Botulism has been observed in horses as a result of the action of potent neurotoxins produced by the soil-borne, spore-forming bacterium, Clostridium spp. Botulinum neurotoxin is the most potent biological toxin known and acts by blocking transmission of impulses from nerves to muscles, resulting in muscle weakness progressing to paralysis, inability to swallow, and frequently, death. Of the neurotoxins produced, serotypes A, B and C are associated with most outbreaks of botulism in horses.
Three syndromes of disease have been identified:
Wound botulism results from germination of spores of serotype B and subsequent production of toxin in contaminated wounds.
Shaker Foal Syndrome (toxicoinfectious) results from toxin produced by germination of ingested spores in the intestinal tract. Shaker Foal Syndrome is a significant problem in Kentucky and in the mid-Atlantic states in foals between 2 weeks and 8 months of age, due to serotype B.
Forage poisoning results from ingestion of preformed toxin produced in decaying plant material, including improperly preserved hay or haylage, or silage. Commonly animal carcass remnants present in feed are the cause.
A killed vaccine (toxoid) directed against Cl. botulinum serotype B is licensed for use in horses in the United States. Vaccination against botulism is currently not a core vaccine in horses. Vaccination is a risk-based decision for horses at increased risk of developing botulism due to residence in (or travel to) endemic regions, including (Kentucky and the Mid-Atlantic states). Particularly susceptible groups within those regions include adult horses fed high-risk forages and foals born to unvaccinated mares.
Foals born in endemic regions are at risk for toxic infectious botulism unless protected by colostral transfer of antibodies produced by vaccination of the pregnant mare. Limited information suggests that foals vaccinated with the toxoid at 2 weeks, 4 weeks and at 8 weeks of age developed adequate serologic response, even in the presence of passive maternal antibodies.
Currently, no licensed vaccines are available for preventing botulism due to serotypes A or C or other serotypes of toxins. Cross-protection between subtypes does not occur.
Unvaccinated adult horses: Vaccinate with 3 doses at 4-week intervals.
Previously vaccinated adult horses: Revaccinate annually.
Previously vaccinated pregnant mares: Revaccinate annually with a single dose, 2 to 4 weeks before foaling.
Previously unvaccinated pregnant mares: Vaccinate during gestation with 3 doses administered at 4-week intervals. Vaccination should be scheduled so that the last dose will be administered 2 to 4 weeks before foaling to enhance concentrations of immunoglobulin in colostrum.
Foals: Foals at high risk may have the vaccination series initiated as early as 2 weeks of age.
Horses with history of natural exposure: A vaccination protocol should be initiated once antitoxin immunoglobulins are depleted.
Reviewed and revised 2019