Botulism in horses: Veterinarians should be cognizant in their diagnosis
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
Early diagnosis is keyJohnson 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."
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.
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.