Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

An outbreak of botulism has killed 5 horses on a farm in Wisconsin, and is apparently also affecting cattle in the area.

It is suspected that the spring melt played a role in this outbreak, as was reported in a review of Type A botulism cases in horses that I discussed the other day. Botulism occurs when the bacterium Clostridium botulinum grows and produces toxins. To grow, it needs an oxygen-free environment. Clostridium botulinum can live in the environment in its dormant spore form for years, waiting for the right conditions to start multiplying. With a spring melt, you can get accumulations of hay that are soaked in water and starting to decompose. That can create a nice moist, oxygen-free environment with a good bacterial food source - perfect conditions for C. botulinum. If horses eat hay that the bacteria are growing in, they can get botulism. Other possible sources include feed that is already contaminated with toxin, something that is of particularly concern with large bales of hay, haylage and silage. Horses are exquisitely susceptible to botulinum toxin and extremely small amounts can cause disease. Identifying the actual source can be tough, but hopefully efforts to do so are underway in Wisconsin.

Three of the 8 horses on this farm survived. They were all treated with anti-toxin, which is very expensive (thousands of dollars). It's hard to say whether the antitoxin worked or whether the surviving horses actually weren't exposed to the toxin as the others were, but treating all potentially exposed horses is the preferred approach if you have rapid access to antitoxin and can afford it.

Photo soucre: www.edu.gov.mb.ca

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Botulism is bad. Horses are particularly susceptible to this disease, and mortality rates are very high.  Botulism is caused by the bacterium Clostridium botulinum. Specifically, it's caused by toxins that this bacterium can produce. Botulinum toxin is the most potent toxin ever discovered, and even minuscule amounts can kill a horse.

The disease comes in two main forms:

• Toxicoinfectious botulism: This occurs when the C. botulinum bacterium grows in the intestinal tract and produces toxins. This happens most often in foals and is also called "shaker foal syndrome."
• Ingestion of preformed toxin: This occurs when horses ingest feed that has been contaminated with botulinum toxin, such as haylage or silage that has been improperly produced or stored, or that has been contaminated by an animal that has died of botulism. This form can affect horses of any age.

There are also different types of C. botulinum, designated A through G (with 2 different C's, making a total of 8 types). In North America, most infection are caused by type B, but some type A and C infections also occur. A recent paper in the Journal of Veterinary Diagnostic Investigation (Johnson et al. 2010) describes 10 years of type A botulism cases in horses that were identified by the University of Pennsylvania's Botulism Laboratory. Here are some of the highlights:

• Three isolated cases and eight outbreaks were identified. All of the isolated cases were in foals.
• One case of wound botulism was identified in a foal. This is a rare situation where the C. botulinum bacteria grows in a wound and produces toxins. This occurred in a foal with an umbilical infection.
• In the outbreaks, the number of affected horses per farm ranged from 2 to 24. Anywhere from 9 to 100% of horses on farms with outbreaks got sick. Mortality (death) rates ranged from 67 to 100%, with an average of 90%. One farm lost 24 horses. (So, with up to 100% of horses affected on a farm and up to 100% mortality, you can see why outbreaks, while rare, are still a major concern).
• Contaminated hay or silage was identified as the cause of 6 of the 8 outbreaks. The source wasn't identified in the other 2.

There were some interesting aspects of the different outbreaks too:

• One outbreak was associated with feeding large (~ 1 ton) bales of hay. The owners reported that conditions were warm and wet and the hay was wet and matted. This is the farm that lost 24 horses (20 in the first 3 days).
• Another outbreak also involved 1-ton bales, and the owners reported the area where the bales were fed was wet from an abnormally warm January and melting snow. Apparently, hay that fell became frozen in the ground around the feeders and horses were noted to have eaten it when it thawed out in the spring. Those were the same horses that got sick.
• The other outbreaks involved spoiled feed sources, not weather conditions that might have resulted in growth of C. botulinum. These included a 1-ton bale of hay parts of which were damp and moldy, one 3-year-old bale of silage, a bale of haylage with a pH of 5.6 (which would allow for C. botulinum growth) and ten 1-ton square bales with wet. moldy hay piled on top of them.

As you can see, there are some clear similarities, mainly involving feeding large bales and wet/moldy hay. Clostridium botulinum is not a mold, but moldy hay indicates improper production or storage of the hay and the potential for growth of microorganisms like C. botulinum. Particular care must be taken when using large bales or feeding silage, since it is easier from an anaerobic (oxygen-free) environment to be created (which is another requirement for C. botulinum growth). These are important risk factors for botulism, and while it's a rare disease, it can devastate a farm. It's much better and cheaper to discard questionable forage than feed it and risk an outbreak. Mortality rates are high, and successful treatment is difficult and extremely expensive. If in doubt, throw it out.

