Botulism in Horses: An Introduction

Forty years ago, in the ominously titled research paper “Bacterial Toxins: A Table of Lethal Amounts,” D. Michael Gill described the Clostridium botulinum neurotoxin as the most lethal toxin known to mankind.^* The dangers of the neurotoxin extend to all mammals and some, like horses, are especially susceptible to intoxication, a disease called botulism.

C. botulinum bacteria are dispersed widely in soil and water throughout the world. Because of this, the risk of exposure and potential toxicity looms large. Horses with botulism acquire the disease in one of three ways: (1) ingestion of toxin with forage or feed by mature horses; (2) ingestion of C. botulinum spores that later proliferate in the gastrointestinal tract of foals; and (3) contamination of wounds with C. botulinum with consequent bacterial growth and toxin release.

Scientists recognize different C. botulinum neurotoxins, each denoted by a letter (A, B, C-1, C-2, D, E, F, G). Horses are known to be affected by only three of the eight types: A, B, and C.

Type B is the most common cause of botulism in horses and typically occurs as a result of consuming moldy hay or improperly fermented haylage, though grain has been implicated in some cases. Decomposing animal carcasses have also been a source of C. botulinum intoxication, usually type C. Regardless of type, when ingested, the toxin is absorbed from the intestine and spread throughout the body by the circulatory system. Toxins interrupt nerve transmission, which leads to physical manifestation of disease.

Clinical signs of botulism, which usually occur within 24 hours of exposure, include generalized muscle weakness with recumbency, difficulty swallowing, pupil dilation, respiratory changes, and decreased tone of eyelids, tongue, and tail. Because difficulty swallowing is a consistent clinical sign among affected horses, astute horsemen will sometimes report a change in eating behavior in the early stages of the disease—the horse may take longer to consume a meal, leave meals unfinished, or be reluctant to swallow. A presumptive diagnosis of botulism can be made on clinical signs.

Early presumptive diagnosis and swift, aggressive treatment is warranted in suspected cases of botulism. Antitoxin is available in some countries. The goal of intravenous antitoxin therapy is the neutralization of circulating toxin. Antitoxin therapy does not reverse existing clinical signs and does not necessarily derail disease progression. Supportive care at a veterinary clinic often entails frequent turning of recumbent patients to avoid pressure sores, feeding through nasogastric tube, eye lubricants to offset the effects of decreased eyelid tone, and sedation to reduce muscle activity.

In a retrospective study at New Bolton Center, University of Pennsylvania School of Veterinary Medicine, researchers reviewed the medical records of 92 horses with botulism in an attempt to identify variables associated with survival based on information gathered at admission and clinical findings during hospitalization.^** Higher rectal temperature at the time of admission and treatment with antitoxin increased the chance of survival. On the contrary, an inability to stand or abnormal respiratory effort signaled decreased likelihood for survival. Of the 92 horses, 44 survived. The researchers acknowledged that the most important predictor of survival was maintaining the ability to stand.

Botulism in foals, often called shaker foal syndrome, is a sporadic condition affecting foals of all ages, from only a few days old to several months of age. Foals are thought to consume soil contaminated with the toxin, usually type B, which initiates toxicity.

Botulism is often preventable by vaccination. The usual schedule involves three doses at one-month intervals followed by annual boosters.

In addition to vaccination, careful forage selection and storage are paramount. Inspect each bale carefully prior to feeding. Appropriate storage involves a dry, well-ventilated location.

^*Gill, D.M. 1982. Bacterial toxins: A table of lethal amounts. Microbiological Reviews. March, pp. 86-94.

^**Johnson, A.L., S.C. McAdams-Gallagher, and H. Aceto. 2015. Outcome of adult horses with botulism treated at a veterinary hospital: 92 cases (1989-2013). Journal of Equine Veterinary Internal Medicine 29:311-319.

 Reed, S.M. 2004. Botulism. In: S.M. Reed, W.M. Bayly, and D.C. Sellon, editors, Equine internal medicine. 2nd Ed. Saunders, St. Louis, MO, pp. 650-652.