A four-year trial has been unable to show that vaccination
can protect against grass sickness.
Evidence suggests that Equine Grass Sickness (EGS) may be
associated with the bacterium Clostridium botulinum (C. botulinum) type C,
which is found commonly in soil and can produce various toxins.
Vaccines can protect against tetanus and botulism, diseases
caused by closely related bacteria. Hopefully, a similar
vaccine could protect against grass sickness.
As it is not possible to reproduce grass sickness
experimentally, the researchers conducted a field trial to determine the
efficacy of C. botulinum type C vaccination. The aim was to test the ability of the vaccine to prevent
naturally occurring EGS by comparing EGS incidence between groups of vaccinated
and placebo-treated horses and ponies.
The study was co-ordinated by the Animal Health Trust, in
collaboration with the veterinary schools of the Universities of Edinburgh,
Liverpool and Surrey, and with support of the Moredun Foundation Equine Grass
Sickness Fund.
The research team recruited over 1000 horses and ponies
throughout England and Scotland. To be included in the study, the animals had
to live on premises where grass sickness had occurred before. Horses and ponies
were randomly assigned to two groups: vaccination or control. The primary
treatment course comprised three doses of either the vaccine or placebo
administered at 21-day intervals. Twelve months later, a further booster dose
of either vaccine or placebo was given.
As the study was carried out on premises with a history of
grass sickness, the research team anticipated that there would be enough cases
to produce a statistically significant difference between the vaccinated and
control groups. Unfortunately, only nine cases of grass sickness occurred in the
study population – considerably fewer than would have been expected from recent
history.
So, unfortunately, the study was unable to reach any
conclusion about whether the vaccine was effective in preventing
the disease.
However, some useful findings did emerge from the study. The
vaccine was shown to be safe and, unlike the placebo, did produce a significant
antibody response. Most horses and ponies in the vaccine group had a
significant immune response following the primary vaccination course. (C.
botulinum type C antibody levels were on average 2.5 times higher after the
primary course of injections than before the first vaccination.)
The research team also report that, consistent with previous
studies, both young animal age and low C. botulinum type C antibody levels were
significantly associated with an increased risk of EGS.
They add that, 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. The
results highlighted the key role a horse or pony’s immune response has in
their risk of developing EGS.
Dr Richard Newton, Director of Epidemiology and Disease
Surveillance at the Animal Health Trust, said: “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.”
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