional notes:

The Animal Health Trust (AHT) is an independent charity, employing over 250 scientists, vets and support workers. It aims to improve the health and welfare of horses, dogs and cats through research. It also provides specialist referral services and continuous education to vets. Visit the website at www.aht.org.uk. Registered charity 209642.

The Moredun Foundation Equine Grass Sickness Fund is the only UK charity raising funds specifically for research into grass sickness, and is dedicated to advancing research and supporting the nursing of chronic cases. Visit our website www.grasssickness.org.uk. Reg charity No. SC0022515