Photo credit: fivefiveandahalf

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Botulism is suspected in the deaths of 10 horses on a farm in Wyoming, USA. Very few details are available to indicate why botulism is suspected or what the source might be. It was reported that six horses died initially, and four others that were lethargic and unable to stand died within 24 hours. These signs fit with botulism, which is characterized by profound muscle weakness and progressive paralysis. Death is caused by an inability to breathe when the paralysis starts to affect the muscles that control breathing. Making a diagnosis of botulism can be difficult in horses - it is often a "diagnosis of exclusion" whereby the diagnosis is made because everything else that could cause the condition has been ruled out.  Testing of some of the affected horses is underway. It was not reported whether possible sources of the disease (e.g. feed) are also being tested.

Botulism is caused by a toxin produced by the anaerobic bacterium Clostridium botulinum. In adult horses, the disease almost always occurs from consuming the toxin in contaminated feed.  If the bacterium (which can be found in the soil in many regions) gets incorporated into feed, under the right conditions the organism will grow and multiply, and produce the powerful neurotoxins that cause botulism. Fermented feeds like haylage and silage are common sources, because the way they are stored is more likely to create an environment where C. botulinum can grow.  However, various other food sources have also been implicated in some outbreaks.

Botulism is rare, but typically fatal in adult horses. Some basic tips to reduce the risk of botulism are:

• Avoid feeding horses haylage and silage, especially in areas where botulism is more common. There are some benefits to using feeds, so they are commonly used in some regions, but if you use fermented feeds you should discuss the risk of botulism with your local veterinarian, and take measures to reduce the risk of C. botulinum growth.
• Vaccines against botulism are available. Adult horses are usually not vaccinated, other than mares which are vaccinated to reduce the risk of botulism in their foals (known as "shaker foal syndrome"). Adult horses being fed high-risk feeds could be considered for vaccination, but if the feed is so high risk that vaccination is warranted, perhaps the feed program should be reconsidered.  Furthermore, there are several types of C. botulinum, but the vaccines only protect against some of them. If you are thinking about vaccinating, you should make sure that the vaccine you're going to use protects against the botulism strains found in your area.

If you ever have a horse that is showing signs of botulism (e.g. weakness, difficulty rising, dropping food out of its mouth, difficulty swallowing), or if botulism is suspected in a sudden death, immediately stop feeding any potential high risk feeds until the cause of the disease is identified. This might help reduce the number of affected horses.

More information on botulism can be found in our botulism archives. An information sheet about botulism in foals can be found on the equIDblog Resources page.

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Equine dysautonomia, or "grass sickness", is a very important disease in some areas of the world, particularly the United Kingdom and some parts of South America (where it's called mal seco). The cause of the disease is still unclear, but there is increasing evidence implicating the bacterium Clostridium botulinum, which produces the toxin that causes botulism. There are several forms of grass sickness, one of which is the chronic form. This form of grass sickness is potentially survivable (unlike the other forms which are invariably fatal), although treatment can be long and difficult. To help guide people managing horses with chronic grass sickness, a free-access online booklet has been released by World Horse Welfare and the Royal (Dick) School of Veterinary Studies. It is a comprehensive booklet that provides a wealth of information about this disease, with an emphasis on how to care for affected horses. The document can be downloaded here or by visiting the World Horse Welfare website.

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Preliminary evidence has suggested that botulism might be the cause of botulism may be the cause of death of approximately 100 horses at a large breeding facility in Florida housing approximately 400 mares. The horses died over a period of about 7 days, with signs of neurological disease. There are no reports of affected horses at other facilities in the area.  The clinical signs in these horses and lack of evidence of another disease are strongly suggestive of botulism. This farm apparently fed the horses haylage (although some reports seem to alternate between using the words "hay" and "haylage"), which is a high-risk feed source for botulism.

In adult horses, botulism is caused by ingesting a toxin produced by the bacterium Clostridium botulinum. This bacterium will not grow in the presence of oxygen, however in can grow in conditions that are sometimes present in improperly fermented haylage and silage. As the bacterium grows, it produces botulinum toxin, one of the most potent toxins on the planet. Ingestion of botulinum toxin leads to progressive paralysis (i.e. severe weakness and flacid muslces).

Treatment of botulism is difficult and can be expensive, and the mortality rate for this disease is high. Some people choose to feed haylage and silage despite the risk of botulism. If you choose to feed haylage, silage or other high-risk feeds:

• Ensure that haylage/silage is properly prepared and stored.
• Consider vaccinating horses that are fed haylage/silage against botulism. Be aware, however, that vaccines do not protect against all strains of botulism.
• Immediately stop feeding haylage or silage if any horses show signs that could be consistent with botulism (e.g. weakness, problems eating).

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Click on any of the highlighted links below for more information about these horse-related infectious disease topics. Topics that are not highlighted are in development and coming soon. New information will be added as it becomes available, so be sure to check this page regularly for the latest updates.

All information sheets found on this page can be freely downloaded, printed and distributed. The authors only request that this website (www.equIDblog.com) is acknowledged as the source.  The downloadable files on this page can be opened with Adobe® Reader®.  To get the latest version of Adobe® Reader® for free, click here.

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