One of the most lethal diseases in equine medicine, Botulism, is caused by a toxin produced by the bacteria named Clostridium botulinum. The genus (or group) of bacteria that is called Clostridium can affect many species of animals and man. They can be commensal (living together with no disease state) or opportunistic (when conditions are right the bacteria can grow) pathogens affecting many different organ systems in the patient’s body. The organ systems typically affected in all the animal species and man are the gastrointestinal tract (GI), liver, muscles, and/or nerves. So the symptoms can be for GI disease, vomiting, diarrhea or bloody diarrhea; for muscles, swelling and pain and abscessation of the muscle; for nerves, paralysis of peripheral nerves in specific areas of the body but not affecting the spinal cord (unlike the Herpes or various encephalitis viruses which attack the spinal cord and brain). Sometimes the aggressive nature of the Clostridium infection results in peracute death of the patient with little or no outward clinical signs. Botulism is caused by neurotoxins found in the spores of the bacteria not the bacteria itself. Spores are found in the soil worldwide. The strains of Botulism are categorized by the toxin they produce; A, B, C1, D, E, F, and G. Equine Botulism has been reported to be caused by types A through D. Most commonly type B is the most widespread in North America but types A and C can also be involved. Kentucky and the Mid-Atlantic regions are the two most common regions of the United States affected. Two main forms of the disease are typically reported. The first is called “Shaker Foal Syndrome”. It affects foals 1-2 months of age, and less typically as young as 1 week and as old as 6 months. The C. botulinum grows in the foal’s colon, produces spores which in turn produce toxin which then makes the foal sick (unable to nurse and collapsing with muscle weakness). The second form, referred to as “Forage Poisoning”, affects adult horses after ingesting preformed toxin from the environment. This can come from decaying vegetable matter like spoiled hay, haylage, silage, or grain where the bacteria can live. Some reports suggest wild animals, which may harbor the bacteria, die or are accidentally killed in the hay baling or feed storage process, contaminating the feed source for the horse, therefore exposing the horse to the preformed toxin. Because of this, even horses not turned out on soil or pasture (confined at a racetrack or training facility) can be affected because of exposure to potentially contaminated hay and feeds. Most adult horses present as a “choke” with difficulty swallowing, and feed and water being discharged through the nostrils. So what is going on with this neurotoxin? To understand the disease you have to understand the micro-anatomy of the nerve and the muscle. Muscles work because they get a signal from the nerve to make it contract. That occurs with a structure called a neuromuscular junction. There is a chemical released from the nerve called acetylcholine which flows across the junction and stimulates the muscle to contract. Botulism toxin irreversibly binds to the receiving side of the junction not allowing the acetylcholine to stimulate the muscle. Therefore the muscle cannot contract. This results in the muscles losing their tone. Depending on which muscle group or groups are affected results in the clinical signs of flaccid muscle paralysis that we can identify as disease. Most typically the muscle weakness followed by complete paralysis affecting the face, tongue, throat, and muscles of respiration. So foals cannot nurse and adult horses cannot swallow water or feed. This progresses to weakness of skeletal muscles, then paralysis of the intercostal muscles and diaphragm. Some horses present with colic symptoms, as the toxin stops the normal GI tract musculature from contracting with normal peristaltic waves, resulting in gas build up and pain. Depending on the amount of toxin either formed in the gut or ingested, foals and adults are found struggling to swallow and drink or down in the field struggling to breath. The typical course of the disease is about 10 days until death. Treatment is aimed at counteracting the toxin with a specific antitoxin, administered I.V., and massive supportive care to evacuate the GI tract of all remaining toxin, with nutritional and intravenous fluid support. The earlier in the course of disease the treatment is begun the better the chance of a positive outcome. Because Botulism lives everywhere on the East coast, vaccination against botulism affords the best protection. All horses should receive a 3 shot series of the vaccine administered 30 days apart followed by annual boosters. Foals should begin their series starting at 3 months of age. It can be administered at a younger age if botulism is considered endemic on a farm. Pregnant mares should receive the vaccine 30 days before their due date to convey excellent passive immunity to the newborn through the colostrum. The vaccine conveys excellent immunity in young and old horses. Talk to your veterinarian at Brenford Animal Hospital about including Botulism vaccine in your annual vaccination program for your horses. Don’t let this preventable disease cut short the life of a potential world champion. - PFH (1.14.14)
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