Trial of clostridium botulinum type C vaccination for prevention of equine grass sickness


Norman Hayward Fund



Research Period


Area of study



Investigator(s): Jo Ireland

Equine grass sickness (EGS) is a predominantly fatal disease affecting grazing equids. Research suggests that EGS is a toxico-infection involving Clostridium botulinum (C. botulinum), with several studies demonstrating a protective effect of natural immunity to C. botulinum type C. Other clostridial diseases are successfully prevented by vaccination, implying that it should be possible to prevent EGS by vaccination.
This project will determine the efficacy of C. botulinum type C vaccination in preventing EGS by performing a nationwide randomised controlled field trial, comparing EGS incidence between groups of vaccinated and placebo-treated horses/ponies. Recruitment for the EGS vaccine trial commenced in March 2014, enrolling privately owned horses/ponies from EGS-affected premises. Only horses/ponies over six months old, with a valid passport, and kept on premises with a history of at least one EGS case within the preceding three years were eligible for enrolment.
A total of 120 premises are participating in the trial (52% in England and 48% in Scotland), with 84 veterinary practices recruited as site investigators. A total of 1,001 eligible horses and ponies were enrolled by 221 owners. On enrolment, these horses/ponies were randomly selected to enter either the vaccine or placebo treatment group. Of the enrolled animals, 936 horses/ponies have completed the entire primary treatment course (comprising three vaccine or placebo injections administered at 21 day intervals) to date. For horses/ponies enrolled during 2014, annual booster administration commenced in April 2015, and these animals will receive a second annual booster in 2016. Three confirmed EGS cases have occurred within the study population to date. Preliminary interim analyses indicated that an extension to the EGS vaccine trial study period to include a third year is required to meet sample size requirements. Masking of treatment group allocations will be maintained until final data analyses are completed in early 2017, where the incidence of EGS will be compared between the vaccine and placebo treatment groups.
Demonstration of reduced disease incidence in vaccinated animals would provide a major breakthrough in EGS prevention, with an effective vaccine representing the first preventive healthcare measure to reduce the impact of this disease.


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