AHT Seeking Horses for Grass Sickness Vaccine Trial

AHT Seeking Horses for Grass Sickness Vaccine Trial

Horses with the less severe form of EGS experience sudden and extreme weight loss, drying of the nasal membranes, and difficulty in swallowing. Only mild cases that receive intensive care survive.

Photo: Chris J. Proudman, MA, VetMB, PhD, Cert EO, FRCVS

The Animal Health Trust (AHT), in Newmarket, Suffolk, England, is launching a trial of a vaccine for the prevention of a frequently fatal disease affecting horses: equine grass sickness (EGS).

Following the successful completion of a small-scale pilot study in 2013, and with generous funding from 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, Dodson and Horrell, South Essex Insurance Brokers, British Horse Society, and EB Moller Charitable Trust), AHT is recruiting horses in England to be enrolled in the trial. The trial will be conducted by the AHT in collaboration with the Universities of Edinburgh, Liverpool, and Surrey.

EGS is an often fatal neurologic disease affecting primarily young grazing horses. Since its first reported occurrence in Scotland, grass sickness has occurred in most northern European countries and in South America. Almost all cases of EGS occur in horses with access to grazing and it is thought they are exposed to some form of noxious agent in the soil ingested as a contaminant of grass. Horses with more severe forms of the disease experience colic, difficulty swallowing, reflux of stomach contents, excessive salivation, high heart rate, impacted intestines, muscle tremors, and patchy sweating. Horses with the less severe form experience sudden and extreme weight loss, drying of the nasal membranes, and difficulty in swallowing. Only mild cases that receive intensive care survive.

There is growing scientific evidence to suggest that EGS could be caused by the bacterium Clostridium botulinum type C, which is found commonly within soil and is capable of producing a range of toxins, including neurotoxins (toxins that damage the nervous system), to which horses are particularly sensitive.

The current theory is that EGS is a toxico-infectious form of botulism caused by C. botulinum type C, with the disease occurring when a combination of risk factors triggers the production of toxins within the horse’s intestinal tract. As vaccinating horses protects against other similar diseases, such as tetanus and botulism, theoretically it is possible that EGS could be prevented by vaccination. A field vaccine trial is the only way to evaluate whether a vaccine is effective in reducing the risk of EGS.

This vaccine trial will follow 1,100 horses and ponies for two years. Only healthy horses and ponies with a valid passport kept on premises with a history of EGS cases in the previous two years will be eligible to be enrolled.

Jo Ireland, BVMS , PhD, Cert AVP (Equine Internal Medicine), MRCVS, the AHT’s EGS research coordinator, said, “Half of the horses and ponies on the trial will be vaccinated with a C. botulinum type C toxoid vaccine; the other half of the horses and ponies will receive an inactive placebo injection. We will record, review, and compare incidence of EGS between the two groups to determine the efficacy of the vaccine. If we see reduced disease incidence in vaccinated horses, this would provide a major breakthrough in the prevention of EGS.”

To find out more about the vaccine trial, or how to enroll horses and ponies, please visit www.equinegrasssickness.co.uk or email equinegrasssickness@aht.org.uk.

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