During a single month in the spring of 2025, sixteen horses in Delaware died soon after showing neurologic signs. Testing ruled out infection, and postmortem findings pointed to a toxin. Feed analysis finally revealed the culprit: fumonisin [foo-MON-ah-sin], a fungal toxin.

Fumonisin is produced by mold growing on corn — that is, it’s a type of mycotoxin [my-koh-tock-sin]. Although mycotoxins can harm all kinds of animals, horses are among the most sensitive. Telltale damage involves softening of the brain, which causes a variety of symptoms — like head pressing, blindness and a lack of coordination — and can be fatal.

The only treatment is supportive care, so prevention is crucial. That includes buying feed guaranteed to be mycotoxin-free and storing all feed appropriately.