Botulism Outbreak Suspected in Maine Horse Deaths

Agricultural authorities in Maine believe a botulism outbreak is responsible for the deaths of 23 horses at a Gorham farm.

Maine State Veterinarian Donald Hoenig, VMD, said that between April 7 and April 17, 23 of more than 40 horses residing at the Whistlin' Willows Farm in Gorham died; some survivors became ill but recovered, he said. Agricultural authorities suspect the deaths were caused by botulism resulting from the presence of the causative agent, Clostridium botulinum, in the horses' baleage or round bale hay supplies, Hoenig said.

"This is the largest botulism outbreak any veterinarian in Maine has ever seen," he said.

No one at Whistlin' Willows Farm was available for comment.

Keith Poulsen, DVM, Dipl. ACVIM, of the University of Wisconsin-Madison School of Veterinary Medicine, said that baleage, also known as haylage or silage, is produced by cutting young high-quality grasses and placing the cuttings into an airtight plastic bag to promote fermentation, which lowers the grasses' pH. At a pH of 5, the grass enters a suspended state.

"The option allows horse owners to keep high quantities of feed for years." Poulsen said.

Botulism-causing bacteria can enter bale age supplies if plastic bags are punctured, if grasses are improperly packaged, or if the remains of a dead animal were inadvertently mixed with the grasses during the cutting process, he relayed.

"It can also happen if a cat or possum dies in the area of the barn where when baleage or hay bales are stored," Poulsen said.

Hoenig said samples from baleage and round bale hay supplies found at the property are being tested for the presence of C. botulinum. Pending these results, Hoenig said investigators might never know the exact cause of the animals' deaths.

"We could not necropsy the horses because we found out about the deaths too late and the horses had already been buried," Hoenig said. "As far as the samples are concerned, the more time that goes by, the less chance we have of finding out what happened."

Poulsen said horse owners who feed baleage should be sure they purchase their supplies from a reputable dealer. They should also be sure their horses, especially pregnant mares, receive botulism vaccinations.

"Also, they should call their veterinarian immediately if multiple horses exhibit signs of weakness," Poulsen said.

Once C. botulinum enters the horse's body, his health typically deteriorates rapidly. Clinical signs of botulism in horses are weakness; decreased muscle tone of the tail, eyelids, and tongue; trembling; dilated pupils; lying down; difficulty in swallowing; drooling; and green or milky nasal discharge. The horse will eventually become recumbent, as the toxins target only the nerves responsible for motor function. As the chest muscles and diaphragm become weakened, respiratory failure can set in, killing the horse via asphyxiation.

Botulism is often fatal in horses because of the rapidity of disease onset. When treatment is an option, intensive supportive care and the administration of botulism antitoxin are the main options.

About the Author

Pat Raia

Pat Raia is a veteran journalist who enjoys covering equine welfare, industry, and news. In her spare time, she enjoys riding her Tennessee Walking Horse, Sonny.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More