The Horse

DEC 2018

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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24 December 2018 The Horse | TheHorse.com distressed, and wanting to lie down. Her heart rate was high, 60-80 beats per min- ute, and she didn't have very good gastro- intestinal (GI) sounds when we listened to her. She had no fever, and when we passed a nasogastric tube, we didn't obtain any reflux from her stomach (as you might expect to find in a colicking horse). One of the first signs that led to our botulism diagnosis was that the horse had difficulty swallowing the stomach tube (affected horses typically don't swal- low well), she says. The second clue was that when she lay down, she lay perfectly still. The sweating and shaking stopped; she looked much more relaxed. Horse with botulism are using all their energy reserves to recruit muscle fibers to keep them on their feet. When they lie down, they aren't fighting gravity anymore and can relax. Another clue was that because botulism causes weakness rather than pain, typical painkillers used for colic (sedatives such as xylazine or analgesics such as flunixin meglumine) won't make a horse with botulism look or feel any better. Once we have a clinical suspicion of botulism, we'll typically do a tongue stress test—pulling the horse's tongue out of his mouth to see how quickly he's able to retract it. We might also do a grain test where we offer 8 ounces of sweet feed in a pan to see if he's able to eat any and how quickly he consumes it. A normal horse will almost always consume it within two minutes. But horses with botulism, even if they're interested in the grain, will take a couple of bites and chew and chew, but because they're unable to swallow will drop it from their mouth. It might even come back out through their nose. Treatment Johnson describes her ap- proach: The toxin binds to the part of the neuron—the lower motor neuron—that tells muscle fibers to contract. Once the toxin begins to take effect, you see in- creasing muscle weakness progressing to very flaccid paralysis. Every hour or two you wait, if the horse is still absorbing toxin from his GI tract, the disease will continue to worsen. Once we're convinced the clinical diag- nosis is botulism, we give the horse a dose of antitoxin, a plasma product from anoth- er horse that has been hyperimmunized and built antibodies against the toxins. The antitoxin doesn't reverse clinical signs; it halts disease progression by at- taching to botulinum toxin still circulating in the bloodstream and binding it before it can get to the lower motor neurons. The mortality rate is high among horses exposed to a large amount of toxin and not treated quickly. And the critical treatment window varies with the type and amount of toxin consumed. The only thing that can clue you in is the rate of progression. If a horse ingested just a small amount of toxin, he might eat or drink abnormally for several days but still have the ability to walk around. He won't spend much time lying down and will only have a little difficulty swallow- ing. But if the horse has ingested a lot of toxin, he can go from appearing totally normal to being recumbent (unable to rise) in less than 12 hours. The good news is, if you can stop disease progression, horses can recover fully and return to their previous level of athletic performance, usually with no long-term effects. Along with antitoxin, supportive care via a nasogastric tube, due to the horse's inability to swallow, can maintain hydra- tion and nutrition. Sometimes veterinar- ians give intravenous (IV) fluids. And sometimes, because of the horse's weak swallow, aspiration pneumonia makes antibiotics necessary. Also, painkillers can alleviate discomfort from not being able to shift position while lying down. Finally, if the horse doesn't have a strong blink we can apply eye lubrication. Outcome Usually it takes 10-14 days for a horse to regain full ability to swal- low, so you might need to tube the horse or provide supplemental nutrition and hydration for as long as two weeks. If the horse has lost his ability to get up, he might need the help of a sling for two, three, even four weeks until he's regained that ability. Standing even for 15 minutes to an hour gets the blood flowing in all those muscle groups and prevents com- plications, such as limb contractures, that can come from recumbency. Happily, the case mare recovered com- pletely and is now a successful brood- mare in Kentucky. Equine Protozoal Myeloencephalitis (EPM) Treating veterinarian: Bill Gilsenan, VMD, Dipl. ACVIM, internal medicine spe- cialist at Rood & Riddle Equine Hospital in Lexington, Kentucky Frequency Rood & Riddle veterinarians see 25-50 cases of this neurologic disease per year. While exposure to its causative agents is fairly common, only a small per- centage of horses develop clinical signs. Etiology Two agents cause EPM: the protozoon Sarcocystis neurona (the most common) and Neospora hughesi. Opos- sums, the definitive host, shed infective Dire Diagnoses Veterinarians will perform a neurologic exam on horses suspected of having EPM. DUSTY PERIN

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