Accuracy of a Mouse Bioassay for the Diagnosis of Botulism in Horses

Abstract

BackgroundThe laboratory diagnosis of botulism in horses traditionally has relied upon the mouse bioassay (MBA). The accuracy of this test for the diagnosis of botulism in horses is unknown.Hypothesis/ObjectivesOur goal was to determine the sensitivity, specificity, positive predictive value, and negative predictive value of the MBA on laboratory‐processed fecal and gastrointestinal samples for foals and adult horses.AnimalsCases included all horses with a final clinical diagnosis of botulism that were admitted between 1986 and 2011 and had MBA testing performed. Controls included horses without botulism that were admitted during the same time period and had MBA testing performed.MethodsRetrospective study. Horses suspected of having botulism had fecal or (less commonly) gastrointestinal content samples tested using MBA. For every hospitalized botulism suspect, control samples were obtained from ≥1 additional hospitalized horses not suspected to have botulism.ResultsOne hundred and twenty‐nine adult horses and 253 adult controls were identified. Overall sensitivity of the MBA was only 32% but specificity was 97%. Forty‐three foal cases and 21 foal controls were evaluated; sensitivity of the MBA was 53% and specificity was 100%. Positive predictive value was substantially higher (100% for foals and 89% for adults) than negative predictive value (51% for foals and 67% for adults).Conclusions and Clinical ImportanceMouse bioassay has low sensitivity but high specificity for the diagnosis of botulism in horses. Positive results are highly suggestive of botulism but negative results do not exclude the diagnosis. Unaffected horses and foals rarely shed C. botulinum in their feces.

Presumptive tick paralysis in 2 American Miniature horses in the United States

“…19 Given the insensitivity of available antemortem diagnostic tests for botulism and the importance of rapid treatment, trivalent equine botulinum antitoxin was given to both horses within hours of presentation. 21 However, the rapid improvement in neurologic status observed in these 2 horses made botulism unlikely because recovery requires regeneration of new motor end plates, which can take up to 3 weeks. 22 Neurotoxicity associated with ingestion of locoweed, yellow star thistle (Centaurea solstitialis), moldy corn, or Bermuda or rye grass ("grass staggers") was not considered because of the absence of forebrain signs.…”

Equine feed contamination and toxicology

“…Botulism is caused by botulinum neurotoxins ( BoNTs ) produced by C. botulinum , an environmentally ubiquitous spore-forming, anaerobic gram-positive rod. Each of several serotypes produces their own specific BoNT: serotype A (BoNT A) and B (BoNT B) are found in soil and are the most common forms that affect horses ( Johnson et al, 2015 , 2016 ). Serotype C (BoNT C) has been proposed as a cause for dysautonomia or mal seco in horses in Europe and South America, respectively, but convincing evidence is lacking ( Böhnel et al, 2003 ).…”

“…Clostridium botulinum thrives in anerobic conditions and devitalized tissues, and poisoning incidents in horses and other livestock are usually associated with carrion in the feed, though hot spots for botulism due to high C . botulinum spore loads can occur in specific locations, for example, stalls within a barn ( Johnson et al, 2016 ). Botulism in horses occurs most commonly when haylage is fed, though small animal carcasses can inadvertently be incorporated in to any kind of feed.…”

“…Because horses are exceptionally sensitive to BoNT, getting a definitive diagnosis of botulism is extremely challenging. Bioassays were once common, but could probably only detect about 30% of horses with botulism ( Johnson et al, 2016 ). ELISA serology has also been used in horses.…”

Botulism: Knowledge and preparation improve outcome for horses

“…Even in laboratories accustomed to working with equine samples, mouse bioassay only yields positive results for approximately one‐third of clinically diagnosed cases (Johnson et al . ). Veterinarians should never await laboratory confirmation prior to treating suspect cases appropriately.…”