The tragic thing about equine botulism is that horses might die from it before it’s ever diagnosed—or, in areas where it’s uncommon, even seriously suspected. That’s the case with a Florida outbreak that began earlier this month.

In this outbreak, veterinarians initially suspected horses found dead in their pastures had ingested a toxic plant or an incorrectly formulated feed. But when horses were admitted at the University of Florida (UF) with signs of weakness, poor tongue tone, and muscle tremors, botulism immediately became the top differential.

Botulism is caused by toxins produced by the bacterium Clostridium botulinum. Botulism Type B, associated with hay that has been allowed to reach the wrong pH or contains excessive moisture, is more commonly found in the Northeast and Kentucky. (Type A usually occurs in the West and is associated with feeding grass clippings, whereas Type C typically happens when horses eat hay contaminated with a dead animal.)

Although the vaccine used to prevent botulism is only effective against Type B, the antitoxin—a plasma product that veterinarians administer to treat botulism—works on all three types. “We administer the antitoxin to horses with clinical signs of botulism or known exposure to a source and know that they’re covered for all three types of botulinum toxins regardless of whether we have confirmed the diagnosis yet,” said Sally DeNotta, DVM, PhD, Dipl. ACVIM, an assistant professor of large internal medicine at UF’s College of Veterinary Medicine, and the state’s equine extension veterinarian. Botulism is diagnosed in most horses on clinical signs alone; C. botulinum toxin identification tests, which will help the veterinarians know exactly what type (A, B, or C) each horse has, are pending at the national laboratory for some of the affected horses.

How does the antitoxin work?

Under certain conditions C. botulinum bacteria produce the neurotoxins, which enter the horse’s bloodstream and block neuromuscular junctions, preventing communication between neurons and muscles and eventually causing paralysis and death.

DeNotta said the antitoxin used to treat affected horses binds to and clears the C. botulinum toxin from the bloodstream and prevents it from reaching the spinal cord and peripheral nerves: “One dose of antitoxin will circulate long enough to mitigate any free toxin but doesn’t have any effect on toxin that’s already bound to the neuromuscular junction. So, it’s important to administer the antitoxin as early as possible, ideally before the horse becomes too weak to stand.”

How can you protect your horses against botulism?

1. Inspect all hay before feeding. Never feed hay that looks moldy, and discard any bales found to contain rodents or other dead animals. Avoid feeding round bales, block or cubed hay, or silage to horses, because these types of forage are considered to put horses at high risk for botulism, DeNotta said.
2. Watch for signs. Horses might be lying down more than usual, which is particularly noticeable if more than one horse is involved; they might display muscle tremors, weakness resulting in inability to stand, loss of tongue control, inability to swallow leading to drooling and difficulty eating and drinking, a stiff, short stride or stumbling gait, and loss of tail tone.
3. When signs occur, act quickly. One boarding facility owner in Florida who feeds alfalfa blocks (compressed bales that are 500-1,000-pounds) and experienced loss in this outbreak, said she talked with her veterinarian about possible causes after the first horse died, then four days later had the vet out after another horse died and a third horse went down within hours. She pulled all the horses from that pasture, began feeding baled hay and trucked in city water for the horses. Within a week she had found an alternate facility, installed fencing, and moved the horses. Still, six horses were affected, three of which died, while others were treated at UF.
4. Ask your veterinarian if vaccination is right for your horse. If you live in a region where botulism is endemic, or if you feed hay types known to put horses at risk for botulism, preventive vaccination for Type B may be helpful. DeNotta warned, however, that vaccination is not likely to be helpful in an outbreak situation, and underscores it’s not effective against Types A or C.

Should you have your hay tested for C. botulinum toxin?

In an outbreak situation it is natural to want to test hay for this deadly toxin to protect your horses from potential exposure. However, DeNotta said, “Unfortunately, because the toxin can be distributed very sporadically throughout only some bales within a batch of hay, or even in only certain areas within a single bale, there’s really no way to take a single hay sample and accurately assess your horse’s risk.

“We test feces, intestinal contents, and feed from horses we know are clinically affected to confirm the diagnosis and determine the type of botulinum toxin involved, but there’s no reasonable way to screen a batch of hay to know it’s safe for your horses.”

The safest feeding practice is to feed good-quality baled hay and avoid haylage, round bales, and other types of hay that are better off fed to livestock. “When you’re feeding small bales, you can visually inspect each flake as you feed so if there’s excessive moisture, weed contamination or mold, or if an animal was trapped in the baling process, you can discard that bale without feeding it,” said DeNotta.

Editor’s note: At the time of publishing, this article had been reviewed for accuracy, no additional cases had been reported, and DeNotta reported that the veterinary team hopes this is an isolated incident.