Review Of Botulism In Horses
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
