What is botulism?
- Botulism is the result of a neurotoxin released by Clostridium botulinum spores.
- The toxin blocks the communication between nerves and muscles, resulting in progressive flaccid paralysis.
- Horses are more susceptible than other species and botulism is highly fatal unless treated with antitoxin.
- Three versions of the disease have been identified in horses. Toxin-producing spores are ingested, infect a wound, or germinate in the intestinal tract of foals.
- A vaccine against type-B is available and is beneficial for horses in endemic areas, but there is no cross-protection between other types.
Botulism is caused when Clostridium botulinum spores are ingested (forage poisoning) or infect a wound (wound botulism) and release a neurotoxin. Another form of the disease is shaker foal syndrome, which results when spores germinate and produce toxin in foals’ intestinal tracts. The toxin blocks the communication between nerves and muscles, resulting in progressive flaccid paralysis and reduced muscle strength. Although rare, it is highly fatal unless treated with antitoxin. Horses are more susceptible to botulism than many other species.
Strains of C. botulinum can produce seven neurotoxins (A through G), which are generally linked to different geographic locations. Not all of these neurotoxin types are toxic to horses. Types A, B, and C have been reported in relation to equine illness, although type A toxicity is uncommon. Shaker foal syndrome is due to type B and is primarily of concern in Kentucky and other mid-Atlantic states. Type C botulism is often linked to forage contaminated with an infected animal carcass.
Spores of C. botulinum are found in the environment, including in the soil, where they can exist for many years without incident. Poisoning can occur when the spores are in an environment favorable to toxin production - moist conditions with low oxygen, high protein, and low acidity - such as decaying plant matter. Feed contamination can also occur due to animal carcasses or birds carrying animal material from off-site.
What are the clinical signs of botulism?
Clinical signs of botulism in horses can include weakness, exercise intolerance, muscle tremors, drooling, and reduced tongue strength. Progressive paralysis can lead to labored breathing and difficulty swallowing. Death can occur from respiratory paralysis and heart failure.
How is botulism diagnosed?
A botulism diagnosis is challenging and is often primarily based on clinical signs and exclusion of other potential neurological (such as tick paralysis, hypermagnesemia, West Nile virus) and toxicological (such as lead toxicity) causes.
Laboratory assays are often inconclusive due to low circulating levels of the neurotoxin. The available mouse bio-assay requires at least six milliliters of plasma or serum from an affected horse prior to antitoxin treatment. The levels of circulating toxin are often below the detection threshold for this assay.
The most accurate diagnosis comes from being able to identify the bacteria in the horse’s feed or environment. This is more likely to be successfully determined in outbreaks, rather than when only one or two horses from a property are affected. Sometimes C. botulinum spores can be identified in intestinal contents of horses with clinical signs. An electrophysiological test (nerve testing) developed for large animals at UC Davis has been successful in strongly supporting botulism as a clinical diagnosis.
How is botulism treated?
Affected horses should be treated with plasma containing antitoxin as soon as possible. Polyvalent plasma contains antitoxin for neurotoxins A, B, C, D, and E (approximate cost is $2,500 - $3,000.00 USD per unit). A trivalent plasma contains antitoxin for A, B, and C neurotoxins and is less than $1,000.00 per unit. Only one dose is needed and can provide protection for at least 60 days. For horses with mild disease, recovery may be possible without antitoxin. Horses should be placed on stall rest to minimize muscle activity. Supportive care may include antimicrobial therapy for secondary complications such as aspiration pneumonia.
What is the prognosis for botulism?
Recovery from botulism depends on the dose of toxin and the severity of the clinical signs. Horses mildly affected with botulism may recover, although associated muscle wasting can take weeks to resolve. Without timely treatment and supportive care, botulism in horses is usually fatal. The prognosis for horses that become recumbent and are unable to stand is usually grave. Early recognition and prompt therapy, including antitoxin plasma, improves the outcome.
How can botulism be prevented?
The best way to prevent botulism is through good husbandry practices. This includes properly storing and inspecting forage and avoiding feeding silage or fermented forages. Keep surrounding areas free of decaying vegetation and animal carcasses. Control rodents, birds, and other animals that might carry the toxin. Properly care for wounds to avoid infection.
A vaccine is available for type B botulism and is recommended annually in endemic areas. There are no vaccines that currently protect against multiple types of botulism.
For more information
Risk-based vaccination guidelines, botulism, American Association of Equine Practitioners
Aleman M, Williams DC, Nieto JE, Magdesian KG, Feary D, Hilton H, Kozikowski T, Higgins JK, Brosnan RJ, Madigan JE, LeCouteur RA. Repetitive stimulation of the common peroneal nerve as a diagnostic aid for botulism in foals. J Vet Intern Med. 2011;25(2):365-372.
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