Last year there were many questions about using Botulism vaccine. We have discussed it here and with colleagues and have decided to recommend its use to our clients. We vaccinated all of our own horses last Fall and the process was smooth without any complications.
The disease is not new. It is caused by botulism toxin, produced by a bacteria, Clostridium botulinum. The resultant disease is in the form of paralysis. This toxin is a neuroparalytic unlike its “cousin” tetanus (Clostridium tetani) which is a neurostimulant. This disease is just as deadly as tetanus and horses are exquisitely susceptible. Like with tetanus, botulism is ubiquitous in the environment and all horses are at risk. It is not something we see commonly but recent weather changes and hay availability seem to be helping create an increase in the number of cases – causing us to rethink the value of vaccination.
Adult horses most often see botulism as forage poisoning. This results when the horse ingests preformed botulism toxin and then absorbs it through the intestine. Typically this occurs due to decaying vegetable material with bacterial proliferation and toxin production, rather than from contamination by a dead animal. A carcass in hay can still serve as a disease source for your horse. Some animals have the toxin in their intestinal tracts and as they decay that toxin can contaminate the hay. As mentioned, horses are very susceptible to this toxin and hay that cattle ate safely could still poison and kill a horse. The most frequent culprit is ensilage, or plastic wrapped haylage bales, but any improperly cured hay, or hay containing a carcass, could pose a threat.
Foals are more commonly affected by a variant of the disease called toxicoinfectious botulism. The C. botulinum bacteria persist in soils for a very long time, as spores. Foals may ingest these spores and then toxin is produced in their digestive systems. Adult horses are more protected by a robust intestinal flora as well as more developed digestive enzymes and acids (think of human babies that should not be fed honey while adults can).
Horses affected by this toxin usually show signs within 24 hours. Early signs may be subtle, like difficulty swallowing, more saliva, and taking longer to eat. Dilated pupils may be noticed. As the disease progresses more obvious weakness is apparent. In this case, just weakness, not the clumsiness or ataxia we see with other neurologic diseases like EPM. Muscle tremors or fasciculations may be observed (affected foals are called “shaker foals”). Eventually the horse will go down and die from respiratory failure.
Treatment of this disease is often unrewarding. An antiserum exists but is expensive to use and difficult to acquire. Aggressive and intensive nursing and supportive care are needed – such as tube feeding and a sling to help the horse get up and down. The prognosis is poor and most of these cases do not recover.
Botulism, while difficult and unrewarding to treat, is very preventable. The vaccine has been used for many years in places where the disease was thought to be more prevalent (most pregnant mares in KY are vaccinated to protect foals). With the rise in cases here and the increasing difficulty in ensuring a good supply of quality hay, we think it makes sense to protect your horses with this vaccine. The vaccine is given as a series of 3 doses, one month apart, the first year. Thereafter, a single annual booster will maintain immunity. Pregnant mares should be vaccinated in months 8, 9, 10 of gestation. Most adult horses could have this vaccine added as part of their Fall vaccines.