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Most often contracted through contaminated or decaying foodstuffs, the botulinum toxin enters the bloodstream through the gastrointestinal tract, where it is dispersed to the nerves throughout the body. The toxin blocks the release of acetylcholine, a neurotransmitter that sends impulses from the nerve cells to the muscle cells, thus impairing nerve and motor function. Since the muscles are unable to contract, weakness and the inability to eat are some of the first signs. As the motor impairment progresses to paralysis of the muscles, and eventually to the diaphragm, it can affect the ability to breathe. Death can be rapid, and can occur 1 to 2 days from contraction of the bacteria.
Botulism occurs when a horse ingests the neurotoxic bacterium Clostridium botulinum, which is found in soils, sediments, plant matter, and in the intestinal tracts of animals. Toxins are created by this bacteria in an environment of decaying plants or animals that can affect nerve function. While sometimes, no symptoms are seen before an animal is found dead, typically there is a progressive motor impairment that leads to paralysis. Though the disease is not common, it is fatal and needs to be treated immediately.
The symptoms involved with botulism are caused by muscle paralysis, and include:
Additional symptoms seen in foals include:
There are three main types of botulism seen in horses, with a couple subsets.
Also called forage poisoning, this refers to when the Clostridium botulinum toxin is contracted through eating hay or silage contaminated with it. Contamination can occur from decaying vegetable matter, or from decaying carcasses that have accidentally cross-contaminated the horse’s foodstuffs. This is the most common type of botulism.
This is when the bacteria grows inside living animal tissues and produces toxins inside the animal. It can cause further internal symptoms of gastric ulcers, lesions in the GI tract, abscesses in the lungs, and other wounds. There are a few subsets of botulism that fall under this type. Shaker foal syndrome is a form of botulism affecting foals older than 4 weeks, often those that have not yet been vaccinated. Muscle trembling gives its name to this form that causes similar symptoms of motor paralysis. Death often results with 24 to 72 hours after the first signs are seen. Equine grass sickness, or equine dysautonomia, occurs when the bacteria is ingested from the soil and produces the toxin inside the gastrointestinal tract.
This is least common type, and results from wounds becoming contaminated with spores of the bacteria. The bacteria then grows and produces the toxin in the wound itself, and then is absorbed through the bloodstream.
There are three ways in which horses can acquire the Clostridium botulinum toxin.
Botulism can be difficult to diagnose, either because it can be fatal before symptoms are present, or because it can mimic other problems, such as choke or colic. Often, a diagnosis is come upon after eliminating other potential causes of paralysis. Another problem is that the bacteria is hard to detect with current testing methods. Relating all information about symptoms and behavior of your horse to your veterinarian can be invaluable, as this disease progresses swiftly.
After a basic exam, routine blood tests, including a CBC and serum testing, are performed. Often, negative results from these tests can clue a vet in to a case of possible botulism. A PCR test may be given, but since the results can take a few days to receive, if botulism is suspected, treatment needs to begin before the results are seen. Samples of the horse’s foodstuffs may be taken to try and identify the toxin. A tissue sample of your horse may also be tested for the toxin.
To differentiate between botulism and choke, your veterinarian may try to pass a nasogastric tube down your horse’s throat to see if there is an obstruction. If there isn’t, it could be botulism. Further differential tests include a tongue stress test, which assesses your horse’s ability to retract his tongue, and a feed test, which times how long it takes for your horse to eat.
The main treatment for botulism in horses is the rapid and early administration of a hyperimmune plasma that contains an antitoxin. It is important to know what toxin is being treated, but since the turnaround time for test results is longer than the lifespan of an affected horse, giving an antitoxin that is the most likely to be present may be the best course of action to ensure your horse’s survival. The plasma is obtained from immunized horses, and the antitoxin works by binding the toxin molecules before they can bind to nerve cells. Once bound, the toxin cannot be unbound. However, horses can make new neuromuscular junctions to replace those the toxins are bound to within 7 to 10 days.
During that time, your horse will need supportive care as it lies recumbent, and possibly unable to eat or drink. Adult horses can develop pressure sores and muscle damage, and may need to be rolled. Maintain fluids intravenously or through a stomach tube with a feed slurry. Antimicrobials may be prescribed to deal with wounds or subsequent pneumonia. Any other supportive care, such as eye ointments, bandaging, or a urinary catheter, are given on an as needed basis. Ventilation may be needed if breathing muscles are affected. While easier in the foal, there isn’t currently technology to ventilate an adult horse. In those cases, it is often recommended to euthanize.
Recovery is good if administration of the antitoxin is early enough. If the disease has progressed too far, and your horse is unable to breathe, the condition can be fatal. Make sure any dietary issues are taken care of, and any contaminated foodstuffs are disposed of. Any decaying grass should be removed. Prevent this fatal infection through vaccination. Vaccination in pregnant mares can prevent botulism in foals.
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