Botulism

• Wound botulism results from vegetation of spores of Cl. botulinum and subsequent production of toxin in contaminated wounds.
• Shaker Foal Syndrome (toxicoinfectious) results from toxin produced by vegetation of ingested spores in the intestinal tract.
• Forage poisoning results from ingestion of preformed toxin produced by decaying plant material, including improperly preserved hay or haylage, or animal carcass remnants present in feed.
• Equine Grass Sickness (Equine Dysautonomia) is considered a form of botulism resulting from the overgrowth of Cl. botulinum type C in the intestinal tract, especially the ileum. There are reports of isolated cases of the disease occurring in the U.S.

Botulinum toxin is the most potent biological toxin known and acts by blocking transmission of impulses in nerves, resulting in weakness progressing to paralysis, inability to swallow, and frequently, death. Of the 8 distinct toxins produced by sub-types of Cl. botulinum, types B and C are associated with most outbreaks of botulism in horses.

Vaccine:

A killed vaccine (toxoid) directed against Cl. botulinum type B only is licensed for use in horses in the United States. Its primary indication is for prevention of the Shaker Foal Syndrome by colostral transfer of antibodies produced by vaccination of the pregnant mare. Almost all cases of Shaker Foal Syndrome, a significant problem in Kentucky and in the mid-Atlantic seaboard states in foals between 2 weeks and 8 months of age, are caused by Cl. botulinum type B. 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.

There are no licensed vaccines available for preventing botulism due to Cl. botulinum type C or other subtypes of toxins. Cross-protection between the B and C subtypes does not occur; thus routine vaccination against Cl. botulinum type C is not currently practiced.

Vaccination Schedule:

Previously vaccinated pregnant mares:  Vaccinate annually with a single dose 4 to 6 weeks before foaling.

Previously unvaccinated pregnant mares:  Vaccinate during gestation with a primary series of 3 doses administered at 4-week intervals and scheduled so that the last dose will be administered 4 to 6 weeks before foaling to enhance concentrations of immunoglobulin in colostrum (i.e. months 8, 9, 10 of gestation).

Foals of vaccinated mares (in endemic areas):  Administer a primary series of 3 doses, at 4-week intervals, starting at 2 to 3 months of age. As maternal antibody does not interfere with vaccine response, foals at high risk may have the vaccination series initiated as early as 2 weeks of age.

Foals of unvaccinated mares (born in, or moving to, endemic areas):  Administer a primary series of 3 doses, at 4-week intervals, beginning at 1 to 3 months of age. Foals at high risk may have the vaccination series initiated as early as 2 weeks of age.  Foals of unvaccinated mares may benefit from transfusion of plasma from a vaccinated horse or from administration of Cl. botulinum type B antitoxin. The efficacy of these practices needs further study.

All other horses (where indicated):  Administer a primary series of 3 doses of vaccine given at 4-week intervals and followed by annual revaccination.