Multi-state equine botulism outbreak in the United States linked to contaminated feed: an epidemiologic, clinicopathologic, and regulatory investigation.

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.