Grass Sickness Linked to Clostridium botulinum Bacterium

United Kingdom (U.K.) researchers recently found that grass sickness is strongly associated with low antibody levels to the bacterium Clostridium botulinum, indicating that affected horses couldn't fight off infection. Findings from the study, which was conducted at the U.K.'s University of Liverpool (UL), 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, of UL, said, "The disease involves antibody levels in the horse, which we can influence--this opens the door to vaccination as a preventive measure."

Grass sickness usually is fatal and presents itself in two different ways--either as severe colic, or as weight loss and difficulty eating. Both manifestations are a result of intestinal nerve damage that partially or completely paralyzes the gastrointestinal tract, rendering it unable to digest food. No U.S. cases have been found.

Proudman's team collected biological samples from horses affected by grass sickness and control horses (without grass sickness)--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, any other disease that horse suffered recently, anthelmintic (dewormer) use, vaccine history, and all aspects of the horse's management," he said.

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 age association--the horses at highest risk of grass sickness were 4- to 5-year-olds. Tertiary findings included management factors. The research confirmed prior findings that recent feed change was a risk factor, feeding hay or haylage in the diet was protective, and grass sickness was associated with ivermectin use.

"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," he said. The higher the fecal egg count, the lower the risk of grass sickness. He can't make recommendations regarding ivermectin use and grass sickness--further research is needed to understand this finding.

Can U.K. horse owners assess their horses' risk? Only at a crude level, said Proudman. "A horse that is young and is grazing a premises that has had grass sickness previously--that horse would be at an increased risk." (See for accompanying grass sickness studies.)

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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