MARION COUNTY, Fla. -- It's a tragedy that state veterinarians have never seen before. At least 100 horses died in just a week and a half at a Marion County farm.

No one was more devastated when death came to the 87-acre horse farm than the owner.

"It was a surprise to everyone. It was an isolated incident. It's very sad. We rescued those horses to be surrogate mothers," said Francis Ramirez of Equitransfer.

In all, 100 mares suddenly experienced seizures, suffering spasms and twitching muscles, before they fell to the ground. Ramirez euthanized many of the animals.

Veterinarians in Marion County said they've never seen such a mass outbreak. The feeling is that it's not contagious and may have come from the horses' food.

"My best guess is that it was botulism and that's probably the most common food contaminant," said Dr. Carol Clark, veterinarian.

Veterinarians that Eyewitness News spoke with said the killer illness may have come from hay bails left out in the wet weather where mold spores and bacteria that horses are sensitive to, can thrive.

Green, semi-fermented hay called haylage is intended for cattle because of its higher protein content. The suspect haylage was destroyed and the owners say it came from a source that grows haylage specifically for their operation.

Other horse owners Eyewitness News talked with said they never feed haylage to their prized animals because of the risk of toxins.

"Why were you feeding the horses haylage and not dry hay in the first place?" WFTV reporter Robert Maxwell asked Ramirez.

"They're fed multiple types of hay," she said.

The owners at Equitransfer estimate the loss in the hundreds of thousands of dollars.

The state of Florida does not regulate hay or haylage, only processed animal feed, so it is not investigating the horse deaths.

The owners sent samples from the suspect hay and two of the dead horses to toxicology labs in the northeast.