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Multi-state equine botulism outbreak in the United States linked to contaminated feed: an epidemiologic, clinicopathologic, and regulatory investigation.
Abstract: A multi-state outbreak of equine botulism occurred between December 2022 and March 2023 in the United States. Follow-up and testing were performed on 42 horses, including 24 that died or were euthanized in the outbreak that affected ~98 horses. Affected horses had all been exposed to the same commercial feed. Clinical signs included progressive muscle weakness and tremors, recumbency, and colic. No significant gross or microscopic abnormalities were observed on autopsy. Feces and gastrointestinal content were tested for various infectious agents, including botulinum toxin; fecal samples from 2 horses tested positive for botulinum neurotoxin (BoNT) type C using the mouse bioassay (MBA). Feed samples, as well as mammalian tissue found within the feed, were collected and tested; 2 samples were positive for BoNT type C by MBA. Based on these results, a diagnosis of botulism was established, and the contaminated feed was identified as the source of exposure. We highlight the diagnostic challenges associated with equine botulism and the importance of regulatory agencies and interagency collaboration during outbreaks.
Publication Date: 2026-03-25 PubMed ID: 41882921DOI: 10.1177/10406387261430324Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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Overview
- A multi-state outbreak of equine botulism affected about 98 horses in the United States between December 2022 and March 2023.
- The outbreak was traced to contaminated commercial feed containing botulinum neurotoxin type C (BoNT type C).
Background and Context
- Equine botulism is a serious and potentially fatal disease caused by botulinum neurotoxins produced by Clostridium botulinum bacteria.
- These neurotoxins lead to progressive muscle weakness, paralysis, and can result in recumbency (inability to stand) and colic in horses.
- Diagnosing botulism in horses is challenging due to lack of significant gross or microscopic findings upon autopsy and the need for specialized toxin detection assays.
Outbreak Details
- The outbreak spanned multiple states in the U.S. and involved approximately 98 affected horses.
- Follow-up investigations included 42 horses, of which 24 died or were euthanized due to the illness.
- Clinical signs observed were typical of botulism and included:
- Progressive muscle weakness and tremors
- Recumbency (inability to rise)
- Colic (abdominal pain)
Diagnostic Investigations
- Clinical examination and postmortem autopsies revealed no significant gross or microscopic abnormalities to explain the clinical signs.
- Samples collected for testing included:
- Feces and gastrointestinal contents from affected horses
- Samples of the commercial feed suspected as the source of the outbreak
- Mammalian tissue fragments found within the feed
- Testing methods:
- Mouse bioassay (MBA) was employed to detect botulinum neurotoxin type C (BoNT type C)
- Results:
- Two fecal samples tested positive for BoNT type C
- Two feed-related samples also tested positive for the same toxin
Conclusions and Implications
- The findings established a definitive diagnosis of botulism in the affected horses.
- The contaminated commercial feed was identified as the source of the botulinum neurotoxin exposure, confirming it as the outbreak’s origin.
- The investigation emphasized the diagnostic challenges of equine botulism due to subtle clinical and pathological findings and the need for specialized laboratory assays.
- The outbreak highlighted the critical importance of collaboration among veterinary professionals, regulatory agencies, and interagency partners in identifying and controlling feed-borne disease outbreaks.
- Regulatory responses likely included recall of the contaminated feed and enhanced monitoring to prevent further cases.
Cite This Article
APA
Aguirre Siliezar K, Datu C, Proia K, Rotstein D, Nemser SM, Tyson GH, Ragsdale JM, Wilkes RP, Baker RE, Carossino M, Del Piero F, Sasaki E, Yant P, Uzal FA.
(2026).
Multi-state equine botulism outbreak in the United States linked to contaminated feed: an epidemiologic, clinicopathologic, and regulatory investigation.
J Vet Diagn Invest, 10406387261430324.
https://doi.org/10.1177/10406387261430324 Publication
Researcher Affiliations
- Department of Medicine & Epidemiology, University of California-Davis, Davis, CA, USA.
- School of Veterinary Medicine, and Department of Entomology and Nematology, University of California-Davis, Davis, CA, USA.
- Veterinary Laboratory Investigation and Response Network, Center for Veterinary Medicine, U.S. Food and Drug Administration, Laurel, MD, USA.
- Veterinary Laboratory Investigation and Response Network, Center for Veterinary Medicine, U.S. Food and Drug Administration, Laurel, MD, USA.
- Veterinary Laboratory Investigation and Response Network, Center for Veterinary Medicine, U.S. Food and Drug Administration, Laurel, MD, USA.
- Veterinary Laboratory Investigation and Response Network, Center for Veterinary Medicine, U.S. Food and Drug Administration, Laurel, MD, USA.
- Veterinary Diagnostic Services, New Mexico Department of Agriculture, Albuquerque, NM, USA; (Ragsdale).
- Willie M. Reed Animal Disease Diagnostic Laboratory, College of Veterinary Medicine, Purdue University, West Lafayette, IN, USA.
- Departments of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
- Louisiana Animal Disease Diagnostic Laboratory, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
- Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
- Pathobiological Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA.
- California Animal Health and Food Safety Laboratory, San Bernardino branch, University of California-Davis, Davis, CA, USA.
- California Animal Health and Food Safety Laboratory, San Bernardino branch, University of California-Davis, Davis, CA, USA.
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