As the high risk season for equine grass sickness (EGS) approaches, the Animal Health Trust (AHT) is appealing to vets to encourage clients who keep their horses on premises previously affected by the disease to sign up to its vaccine trial.

EGS – a debilitating and often fatal disease affecting horses, ponies and donkeys – occurs predominantly in Europe, with Britain experiencing the highest incidence worldwide.

Last year, 59 cases of EGS were reported through the EGS surveillance scheme, but it is likely this represents only a fraction of cases occurring annually throughout Britain.

Almost all cases affected horses with access to grazing. Growing scientific evidence suggests the disease may be caused by the bacterium Clostridium botulinum type C, commonly found in soil and capable of producing neurotoxins to which horses are particularly sensitive.

EGS occurs when a combination of risk factors trigger the production of these toxins within the horse’s intestinal tract, causing damage to the nervous system and paralysis of the gut.

As similar equine diseases such as tetanus and botulism can be prevented by vaccination, it is theoretically possible a vaccine could prevent EGS.

The AHT, in collaboration with the Universities of Edinburgh, Liverpool and Surrey, launched the EGS field vaccine trial last year to try to establish whether a vaccine could be effective in reducing the risk of the disease.

The trial is funded by a number of sources including Neogen Corporation;  Animal Welfare Foundation;  Horserace Betting Levy Board;  Racing Foundation;  Moredun Foundation Equine Grass Sickness Fund;  Hong Kong Jockey Club;  British Horse Society; and the EB Moller Charitable Trust.

Now entering its second year, the trial involves enrolled horses and ponies receiving either a course of the C botulinum type C toxoid vaccine or an inactive placebo, as well as an annual booster.

The incidence of EGS is monitored throughout the trial to determine the efficacy of the vaccine, and enrolled horses and ponies are closely monitored through regular follow-ups for the duration of the trial.

With 60% of EGS cases occurring during April, May and June, the AHT is appealing to practices with clients who keep their horses on premises that have had at least one case of EGS in the past three years to encourage enrolment of those animals in the vaccine trial.

The AHT’s EGS research coordinator Jo Ireland said it had been delighted with the “overwhelmingly positive” response to the vaccine trial.

“Many owners have welcomed the chance to be a part of such important research,” Dr Ireland said. “We don’t want any eligible horse owners to miss out on the opportunity to contribute towards a potential breakthrough in the prevention of EGS, so we are encouraging more owners to enrol their horses on the trial this year.”

To find out more about the vaccine trial or how clients can enrol horses and ponies, visit

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