More on the Recent Grass Sickness Link to Clostridium botulinum

Researchers in the United Kingdom recently advanced scientific knowledge of equine grass sickness with a three-year study that found the disease to be strongly associated with low antibody levels to the bacterium Clostridium botulinum. Findings from the study, which was completed at the University of Liverpool, funded by The Home of Rest For Horses, and published in the Equine Veterinary Journal might lead to routine vaccination against C. botulinum in U.K. horses.

Chris Proudman, MA, VetMB, PhD, CertEO, FRCVS, senior lecturer in equine surgery at the U.K.'s University of Liverpool, said, "The research provides some good evidence that the disease is associated with C. botulinum, and from a horse owner's point of view, probably the most important take-home message is that [the disease involves] antibody levels in the horse, which we can influence--this opens the door to vaccination as a preventive measure."

Grass sickness was first identified around 100 years ago, and scientists have since struggled to understand the disease and identify its cause. The disease usually is fatal and presents itself in two different ways--either as severe colic or weight loss and difficulty eating. Both manifestations of grass sickness are as a result of nerve damage to the intestine that partially or completely paralyzes the gastrointestinal tract, rendering it unable to digest food.

According to Proudman, no cases have been found in the United States. He hypothesized that one possible explanation for the regional and geographical distribution of grass sickness is that the causal toxin is carried on a plasmid (a small, independently replicating, piece of DNA outside a cell's nucleus that can be transferred from one organism to another) within C. botulinum Type C. "It could be that the Type C C. botulinum in the United States might not have this particular plasmid," he said.

While there aren't any figures available for the U.K. disease prevalence, it is a very seasonal disorder, according to Proudman, with cases popping up mainly in the spring. Proudman said, "In this hospital, we see approximately 20 cases a year, out of a caseload of about 250 colic cases per year. We take cases from most of the north of England, where the prevalence is higher than down in the south of the country. A informed guess leads me to suggest that hundreds or possibly thousands of horses are affected per year in the United Kingdom, with many horses dying or being euthanized rather than being referred to a hospital for further investigation."

In the current study, Proudman's team collected biological samples from horses affected by grass sickness and control horses (without grass sickness)--these samples included blood for antibody evaluation and fecal samples for evaluation of parasite status. "We then collected a lot of questionnaire data related to the management of that horse, in particular the way it's fed, the grazing history of that horse, and any other disease that that horse suffered recently, anthelmintic (dewormer) use, vaccine history, and all aspects of the horses' management," said Proudman.

The primary finding involved the antibody levels--the lower a horse's antibody levels to C. botulinum, the higher his risk of suffering from the disease. A secondary finding included an association with age--the horses at highest risk for suffering from grass sickness were horses between the ages of four and five.

Tertiary findings included management factors. Proudman said, "Now we know that recent feed change is a risk factor--that's something that has been reported previously, and our research confirmed it. The feeding of hay or haylage in the diet was found to be protective, and also there was an association with use of ivermectin.

"We suspect that the recent use of ivermectin is merely a marker for horses that have had very thorough worm control, their worm burden has been suppressed, but for reasons we don't understand, that has increased the risk of these horses suffering from grass sickness," said Proudman. The higher the fecal egg count in these horses (which signified a higher worm burden within the body), the lower his risk was for suffering from grass sickness. "With a higher worm burden, these horses were less likely to suffer from grass sickness." Proudman said that recommendations regarding ivermectin as related to grass sickness cannot be made--further research must be completed to understand the meaning of this finding.

Can U.K. horse owners assess their horses' risk for grass sickness? Only at a crude level, said Proudman. "A horse that is young, say less than five years old, that is grazing a premises that has had grass sickness previously--we know that horse would be at an increased risk of suffering from grass sickness (more on this in a moment)."

Well-known horses that have suffered from grass sickness include Dubai Millenium, winner of the 2000 Dubai World Cup, who died from an acute case of the disease in 2001, and Mister Baileys, the 1994 Two Thousand Guineas winner, who suffered from the slow, insidious, chronic form of the disease and recovered enough to perform stud duty in the United States before being pensioned in 2003. "He is very much the exception to the rule," said Proudman. "Most horses aren't able to survive the disease."

Management Factors as a Risk for Grass Sickness
In an accompanying study, the research group found that many horse management practices have no effect on the prevalence of grass sickness on a farm. These practices include harrowing, fertilizing, and reseeding. Proudman said, "Some owners have spent a lot of time and money plowing up their fields and reseeding, but there is no evidence that that does anything at all to change the risk of disease.

"The factor that we did find associated was an increase in soil nitrogen content in particular. We don't quite understand how that fits into the causal pathway of the disease, but I think the reason will become apparent in years to come."

Yet another study, this one completed by researchers at the Animal Health Trust and the Universities of Edinburgh and Liverpool, looked at risk factors associated with the recurrence of grass sickness on previously affected farms. The study raised the suspicion of the mechanical spread of the disease through mechanically collecting feces off the pasture--through soil disturbance and the consequent soil contamination of grass.

Future Study
Proudman said, "There's quite an interest group going on in the United Kingdom, and through those means we've been able to make so much progress in the last few years," he said. The Equine Grass Sickness Research fund ( is dedicated to supporting and advancing research into grass sickness and improving the treatment of chronic cases. "It's very helpful because we all have different skills--we're working in connection with microbiologists, and we are clinicians who have contact with clients [affected by the disease] and epidemiological skills. Together we can hopefully put together a useful working party."

Currently, the research groups are in discussion with various pharmaceutical companies and hope to identify a candidate vaccine against grass sickness. "Scientists are seeking funding for a safety evaluation of a vaccine," said Proudman. "If we can [prove its safety], we will then be in a situation to conduct a field trial to see if this vaccine will be able to protect against this disease.

Read more information on grass sickness.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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