Botulism Reviewed for World Equine Vets

An overview of botulism was given at the 2009 WEVA Congress by Nathan M. Slovis DVM, Dipl. ACVIM, CHT (certified in hyperbaric oxygen therapy), director of the McGee Critical Care and Medical Center at Hagyard Equine Medical Institute in Lexington, Ky.

He described botulism as a neuromuscular disease characterized by flaccid paralysis and caused by neurotoxins produced by strains of Clostridium botulinum. He noted horses are one of the most susceptible species to botulism, with both individual and group outbreaks having been reported around the world.

C. botulinum was first identified in 1897 in Belgium during an outbreak of food poisoning traced to imperfectly smoked ham. Clostridium tetani, the organism that causes tetanus, is a close relative. The toxin produced by C. botulinum is one of the most potent known to man--in fact, it has at times been considered a weapon of biological warfare. Since horses are particularly sensitive to botulinum toxin, untreated foals can suffer up to 90% mortality, with adult horses approaching 100%.

"Clostridium botulinum is a Gram-positive, spore-forming anaerobic bacterium," Slovis said. "Spores are found in the soil throughout most of the world, with the distribution of strains dependent on temperature and soil pH. Eight serotypes of botulinum neurotoxin exist and are labeled A, B, C1,C2, D, E, F, and G, all of which have similar toxicity. There is geographic variation in the predominant serotypes. In North America, botulism in horses is most often caused by Type A and B toxins, and less often C."

Slovis said the two main forms of botulism are toxicoinfectious botulism, also known as "shaker foal syndrome," and "forage poisoning," which is usually seen in adult horses.

Shaker foal syndrome occurs as a result of overgrowth of C. botulinum in the intestinal tract, followed by production of neurotoxins, said Slovis. The disease most often affects fast-growing foals that are 1-2 months old. He said the mature protective gastrointestinal microflora of adult horses typically prevent overgrowth of C. botulinum following ingestion of the spores.

Botulism in adult horses generally occurs following ingestion of preformed toxins in spoiled hay or silage. "Silage with a pH greater than 4.5 is favorable for sporulation and toxin production," noted Slovis.

Type C botulism is associated with ingestion of feed or water contaminated by the carcass of a rodent or other small animal. He added that some have suggested that birds might be able to carry preformed toxin from carrion to the feed of horses.

Slovis said botulism also can occur when neurotoxins are produced in wounds infected with C. botulinum, and in gastric ulcers.

"Proliferation of C. botulinum Type B organisms in gastric ulcers, foci of hepatic necrosis, abscesses in the navel or lungs, and wounds in skin and muscle have been associated with toxicoinfectious botulism," he said.

Clinical Signs and Diagnosis

Slovis said the most common clinical signs are symmetrical flaccid paralysis, with the onset and rate of progression dependent on the amount of toxin that is absorbed. Other signs include inability to swallow with excess salivation, weak eyelid and tail tone, and exercise intolerance.

"Affected animals also spend increased amounts of time resting due to generalized muscle weakness, which is also associated with tremors, carpal (knee) buckling, and ataxia," said Slovis. "Severely affected animals die from respiratory paralysis .

Slovis said botulism should be suspected in animals with flaccid paralysis displaying the above clinical signs. "Definitive diagnosis can be achieved by the mouse inoculation test using serum or gastrointestinal contents," he noted. "However, horses are extremely sensitive to the toxin and this test is often negative. The traditional mouse bioassay identifies Clostridium botulinum in only about 30% of feces collected from adult horses with clinical disease. Detection of antibody titers in a recovering unvaccinated horse is also evidence for the diagnosis of botulism."

Slovis said Robert Whitlock, DVM, PhD, of the University of Pennsylvania's New Bolton Center, is developing a quantitative real-time PCR (qPCR, polymerase chain reaction) test for the detection of C. botulinum neurotoxin type B in equine diagnostic samples. "This assay should be more economical, time-efficient, and sensitive than the traditional mouse bioassay," noted Slovis.


Horses diagnosed with botulism based on the above clinical signs and tests require immediate treatment with a polyvalent antitoxin. The antitoxin prevents binding of the circulating toxin to presynaptic membranes, but it can't do anything for the neuromuscular junctions already affected by the toxin. Horses that are severely affected won't show much improvement with the administration of antitoxin.

"Generally, only one dose--200mL of antiserum to foals (30,000 IU) or 500mL (70,000 IU) to adults--of antitoxin is needed and provides passive protection for up to two months," stated Slovis.

Antibiotics should be used if toxicoinfectious botulism is suspected or if there are secondary lesions, such as aspiration pneumonia or decubital ulcers (pressure sores).

"Antibiotics that can cause neuromuscular blockade and possibly exacerbate clinical signs such as aminoglycosides should be avoided, and neurostimulants such as neostigmine should not be used," he cautioned. "Good nursing care, including the provision of a deep bed and a quiet environment, are essential. Frequent turning of recumbent animals, nasogastric feeding and fluid support for animals with pharyngeal and lingual paralysis, frequent catheterization of the urinary bladder, and application of ophthalmic ointments and ventilatory support may all be required."


"A survival rate of 96% has been reported in foals with toxicoinfectious botulism that were provided with intensive nursing care (including mechanical ventilation and botulism antitoxin)," said Slovis. "However, this type of treatment is not available in all areas and is quite expensive. Without aggressive supportive care, the mortality rate is high, with death usually occurring one to three days after the onset of clinical signs."

In adult horses that ingest preformed toxin, prognosis depends on the amount of toxin absorbed and the severity of clinical signs, he said. "Mildly affected animals may recover with minimal treatment, while severely affected animals that become recumbent have a poor prognosis," he explained. "The mortality rate has been reported to be as high as 90% in recumbent (unable to rise) adult horses, with death occurring within hours of the appearance of signs. Over the last eight years at the Hagyard Equine Medical Institute they have documented an approximately 65% success rate in adult horses affected by botulism. In animals that survive, complete recovery is most common. Development of full muscular strength takes weeks to months. Persistent tongue weakness not affecting the ability to eat has been reported." (The Internal Medicine Department at Hagyard Equine Medical Institute has admitted 25 cases of adult botulism and admits a yearly average of 12 foal cases of botulism.)


In areas where there is known type B botulism, there is a type B toxoid available. It is important if you are going to ship a broodmare into a botulism-endemic area for foaling and breeding that you have her properly vaccinated.

"Vaccination is particularly important in areas where neonatal botulism occurs," said Slovis. "Widespread vaccination of mares in certain high-risk areas has dramatically decreased the incidence of neonatal botulism. An initial series of three vaccinations a month apart followed by annual boosters has been recommended. Pregnant mares should receive a booster four weeks prior to foaling to ensure adequate antibody levels in colostrum."

He noted that type B vaccine only provides protection against type B toxin; there is no cross-protection against type C toxin, and type C toxoid is not licensed for use in North America.

Silage, haylage, and other fermented feeds should not be fed to horses because of the risk they pose for botulism infection.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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