Botulism in horses: Veterinarians should be cognizant in their diagnosis

Knowing what to look for and being diligent in gathering the patient's history will help diagnose this often overlooked condition.
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Oct 01, 2013

When exposed to the neurotoxin Clostridium botulinum, horses develop botulism, a disease of progressive flaccid paresis and cranial nerve deficits. The disease is almost always acquired in one of three ways: through the ingestion of preformed toxin with food (food-borne botulism in adults), through the ingestion of C. botulinum spores that subsequently germinate in the gastrointestinal tract and elaborate toxin (toxicoinfectioius botulism in foals) or through contamination of wounds with C. botulinum and subsequent bacterial growth with toxin release (wound botulism).1


Horses with botulism tend to drop food and saliva from their mouth and nose as they attempt to eat. (PHOTOS COURTESY OF DR. SUSAN MCADAMS-GALLAGHER, NEW BOLTON CENTER AT THE UNIVERSITY OF PENNSYLVANIA)
According to Amy Johnson, DVM, DACVIM, assistant professor of large animal medicine and neurology at the New Bolton Center at the University of Pennsylvania School of Veterinary Medicine, botulism in horses is commonly seen in the mid-Atlantic states and Kentucky as serotype B and as type A in the western states, which include California, Washington, Oregon, Idaho, Montana, Wyoming and Nebraska. Type C is sporadically seen across the United States. Types A and B are associated with soil, and type C is associated with carrion. "Equine type B cases account for more than 85 percent of equine cases diagnosed in the U.S.," states Johnson.

Early diagnosis is key

Johnson states that because botulism can easily mimic other diseases, such as esophageal obstruction (choke) or colic, it's important to always have it on your differential diagnosis list. Practitioners should focus on the types of cases in which they should consider botulism and differentiate it from other diseases. "Many veterinarians do not commonly see botulism cases in veterinary school," says Johnson. "And if they don't recognize the signs, they might not have it on their list and, therefore, not easily diagnose it."


The clinical signs of botulism often include difficulty eating and poor hay consumption due to lack of strong tongue tone.
Early diagnosis is key to providing appropriate treatment and, hopefully, saving the horse, but the signs aren't always clear-cut in the early stages of disease, says Johnson. "By the time the horse is recumbent and clearly very weak and dysphagic, it's easy to diagnose," she says. "But the horse that presents not eating very well and looking a little bit colicky, or the horse that's a little bit dysphagic without any other signs of weakness—those are the cases that may be missed."

Botulism vs. choke

Botulism can easily be mistaken for choke because frequently the horses will have food coming out of their mouths and noses. And though often they'll have weak tongue tone, it's not necessarily obvious in the very beginning, says Johnson.


A grain test, which involves timing a horse to see how long it takes him to consume feed, can help diagnose botulism.
One thing that can help differentiate botulism from a choke case is if the practitioner passes a nasogastric tube and it passes easily. There are some choke cases that can be relieved on the first pass, but if it appears that there's not a very significant obstruction, the practitioner should consider whether it's actually a botulism case, Johnson advises.

If botulism is suspected, a tongue stress test and a feed test can be performed to determine if it's dysphagia and not an esophageal obstruction. The tongue tone or tongue stress test is done by gently withdrawing the tongue from the horse's mouth and assessing the horse's ability to pull it back while holding the jaw closed. With botulism, the tongue will tend to hang, not having the strength to be easily retracted.

The feed or grain test involves feeding the horse eight ounces of grain in a bucket while timing how long it takes for the horse to consume the feed. Healthy horses finish their feed within two minutes, whereas horses with dysphagia do not. Not only do they not consume their feed easily, they also tend to drop their feed and release saliva from their mouth and nose as they attempt to eat, leaving a trail of saliva in the bottom of the feed tub.

"Not every botulism horse wants to eat," Johnson notes. "Some of them come in slightly depressed. They stop trying to eat, which is another reason that the disease is not so obvious when first presented."

That's when it's really important to critically evaluate the tongue, she says. Another possible clue to botulism is that the horses will often, but not always, displace their palates and display dysphagia. In a choke or colic case, that would be an unlikely finding.