Two Illnesses Might Have Cyanide Link

Cyanide has been identified as a potential risk or contributing factor in two illnesses that have dominated horse industry headlines this year. Scientists at the University of Kentucky’s Maxwell H. Gluck Equine Research Center in Lexington, Ky., have been examining the role of cyanide in mare reproductive loss syndrome (MRLS), and will be incorporating what they learn from cyanide’s potential involvement in this syndrome to factors associated with the development of grass sickness (primarily seen in the United Kingdom). Thomas Tobin, MVB, MSc, PhD, MRCVS, DABT, a veterinary pharmacologist/toxicologist at the Gluck Center, is working on the basic toxicology of cyanide in horses, and this information should help couple these research areas.

The project to determine the cause of grass sickness and possible preventive measures is already underway, combining the work of research groups in Dubai, Northern Europe, and Lexington.

Grass sickness can be acute or chronic, partially or completely paralyzing the gastrointestinal tract, and rendering it unable to digest food. The disease is usually fatal. It might be linked to a neurotoxin produced by anaerobic bacteria of the Clostridium botulinum family, members of which are associated with botulism in the horse. Horses can contract botulism by ingesting the toxin in contaminated feed or water, through the entrance of C. botulinum in a wound; or by the entrance of the organism through the necrotic (dead) tissue of the umbilical stump in foals. Grass sickness was first described in Europe about 100 years ago, and it isn’t totally clear why cases aren’t seen in the United States. Grass sickness caused the demise earlier this year of Dubai Millenium, a champion English Thoroughbred racehorse.

Tobin explained why cyanide is being targeted in the fight against grass sickness. "There is some evidence to support suggestions that if you have exposure to higher levels of cyanide, there may be an alteration of gut flora, which encourages or allows certain bacterial species to proliferate. In this model, the pathogenesis of grass sickness would be a two-step mechanism -- the effects of the cyanide, and the effects of Clostridial proliferation." Tobin also will research what might trigger the increased concentration of cyanide levels in the pasture. White clover is being investigated as it can harbor high levels of cyanide.

Scientists found increased levels of cyanide in certain tissues from three fetuses killed by MRLS this year, and many believe it is linked to the leaves of black cherry trees. According to Tobin, the university research represents the first attempt at defining normal levels of cyanide in the horse in Kentucky. What they learn will feed into the MRLS and grass sickness studies. "There’s not a lot of analytical work reported in the literature on cyanide in the horse, other than the fact that you can get acute toxicities. So there isn’t a lot of analytical data available, particularly at low levels of exposure," he says.

The researchers will also evaluate what conditions (climatic or otherwise) might cause the concentrations of cyanide to reach levels that would cause biochemical and other responses in the horse.

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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