Following the successful completion of a Grass Sickness vaccine pilot trial last year, a full-scale trial is due to start shortly throughout Great Britain.
Equine grass sickness (EGS) affects grazing horses, ponies and donkeys, and is nearly always fatal. Britain has the highest incidence worldwide. The current theory is that EGS is a toxico-infection involving Clostridium botulinum type C. Several studies have shown horses with natural immunity to Clostridium botulinum type C are less likely to get the disease.
Other equine clostridial diseases are successfully prevented by vaccination, so it should be possible to prevent EGS by vaccination.
The main purpose of last year's pilot study was to assess whether a larger-scale trial would be possible. So it was designed to test the methods and systems used rather than the vaccine itself.
A total of 48 horses and ponies were randomly assigned to the vaccine group and 47 were assigned to the placebo treatment group. All horses in the study completed the primary treatment course of three injections given 21 days part.
The researchers report that the pilot field vaccine trial was a huge success, meeting all of the study objectives. The findings have already been used to revise sample size calculations and trial methodology for the full-scale nationwide randomised placebo-controlled field vaccine trial.
Both the C. botulinum type C toxoid vaccine and placebo injection were shown to be safe.
No systemic adverse reactions (where the entire body may be affected and the horse or pony may become unwell) were reported following any injections administered during the pilot vaccine trial. Minor local injection site abnormalities (such as localised heat, pain or swelling at the injection site) were reported in 19 of a total of 372 injections administered during the study. None of them required treatment or veterinary attention.
There was no significant difference in the number of minor injection site abnormalities between the vaccine or placebo treatment groups.
The researchers report that the response to the vaccine varied, as they expected it would. Some horses showed an increase in antibody levels of up to eight times, but others showed a less marked response. This is in fact what tends to occur with other vaccines, such as influenza. However, it does mean that not all horses would be fully protected by the vaccination.
This ground-breaking EGS vaccine trial aims to determine the efficacy of Clostridium botulinum type C vaccination in preventing EGS by comparing EGS incidence between groups of horses receiving vaccination or a placebo.
The plan is to recruit 1100 horses for a two year period from premises that have previously had a high incidence of EGS. Demonstration of reduced disease incidence in vaccinated horses would provide a major breakthrough in the prevention of EGS.
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