Botulism in Horses
The word Botulism may sound familiar to you. Botox, anyone? This disease in horses is caused by a neurotoxin produced by Clostridium botulinum (same genus as the bacteria that causes Tetanus). This neurotoxin is the most lethal toxin currently known and comes in types A-G. Horses are also the most susceptible species to this toxin. These bacteria are spore-forming, anaerobic organisms. This means when the environment is right (no oxygen) they make spores and their neurotoxin. The right environment can be anything from an animal carcass or decaying, fermenting plant material. These bacteria live in the soil across the entire United States.
How do horses get Botulism? Most horses get this disease through the ingestion of the toxin from a food source. Horses most commonly are intoxicated with Type B, while Types A and C also occur. History usually indicates the ingestion of silage, round bale hay or other contaminated forage. The horses may have gotten out and eaten cattle forage, silage or hay. Rarely, horses can get the disease through deep puncture wounds. It is also possible for foals to get this disease termed “Shaker Foal”. They ingest the bacteria itself and then the toxin is produced in their gut. This is possible due to the difference in the gut of a milk drinker (foal) and that of a forage eater (adult).
Clinical signs are weakness with a difficulty in eating and swallowing. Horses will also develop muscle tremors, weak tongue and anal tone. Excessive salivation with saliva and feed present in the nostrils is another sign. Eventually a flaccid paralysis occurs. Horses will progress to a complete recumbency (down and cannot rise). Death is often due to respiratory failure. Often times owners confuse early clinical signs with choke or colic. The neurotoxin causes these clinical signs because it binds to the ends of nerves and blocks transmission of nerve impulses.
Diagnosis can be difficult. Not all horses progress at the same rate and early clinical signs can be deceiving. An 8 oz grain test is often performed. Horses should be able to easily eat and swallow 8 oz of grain in 2 minutes. Blood testing can be performed however the test is not very sensitive and it takes a very long time for results to come in. Often, by this point the disease will have progressed to a point of no return.
Treatment is time consuming and expensive. If a horse remains standing, the prognosis is still guarded. Once completely down, the prognosis is grave. There is an anti-toxin made by several companies that if given early in the course of the disease will bind any circulating toxin. Horses may still progress for several days as toxin already bound is not affected. These horses often require intravenous or oral feeding. Slings can be used to help assist horses to stand however exertion will make muscle weakness worse. Down horses will need re-positioned and often develop secondary infections. Eventually the nerves will regenerate but this can take a week to 10 days.
Prevention is possible. A vaccine is available however for only Type B. This is a 3 dose series given every 4 weeks then yearly afterwards. The vaccine is very effective but no cross protection for other types is noted. Do not compromise hay quality for price. If you find a dead animal in your hay then you should throw out that bale. However, many times an animal may not be seen. If your horse gets out with cows and eats silage or haylage then you should contact your veterinarian so you can go over symptoms of botulism. Clinical signs can occur as soon as 12 hours and as far out as 10 days from ingestion depending on the amount of toxin ingested.