A rare botulism outbreak has killed 23 horses at Whistlin’ Willow Farm in Gorham, Maine, Portland’s Press Herald reports.

No mistreatment has been reported. Some sickened horses recovered, while at least another 40 horses at the farm never became sick.

State health officials believe the toxin developed in bales of silage — feed grass packaged while still moist.

The illnesses occurred between April 7 and 17.

The state veterinarian told the Press Herald that there is little veterinarians can do for horses once they exhibit symptoms of botulism poisoning. The toxin can kill horses within hours of ingestion.

State health officials advise against feeding animals moldy hay.