Vaccination Against Equine Grass Sickness: Piloting a Clinical Field Trial of a Clostridium botulinum Type-C Toxoid in Scotland in 2012–13





To conduct a pilot study to inform sample size and trial methodology for a proposed triple-blinded randomised placebo-controlled field trial for a Clostridium botulinum type C toxoid vaccine in the prevention of equine grass sickness (EGS).


Only healthy horses/ponies residing on premises previously affected by a high incidence and frequency of EGS were included. Enrolled horses/ponies were randomly allocated, at premises-level, stratified by age, to vaccine or placebo treatment groups at a 1:1 ratio. Baseline and follow-up premises and horse data were obtained via telephone questionnaires. Veterinary surgeons responsible for the primary care of enrolled animals administered the primary course of vaccine/placebo on Days 0, 21, 42. Following appropriate training, owners completed post treatment daily observations for 7 days following each treatment.


There were 5 participating practices, recruiting 10 EGS-affected premises in eastern Scotland. A total of 109 horses/ponies were enrolled: 13 were excluded prior to randomisation, and one was excluded following randomisation due to absence of a valid passport. Median age at enrolment was 5.5 years. Age (P = 0.34), gender (P = 0.19) and breed (P = 0.94) distributions did not differ significantly between vaccine and placebo groups. Ninety-five horses/ponies completed the primary treatment course. No significant adverse events were reported and overall prevalence of minor injection site reactions was 1.4% (n = 4/285; 95% CI 0.4–2.8%).

Conclusions and practical significance

Participant compliance has been excellent, and findings of this pilot study will be used to refine sample size calculations and inform modifications to trial methodology for a proposed future full-scale vaccine trial.

Ethical animal research

This study was conducted under an Animal Test Certificate and granted institutional ethical approval. Informed written owner consent was obtained for each animal, and procedures were in line with recognised veterinary practice as defined in The Animals (Scientific Procedures) Act 1986. Sources of funding: This study was generously funded by Neogen Corporation. Competing interests: None.